- Abraham's Tree
- Yes, the first father of all Hebrews from the Old Testament (Genesis 17), resting at the
base of the Hebrew family tree, has a weather feature named after him.
"Abraham's tree" is the name given to a cloud form which consists of feather and
plume like appendages of cirrus emanating from a point on the distant horizon. In mid
latitudes, this form is most often seen on the western or southern horizon with the
approach of a warm front or as cirrus blows out from the top of a distant thunderstorm
cloud (cumulonimbus). Thus, from the observer's perspective, rain is not very far away. In
spring, as cloud ceilings rise and convective precipitation becomes more frequent, we may
see examples of Abraham's tree in Minnesota. Some of the old forecast rules are "when
Abraham's tree has its foot in the water" (emanates from a dark cloud base) "it
will soon rain," or "Abraham's tree is blooming; it is going to rain."
- Ac
- Most people who see or hear these letters spoken in the summer immediately think of air
conditioning. However, meteorologists (who think differently from normal people) readily
associate these letters with the international observation code for altocumulus clouds.
These are common summertime clouds which form in the middle layers of the atmosphere. They
are puffy, rounded masses, some with considerable vertical development. They typically
form between 6500 ft and 25,000 ft. They indicate moderate instability and turbulence in
the middle layers of the atmosphere, but they are not associated with severe weather.
- Acicular Ice
- After the Latin word "aciculus," meaning needle-like, "acicular ice"
refers to a type of ice which forms in fresh water (lakes and slow moving rivers) between
the bottom of the ice layer and the contact point with the underlying water surface. This
form of ice, sometimes also called "fibrous ice" or "satin ice"
consists of long crystals or needles and sometimes hollow tubes extended from the
undersurface of the ice cover. It is more visible in the late winter or early spring as
lake ice cover begins to thin out.
- Aeolian Sounds
- Aeolian sounds are produced by the action or effect of the wind. They tend to become
more perceptible in the fall as wind speeds increase. Eddies or currents of air formed
immediately beyond an obstructing object (such as the roof of a building, a chimney, a
tree, or telephone wires) can produce their own sound, with a pitch that varies directly
with the wind speed and inversely with the diameter of the object obstructing the wind.
This is what leads to the humming of wires, the whispering of pine trees, and the howling
around rooftops. Other aeolian sounds (such as the rattle of dried corn stalks in the
field, the rustle of leaves down the sidewalk, the creaking and groaning of tree branches,
and the flapping of flags) are due to combinations of wind and other factors.
- Afterheat and Aftersummer
- These are very old terms used to refer to the warm pleasant days in the late autumn (at
or after Indian Summer). It was thought that the residual heat of summer stored in the
ground was released under sunny skies and helped produce temperatures that were well above
normal for the late autumn. Though the soil does indeed accumulate and store heat during
the summer months, periods of late autumn warmth are more often attributed to advection -
the regional-scale transport of heat carried in the surface winds from one area to
another. Temperatures which reach 10 to 20 degrees above normal this time of year cannot
be generated locally by the low sun beating down on the Earth or by heat released from the
ground.
- Agglomeration
- Contrary to popular opinion, agglomeration is not what they served the frontier soldiers
at the Fort Snelling Commissary!
"Agglomeration" is a term used in cloud physics to describe a precipitation
process whereby water droplets or ice crystals grow in size by collision and assimilation
with other precipitation particles. When two colliding water droplets form a new larger
droplet, this agglomeration process is called "coalescence." When an ice crystal
collides with a supercooled water droplet which is assimilated and freezes, this
agglomeration process is called "accretion." Accretion may eventually lead to
the formation of hail. Snowflakes are agglomerations of various ice crystals as well.
- Albedo
- This term is derived from the Latin word, albus which means whiteness. Meteorologists
and climatologists use this term to refer to the characteristic reflectivity of a surface
or composite landscape. Technically, albedo refers to the ratio of the amount of radiation
(visible or total solar) reflected by a surface to the amount of radiation incident upon
it. Albedo can make a big difference during the winter season, as bare ground or forest
canopies may have an albedo of 10 to 20 percent, while fresh snow cover over prairies or
flat agricultural fields may have an albedo of 80 to 90 percent. Thus, the absorption of
the sun's radiation at the surface, which provides the energy to heat the air, may be
greatly reduced by snow cover. As measured from space borne instruments, the planetary
albedo including the radiation reflected by the composite land surface of Earth and its
atmosphere is about 30 percent.
- Anhyetism
- This is not a disease, but in many parts of Minnesota people are suffering from this.
Hyetos is the Greek term for rain, so the meaning is deductive - lack of rainfall. The
term is rarely used anymore, but was used at one time by academics who loathed the use of
the term drought to describe short periods without rainfall.
- Apparent temperatures
- This has been used more commonly since about 1980 to refer to what various temperature
and humidity combinations feel like based on human physiology and clothing science and the
need for the body to maintain a thermal equilibrium. It particularly applies to the summer
months, because relative humidity is much less important to human comfort when air
temperatures are below 40 degrees F.
When relative humidities are very low, as in arid conditions, say below 30 percent, the
air can actually feel cooler than indicated by a thermometer, because of evaporative
cooling effects on the skin. On the other hand, when relative humidities are high, say
above 60 percent, then we can feel warmer than the air temperature indicated by a
thermometer because of the increased resistence to moisture and heat loss by our bodies
(particularly if there is little air movement).
- ASOS
- This is a National Weather Service acronym for Automated Surface Observing System. It is
a system of weather sensors, data collection hardware, acquisition control modules,
communications devices, and peripherals and displays for monitoring local environmental
conditions, primarily at airports. Over 1700 such units are being deployed around the
country to replace manual observations. There is now one operating at the MSP airport,
reporting air temperature, cloud ceiling, visibility, dew point, and wind among other
things. Forecasters use the frequent ASOS reports to refine and update forecasts. In
addition, for some locations computer generated voice reports are available for pilots to
dial in and get airport conditions. ASOS systems are being evaluated, but have thus far
been subjected to a number of criticisms, including the omission of important climate
observations, such as snowfall, snow depth, snow water equivalence, cloud ceiling above
12,000 ft and obstructions to visibility (like smoke or dust). In some cases at busy
airports, ASOS data transmissions are augmented by supplementary manual reports on other
elements of the climate. It may be a number of years before data users are satisfied that
ASOS data and reports are comprehensive enough to document local climatological
conditions.
- Attery
- This is taken from the old Anglo Saxon word 'atter' meaning poison or inflammation. The
Scottish Meteorological Office will still occasionally use this term to describe a spell
of stormy weather, implying that it is like a poisonous, or inflamed condition of the
atmosphere. Usually November brings "attery" weather to Minnesota, but there has
been a conspicuous absence of it so far this month.
- Bai-u, Plum Rains, and Mold Rains
- "Bai-u" (in Chinese pronounced "mai-yoo") is the name of the rainy
season in southern Japan and eastern China (generally April through July), when abundant
rainfall suitable for the cultivation and transplanting of rice usually occurs. These
rains are sometimes referred to as "plum rains" when they coincide with the
ripening of the plum crop, or "mold rains" when they lead to the outbreak of
this form of plant disease.
This year, these rains have been more than simply abundant. They have been catastrophic in
southeastern China and southern Japan, especially on the island of Kyushu. Severe flooding
and mudslides have caused casualties, ruined crops, and washed out roads. While we have
been complaining of our wet July in Minnesota, southeastern China has been reporting
rainfalls of 6 to 14 inches per week, while southern Japan has reported mold rains of up
to 16 to 30 inches per week.
- Beaufort weather notation
- In addition to devising a scheme to visually estimate wind speed (the Beaufort wind
scale) British Admiral Francis Beaufort (1774- 1857) also invented a system of
abbreviations and symbols used as codes to represent types of weather observed and logged
in diaries and observation books. His system was widely adopted in the 19th century and
later modified by British and American meteorologists. A full explanation of the Beaufort
weather notation system and many examples of the letters and symbols used can be found at
the following web site:
http://www.booty.demon.co.uk/metinfo/bletters.htm
- Belfries, Steeples, Spires, Cupolas, and Barns
- These are obviously not meteorological terms! So what do they have in common? In the
history of weather recording, these are the most common structures for mounting wind vanes
(sometimes called weathervanes). The weathercock, which turns to face the wind and is
perhaps the oldest style of wind vane, was first used in bronze form during the 9th
century on churches in parts of northern Italy. Subsequently, metallic roosters, doves,
eagles and lambs were used for wind vanes on many church and public buildings throughout
Europe. In the United States, perhaps the best variety of wind vanes can still be seen
across the agricultural landscape mounted on the tops of barns.
- Billow Clouds
- These are clouds which appear as if they are a series of breaking ocean waves. They are
produced by the interaction of a saturated stable air layer, usually an inversion, and a
pattern of vertical wind shear, which results in somewhat evenly spaced zones of updraft
(where cloud tops crest in a wave-like pattern) and subsidence (where cloud droplets
evaporate as they descind back to the stable layer. The height of the billows may vary
from tens of feet to hundreds of feet and they may be spaced hundreds to thousands of feet
apart horizontally.
Some classic pictures of billows and other cloud formations are
available on the Internet at nature photographer Kay
Ekwall's web site
- Birainy
- This is a term used in climate classification to refer to a place on Earth that has two
distinct rainy seasons within a year. Most often these are locations near the equator
which measure more abundant rainfall at or shortly after the equinoxes (March and
September), a period of high sun. Some equatorial African countries and equatorial South
American Countries have such climates, including Zanzibar (east Africa) and Bogota
(Cloumbia). Tropical rain forest vegitation thrives in this type of climate.
- Blirty
- This is not unlike the word "blurt" which refers to a sudden, implusive, and
often unexpected verbal response from person. This term is used primarily in Scotland to
refer to sudden spells of wind gusts and rain which occur during unsettled highly overcast
conditions. Blirty weather certainly fits for many of our early June days in Minnesota
this year, some of which have been dominated by spells of rain and cool wind gusts from
the north and east. We have seen plenty of clouds, fog, drizzle, rain and wind, along with
some record low temperatures already this month.
- Blizzard
- This is a timely word, since western Minnesota and the eastern Dakotas experienced this
type of storm earlier this week. The operational use of this term by the National Weather
Service in winter weather warning statements specifies wind gusts of 35 mph or higher, low
temperatures (generally less than 20 degrees F) and sufficient snow in the air (either
from snow bearing clouds or from blowing snow) to reduce visibility to 0.25 miles or less.
These conditions must be expected to last 3 hours or more.
The origin of the word "blizzard" is not entirely known. Early American settlers
in Virginia used the term "blizz" to refer to a wind driven rain or snow which
reduced visibility. Thus, blizzard may have derived from this term during the 18th
century. On the other hand, real blizzard-like conditions are somewhat rare in Virginia
and far more common in the Dakotas. Early German settlers in the Dakotas borrowed from the
German word "blitzartig" (lightning like) to name sudden and severe winter
storms "blizzards." This may be the more plausible explanation. In fact, at one
time South Dakota was known as the "Blizzard State", but I suppose that did not
prove to be a very marketable nickname, so it is not used much anymore.
Several regions have storms analogous to blizzards, but they refer to them by other names.
The buran of Russia, the purga of northern Siberia, and the boulbie
of southern France are of a similar nature to the American blizzard and can be life
threatening.
- Blustery and Tousie
- Blustery is often used by the National Weather Service to refer to a day of strong and
gusty winds, especially cool or cold northerly winds. Similarly the Scottish Weather
Service will refer to such a day as tousie, which is a derivative of the term tousle
meaning to tussle or rough about. While Minnesota often suffers a blustery day in winter
due to strong, cold, dry winds from Canada, Scotland will get cold, damp winds from the
North Sea.
- Bourini
- This is a Greek term used to describe a sudden summer squall in the Mediterranean Sea.
Strong winds up to 60 mph or greater may lead to high swells and destructive coastline
waves and sea spray. Greek fishermen are especially mindful of watching for and reporting
on bourini squalls as they move across the region. Some fishing boats are occaisionally
lost in these types of storms. Bourinis are sometimes the result of a cold air mass moving
rapidly into the Mediterranean off the Balkan Mountains of Bulgaria and colliding with a
warm moist air mass over Greece.
- Brickfielder
- No, this is not the guy who hands the bricks to the bricklayer.
Like Cock-eyed Bob and Willy-Willies , this is a colorful Australian term
for winds. While we are transitioning to winter, downunder they are approaching summer.
Sometimes in summer a hot, dry and dusty wind from the interior deserts blows south and
affects the south coastal areas of Australia, making the people who live there
uncomfortable and irritable. This wind is called the "brickfielder," presumably
because it picks up the red, dusty, brick-like soil of the interior and deposits it over
the coastal areas.
- Buys-Ballot's Law
- Buys-Ballot's Law, sometimes called the baric wind law, is an empirical law in
meteorology that relates the horizontal wind field to the atmospheric pressure pattern. It
was formulated in 1857 by Christoph H.D. Buys Ballot (pronounced Bowis-Ball-ott), then
head of the Dutch Meteorological Services. Basically, the law states that, with your back
to the wind, the pressure to your left is lower than the pressure to your right. This law
is based on the known wind fields which circulate around low pressure and high pressure
systems in the northern hemisphere. It is exactly reversed in the southern hemisphere. In
the absence of any weather forecast, you can at least ascertain the direction of low
pressure, where frontal activity such as precipitation may be occurring. If you place your
back to the wind and are facing north, then low pressure and frontal activity is to the
west and likely to be passing your way in the future. Conversely, if you find that you are
facing south with your back to the wind, then low pressure is to the east and active
weather systems are probably already heading away from you.
- "C" Weather
- "C" is the designator for contact weather, meaning that pilots of aircraft
have sufficient visibility to fly without instruments and use only visual reference to the
ground surface.
- CAPE
- During the thunderstorm season in Minnesota, you may occasionally hear meteorologists
use the term CAPE values. What does that mean? CAPE is an acronym, as is often the case
with the National Weather Service. It stands for Convectively Available Potential Energy,
an index derived from 12 hourly radiosonde data (balloons), to assess the potential for
convective thunderstorms.
As warm air rises it may cool and reach the saturation point where condensation occurs.
This process releases energy to the atmosphere (at the rate of 585 calories per gram of
water). CAPE values are used to rate the potential energy from condensation between the
850 mb level and the 300 mb level in the atmosphere (roughly 5000 to 30,000 ft above the
Earth). These integrated values are expressed in joules/kg. CAPE values in the range from
600 to 2000 are somewhat average. Values less than 600 indicate a very stable atmosphere,
while values over 2000 indicate enough energy to produce convective thunderstorms. Values
in excess of 2500 show enough energy to produce strong thunderstorms and may be used as a
justification for the National Severe Storms Forecast Center (NSSFC) in Norman, OK to
issue a severe thunderstorm watch. Daily forecasts by the NSSFC can be accessed on the
Internet at: National Severe
Storms Forecast Center
- CAT advisory
- What would a meteorologist be doing issuing a CAT advisory? Do they have knowledge about
weather which affects cats in an adverse way? Actually this term has nothing to do with
pet cats. It is used by British meteorologists to designate a warning to pilots about
Clear Air Turbulence, which is usually a cloud-free wind shear zone aloft that can make
for a bumpy plane ride.
- Chase Hotel, Dryline Chaser, Meatwagon, White Elephant, StormTracker, U.S.S. Phoenix
- As we have suggested in earlier commentaries, storm chasers are in a league of their own
when it comes to jargon. These terms have all been used to refer to storm chase vechicles,
usually purchased cheaply, then drastically modified to accommodate a variety of
instruments and cameras. The Chase Hotel was a 1996 Ford F-150 4X4 pickup. It was not only
equipped with special instruments, but it had a sleeping area in back (I guess for the
sometimes long waiting periods while severe weather develops). The Dryline Chaser was a
1991 Dodge Caravan, especially equipped with a computer, scanner, ham radio, color TV,
camcorder, automated mobile weather station, cellular Internet hookup, and globabl
positioning system. The Meatwagon was a 1986 Pontiac Parisienne station wagon which
survived an assault by Hurricane Andrew in Florida. The White Elephant was a 1967 Pontiac
Tempest, while the StormTracker was a 1990 Checy Astro Van with so many antennas hanging
out it looked like a porcupine. One of the most economical storm chasing vehicles is the
U.S.S. Phoenix, a Honda Civic SI hatchback used to chase storms in South Dakota and
Minnesota. More on the culture and jargon of storm chasers can be found at their web site:
http://webchat.chatsystems.com/~tornado/vechicles/index.html
- Chilblains
- Taken from old Middle English and Anglo Saxon terms ("chill," meaning cold or
shiver, and "blains," meaning sore or swelling), this term refers to a distress
of the skin as a result of exposure to cold temperatures. It occurs primarily on exposed
hands, wrists, feet, or ankles, but sometimes ears, nose, or cheeks will show symptoms as
well. Aside from some pain, the symptoms are swelling, itching, burning, or a redness or
bluish mottled appearance. Sometimes the skin will crack. Not as severe as frostbite, this
distress is primarily the result of the contraction in the blood vessels near the surface
of the skin. The medical term used for this condition is "acrocyanosis." Perhaps
in reaction to the arctic air mass this week, some Minnesotans who failed to bundle up
properly are suffering from chilblains.
- Chionophile and Chionophobe
- Taken from the Greek word for snow, "chiono" (ki-o-no), these are combination
words used by ecologists and biologists to describe the character of certain plant or
animal species. A chionophile is a snow-loving species and thrives well in winter snow
cover; while a chionophole is a snow-hating species which does not do well in snow. I
think that most Minnesotans are chionophiles.
- Chocolatero (the chocolate gale)
- "Chocolatero" is the term used to describe a moderate to strong northern wind
in the Gulf region of Mexico. Such a wind ushers in cold high pressure from the
continental United States and temperatures drop markedly. These winds are most common
between November and April. The origin of this term is unclear, but I suspect that it has
something to do with drinking hot chocolate!
- Chronoanemoisothermal diagram
- That's a mouthful, pronounced chrono-anemo-iso-thermal diagram. This refers to a graphic
which depicts the average temperature for a given place at all hours of the day for each
cardinal wind direction (east, south, west, and north). It is especially helpful in
regions where the wind direction has great influence on local temperature such as in a
lake district or sea coast.
- Clear, Scattered, Broken, and Overcast
- These are the common terms used in aviation meteorology to refer to sky conditions. They
are based on the percentage of sky obscured by clouds. The list below shows the criteria
used:
| CLEAR |
less than 10 percent of the sky covered by clouds |
| SCATTERED |
10 to 50 percent of the sky covered by clouds |
| BROKEN |
60 to 90 percent of the sky covered by clouds |
| OVERCAST |
greater than 90 percent of the sky covered by clouds |
- Clinometer, Alidade and Ceiling Projector
- Sometimes frustrated middle school or high school mathematics students will ask,
"What good is this trigonometry, anyway?" The angular and distance relationships
which are learned in trigonometry are very applicable in engineering, navigation,
surveying, and (of course) meteorology.
For example, the ceiling projector (also called
a ceiling light or cloud searchlight) is used in conjunction with a clinometer or alidade
to determine the height of the cloud ceiling at night. The ceiling projector shines a
narrow beam of light up to the cloud base. The angular elevation of this spot of light is
measured by a clinometer (portable instrument) or alidade (fixed instrument) some distance
away from the projector light but on the same horizontal plane. The cloud height Z, is
determined by the relationship
Z = L tan E
where L is the horizontal distance between the projector light and the detecting
instrument and E is the elevation angle of the spot of light.
One problem with the nighttime measurement of cloud ceiling occurs when it happens to
be snowing, when specular reflectance of the falling snow flakes can cause a false light
spot to form well below the cloud base and therefore give a ceiling height that is much
too low.
"Alidade" is derived from French and Latin words which describe the revolving
radius of a circle, while "clinometer" is derived from the Greek words
"clino" (meaning bed or horizontal plane) and "meter" (meaning to
measure).
- Cold Air Funnels
- Cold air funnels are not uncommon in Minnesota, especially in the spring. They are
always quite small in scale, short-lived, and hardly ever touch the ground. Unlike
tornadoes, whose parent clouds are cumulonimbus (thunderheads) and of great vertical
depth, cold air funnels may drop from ordinary cumulus clouds as a result of small scale
local instability aloft. They represent cold air eddies and typically appear as small
conical shaped protuberances, lasting from seconds to minutes. They are not terribly
dangerous, but do contain winds of the same order as dust devils, so they might blow
around items like lawn chairs if they do touch down.
- Cold Soak
- This is what some Minnesotans do after they emerge from several minutes in a hot sauna.
It is also a term used by climatologists and engineers to describe equipment exposure in
cold climates, especially in polar regions. Machinery or engines which are stored or left
idle in cold climates experience a cold soak. Metal becomes more brittle, lubricants
thicken and operational tolerances are diminished. Preheating of the machinery before use
is often prescribed for equipment that has been cold soaked for extended periods.
- COMET
- This is an acronym for the Cooperative Program for Operational Meteorology, Education
and Training administered by the University Corporation for Atmospheric Research. Sponsors
include the National Weather Service, the Air Force Weather Agency, and the Naval
Meteorology and Oceanography Command. The COMET mission statement is.. "to serve as a
premier resource to support, enhance, convey, and stimulate scientific knowledge about the
weather for the benefit of providers, educators, and users of weather information."
COMET was established in 1989 and has sponsored the development of special forecasting
methods and tools. Some previous COMET projects have produced better methods of
forecasting windstorms, marine weather for the Great Lakes, lake effect snow storms, and
flash floods. More information about COMET, including some examples of projects and case
studies can be found at their web site:
COMET
- Comma Cloud
- This term refers to the typical cloud pattern of a midlatitude cyclone (especially
winter storms) when viewed in satellite images. The curvature of the cloud system is like
that of a comma because of the counterclockwise rotation of winds that occurs with a
strong low pressure system. Visually, a sharply curved cloud system, usually indicates a
deep low pressure center and a strong storm. Real-time images of cloud systems over the
entire Earth are available on the Internet through the Space Science and Engineering Center at the
University of Wisconsin, Madison.
- Cyclogenesis and Cyclolosis
- "Cyclogenesis" and "cyclolosis" are terms used by meteorologists to
refer to stages of an extratropical cyclone or low pressure system.
"Cyclogenesis" may refer to the birth of a cyclone or the intensification of
cyclonic flow (counterclockwise circulation) around a low pressure system. Typically the
pressure gradient increases (central pressure drops), winds strengthen, and clouds become
more pronounced during this phase. "Cyclolsis" refers to the death or weakening
stage of a cyclone or low pressure system, when the pressure gradient relaxes, winds
become lighter, and clouds become thinner and less organized.
- Cyclones, Typhoons and Hurricanes
- The recent floods in Mozambique and Madagascar are the result of a very active tropical
storm season in the Indian Ocean this year. In the western South Pacific and the Indian
Ocean severe tropical cyclones (wind speeds of 74 mph or greater) are simply called
cyclones. Australian, Indian, and east African weather services will use this term to
describe such storms. In the western North Pacific these storms are called typhoons, while
in the eastern Pacific Ocean, the Atlantic Ocean, and the Caribbean they are called
hurricanes. The current conditions in the western south Pacific, Indian Ocean, and
Mozambique channel off east Africa show very high ocean surface temperatures, ranging from
80 to 90 degrees F in many places. This serves as fuel for cyclones to develop. In
addition, the jet stream winds are rather weak favoring strong vertical development and
sustainability of thunderstorms. Australia and Madagascar have already been affected by a
number of cyclones this season, but fortunately in areas that are not heavily populated.
On the other hand, Mozambique in east Africa has seen severe flooding brought on by two
cyclones which moved very slowly over land and dropped heavy rainfall. Nearly a million
people have been displaced from their homes there. This week, yet another cyclone (Gloria)
passed over Madagascar and the Mozambique channel dumping excessive rainfall, with strong
winds and high seas (15 to 20 ft waves).
More information about cyclones in the
southern hemisphere can be found on the Internet at the Naval Pacific Meteorological and Oceanography
Center's Joint Typhoon Warning Center operating in Hawaii.
- Depression Storage
- This term is used primarily by hydrologists to refer to the water stored in puddles,
ditches, and other small depressions in the local landscape. During the winter in
agricultural landscapes where moldboard plowing or ridge tillage has been done on the
soil, this small scale depression storage can add up to quite a bit of water which must
either be absorbed by the soil or drained before soils can be worked in the spring. In
winters like this one, depression storage may be substantial and contribute to high
volumes of runoff before soils thaw out enough to allow infiltration.
- Derecho (day-ray cho)
- "Derecho" is a Spanish word for "right ahead" or "straight
ahead." It has become a severe weather term used to describe a windstorm which
accompanies a large mesoscale convective complex (MCC) such as the one which crossed the
state on July 1. These winds can be long-lived and very destructive as they move along
with squall lines and thunderstorm systems. They are part of the family of downburst
winds, often quite cool in terms of temperature. Their destructive pattern tends to be
different than tornadic winds in that they scatter debris in a narrow vector rather than
in all directions. The most severe recent episode of derecho winds was in July of 1995,
when over 6 million trees were damaged or destroyed along a wide path in northern
Minnesota.
- Distinguishing an ice storm warning from a freezing rain/freezing drizzle advisory
- The National Weather Service Forecast Offices provide many notices of significant
weather events to warn the public about conditions which may present threats to public
safety and health. Two of these notices which are sometimes issued during the winter
season in Minnesota are an ice storm warning and a freezing rain/freezing drizzle
advisory. The significant difference between these two notices is that an ice storm
warning means that ice accumulations are expected to equal or exceed 1/4 inch, while a
freezing rain/ freezing drizzle advisory pertains to ice accumulation of less than 1/4
inch. Both types of notices infer difficult driving conditions, while the ice storms also
threaten power disruptions and damage to landscape plants and trees.
- Doctor and Harmattan
- These are old colloquisms from West Africa used to describe winds. The harmattan is
derived from a Spanish term and refers to the northeast winds that blow in the dry season
(November to March). These winds come from the Sahara Desert to the north and bring dry
and dusty air, but air that has much lower dew points for the tropical environments of
West Africa. Therefore this wind is considered a relief from the steamy heat of the rainy
season and restores health to the body and soul.
The doctor is also used in the African tropics to describe a sea breeze which generally
brings cool relief from the heat generated in the interior of the continent. We are in
that time of year in Minnesota, when high dew points (greater than 65 degrees F) sometimes
make the air feel oppressive. This may last for days, until we get a a stronger northerly
wind which typically ushers in much lower dew points and feels like natural air
conditioning. This wind could be called the doctor as well, for it restores our vigor,
lifts our spirits and allows us to sleep more comforably.
- Dog Days
- The "dog days of summer" are usually associated with the greatest heat of the
year, characterized by thunderstorm activity and high dew points. The origin of this term
is both ancient and astrological:
- When Sirius rising with the sun
- Marks the dog days well begun
The ancient Greeks and Romans observed that one of the brightest of the stars, Sirius the
Dog Star (located in the constellation Canis Major, which is Latin for "greater
dog") rose in conjunction with the sun during the six weeks of mid-summer. Hot and
sultry weather, which depleted the energy of humans and caused vegetation to wilt, was
often experienced during this period and was attributed to the evil effects of Sirius. In
the United States, the dog days occur between mid-July and early September, while in
western Europe they run from July 3 to August 11.
This term has been misconstrued to refer to the time of summer when dogs are most apt to
go mad. Like other mammals, dogs exposed to high temperatures and high dew points will
exhibit stress symptoms such as increased panting, change in diet, increased thirst, and
lethargic behavior, but I don't know that they go mad more frequently. Some researchers
have associated higher crime rates in U.S. cities with the dog days, and one researcher
has found that aggression among Major League Baseball players is increased during this
period of summer.
Though we felt the dog days of summer in Minnesota during July, they have been relatively
mild in August so far, with most locations reporting only one or two days with
temperatures of 90 degrees F or greater.
- Doppler Radar
- The Doppler radar is a type of weather survelliance radar which takes advantage of the
Doppler effect. It can determine the radial velocity of atmospheric targets moving
directly toward or away from the radar unit based on the change in the frequency between
the outgoing and returning (reflected) radar signal. Thus wind speeds associated with
thunderstorms can be interpreted, and in particular rotating winds associated with funnel
clouds and tornadoes can be seen on the Dopplar radar displays.
Now that the major league baseball season has begun, Doppler radar are routinely used at
most ballparks. They are positioned either behind home plate or in centerfield to measure
the velocity of pitched balls, which typically range from 70 to 90 mph. In this position
the radar gun cannot measure the speed of a throw from shortstop to first base, since the
thrown ball crosses the radar beam rather than moving toward it or away from it.
- Duff
- "Duff" is an old English term for plum pudding made from a stiff flour
mixture. It is also a term used to describe my golf shots. So how does it relate to
meteorology, you ask? It is an important term in the fire weather program of the US Forest Service, used to describe the partially decayed
organic matter on the forest floor which can become highly combustible during drought
periods and contribute to the longevity and spread of forest fires. In fact, the
Keetch-Byram Drought Index is sometimes referred to as the soil/duff drought index because
it is a measure of how dry the soil and duff layers are.
- Dumbbell or Dumbbelling
- Some MPR listeners think that this refers to the long pause (or dead airwaves) so
evident when Bob Potter stumps me with a question. However, these terms are occasionally
used by forecasters as verbs when describing the behavior of a low pressure system.
Sometimes a low pressure system splits into two circulating lobes that are close to each
other. They behave in the large scale weather pattern as a single system, but they
actually appear on satellite imagery as two distinct rotating cloud masses. Such a system
occurred on Thursday of this week in southern Ontario. Dumbbelling then refers to the
shape of the pressure pattern (like a weight room dumbbell) rather than the character of
the forecaster.
- Dustbusting
- This word may have several meanings, but for agricultural researchers it refers
specifically to management techniques designed to mitigate wind erosion, especially of the
type that produces dust and sandstorms. In many semiarid regions where overgrazing has
occurred or where farmers have chosen to fallow agricutlural fields, wind erosion can lead
to environmental degradation and produce a health and safety hazard. Loss of productive
topsoil and decreased visibility on local roads and highways are two of the more obvious
negative effects. In addition, recent research has revealed that in some areas small
particulate air pollution may come predominately from airborne soil. The EPA is concerned
about this, since particulates with diameters of 10 microns or less are small enough to
penetrate the lungs.
Dustbusting techniques that have been tried and proven in such areas as the desert regions
of California, include construction of barriers such as fences, planting of living
windbreaks such as shrubs and trees, or revegetation of the landscape using native or
adaptive plant species seeded from aircraft. In California, plants such as buckwheat,
saltbushes, Indian ricegrass, and even Califonia poppy have served to revegetate barren
landscapes.
- Dvorak Technique
- In 1975, Vern Dvorak, a meteorologist with the National Environmental Satellite
Services, derived a method to analyze and predict tropical storm intensity based on
real-time satellite imagery. Though over 20 years old, this technique is still used to
assess the strength and project the future intensity of hurricanes, tropical cyclones, and
typhoons. The Dvorak technique is based on the pattern recognition of a storm's size and
shape using satellite imagery, especially the infrared. Some of the more important storm
features used in this technique include the curvature and size of the outerband cloud
circulation, the vertical depth of the clouds which compose these bands, and the
difference in temperature between the warm eye of the storm and the surrounding cloud
tops. Though aircraft reconnaissance provides some of the most important data to assess
storm intensity, the Dvorak technique is still used to examine storms between aircraft
flights or in tropical areas where instrumented aircraft are not available for storm
studies, such as parts of the Pacific Basin. For example, Super Typhoon Rosie in the
western Pacific was analyzed continuously by the Dvorak technique this week as it headed
toward the south coast of Japan.
- Echelon Clouds
- Originally derived from the french word for ladder, the term "echelon cloud"
refers to a cloud form which produces stair-steps or a terrace illusion to the observer.
Aligned clouds, all having the same base elevation much higher than the observer on the
ground, are viewed at a lower elevation angle with distance towards the horizon. The
observer sees an apparent stair- step effect as if the cloud base gets successively lower
with distance to the horizon, even though all the clouds have the same base elevation.
These formations can occur with fair weather cumulus or cirrus clouds over Minnesota.
- Ekman Spiral
- Named for Swedish physicist Walfrid Ekman (1874-1954), the Ekman spiral is an idealized
mathematical representation of how the wind-driven surface currents of the ocean behave
with depth. It's also a representation of how wind speed and direction vary with height
above the ground.
In the oceans of the northern hemisphere, wind-driven surface currents spiral to the right
with depth as a result of friction and the Coriolis force (rotation of the Earth). This
motion traces an imaginary clockwise descending spiral.
In the atmosphere, the Ekman spiral refers to how winds spiral to the right with height
above the ground, as a result of the balance between the Coriolis, pressure gradient, and
friction forces. This motion traces an imaginary clockwise ascending spiral. Thus a wind
from the south (180 degrees) at the ground level may become a stronger wind from the
southwest (225 degrees) at a height of 1000 meters.
The clockwise drift of winds and ocean currents in the northern hemisphere becomes a
counterclockwise drift in the southern hemisphere as a result of the Coriolis force.
- Elfin Forests
- This term is taken from the old Middle English word "elvene" meaning elf-like,
small and sprightly. It is used to describe forests which grow in harsh environments,
typically windblown, perhaps at high elevation, often dry and in very shallow or rocky
soils. Many of the world's mountain ranges have elfin forests. Only dwarf trees can grow
and they appear gnarled and misshapen, bearing little resemblance to members of the same
species that grow in more favorable environments. The bristlecone pines that grow in
Colorado is such a species. In parts of South America they call the elfin trees the Ceja
de la Montana, meaning eyebrow of the forest.
- EMWIN
- This is a National Weather Service acronym for the Emergency Managers Weather
Information Network. Developed in cooperation with FEMA (Federal Emergency Management
Agency), EMWIN uses a variety of sources of data which are transmitted from the
geostationary satellites (GOES 8 and GOES 10) to federal, state, and local emergency
management offices which have satellite receiving dishes. A number of the information
products are also available on the Internet, including severe weather warnings, hurricane
warnings, flood warnings, air pollution statements, specialized forecasts, seismic
activity, iceberg reports, and various advisories. Many can be found at the the following
web site...
http://iwin.nws.noaa.gov/iwin/graphicsversion/rbigmain.html
- English Rule and American Rule of Groundwater Rights
- Historically, under the common law doctrine, rights to utilize groundwater resources
have been granted to landowners in the United States and many European countries. In humid
climates with abundant precipitation and adequate recharge, the English Rule has applied
which grants landowners absolute rights to pump as much water as they wish. This rule does
not recognize the fact the pumping water from beneath one property will deplete acquifers
that are shared with other property owners. In climates with deficit precipitation,
inadequate recharge, and multiple users of groundwater resources, the American Rule
applies, which allows landowners a so-called "reasonable use" taking into
account the impacts on the water rights of others and depletion limits for specific
acquifers. There are still many inconsistencies among states in groundwater rights, but
the number of locations and situations where the English Rule applies is continuing to
decline as a result of pressures to protect and conserve groundwater resources.
- The Equation of Time
- The Equation of Time is a simple way of expressing a complex periodic function. For any
specific time zone, it represents the difference between mean solar time (local noon) and
real solar time (solar noon) measured by the Sun's transit across the sky. The time
between successive transits of the Sun across the same point in the sky varies throughout
the year, by as much as 15 minutes. This is because of orbital features of the Earth, both
its elliptical shape which causes the orbital speed to vary and because of the tilt of the
Earth's axis with respect to the plane of the elliptical orbit. This is a primary reason
why the earliest sunset in the winter is not coincident with the latest sunrise. More on
this complex relationship can be found at the U.S.
Naval Observatory web site.
- Fair Weather
- The word "fair" is derived from old Middle English usage and has many
different meanings. It may be used as an adjective, adverb, verb, or noun. Examples of
usage include "fair skin," "to play fair," "fair catch,"
"to hit a ball fair," and "to go to the fair," as is popular in
Minnesota right now. In this context it is an exhibition of wares, farm products, and
amusements, along with competitions and food, food, food.
- Fair Weather
- This term is commonly used to refer to visibly pleasant though not necessarily
comfortable weather conditions. It is a purely subjective description and may be used with
respect to what is normal or average for a particular location and time of year. When the
National Weather Service includes this term in their forecasts it is supposed to satisfy
some if not all of the following criteria: no precipitation expected; less than 0.4 sky
cover of low clouds; very good visibility; and absence of any strong winds. The use of the
term often has little regard for air temperature, so that a very warm or cold day may
still be referred to as a fair weather day.
The word "fair" has been used to describe a particular weather condition for
ages, as in "the weather faired as the night went on," or "fair skies are
expected for star gazing," or "the parade will go on whether the weather be fair
or foul." Generally speaking, to use the word "fair," a forecaster must
ascertain that the skies will either be clear or contain only a few higher level clouds,
winds will be light, and temperatures will not deviate significantly from seasonal
normals. To be honest, although this term is still used around the world in English
speaking countries, National Weather Service
personnel probably use it less today than they used to. This is due to the public
expectations for more accurate forecasts and the implementation of technology that allows
the forecaster to be more precise about specific weather elements and events, particularly
their timing.
In the early days of the U.S. Weather Bureau (under the USDA), when forecasts were
provided to local communities either by mailing them on postcards or sending them by
telegraph, a system of flag signals was used to post the forecasts in town for local
residents. Each town had a designated forecast displayman (maybe the postmaster, local
weather observer, sheriff, banker, or train station manager). This person would receive
the Weather Bureau Forecast and then display the appropriate flag or flags to designate
expected conditions. A plain white flag alone would indicate fair weather; a blue flag
would indicate that precipitation was expected. Various combinations of square flags,
triangular flags, and colored flags would indicate other types of weather conditions. Even
as late as the 1960s and 1970s a bank building in downtown Minneapolis would display a
colored ball (weather ball) indicating what the weather was going to be.
At some of the early Minnesota State Fairs,
a system of flags was probably used to indicate expected weather conditions. In fact,
there were two Weather Bureau Offices in the Twin Cities during the early 1890s -- one in
St Paul and one in Minneapolis. There was quite a rivalry between the forecasters in the
two offices. There were competitions between the two and public debate about which office
provided the public with the best forecasts.
- Four Basic Thunderstorm Types
- Thunderstorms occur in a variety of forms, sometimes as an isolated cumulonimbus cloud
(anvil shaped), sometimes as a cluster of clouds, sometimes as a squall line, and
sometimes as a supercell (massive convective cloud system). The first type is known as a
single cell storm usually composed of a convective cloud containing one updraft and one
downdraft segment. These may produce some heavy rain, hail, or even a weak tornado, but
they are usually short-lived (30 minutes or less). The second type is known as a multicell
cluster composed of a group of convective clouds that move together as a single unit.
There may be multiple updraft and downdraft segments, highly variable rates of rainfall,
and some moderate hail. These systems may last for hours and produce flash flooding or
weak tornadoes. The third type is the squall line composed of a line of convective clouds
which share a common gust front along the leading edge (sometimes seen as a wall cloud).
They can move at rapid speeds and produce heavy rainfall and moderate hail, sometimes
resulting in flash flooding. Tornadoes may occur behind the squall line as well. The
fourth type of thunderstorm is the most damaging, that is the supercell, which is composed
of a system of clouds which rotate as one unit containing imbedded strong updrafts and
downdrafts, large hail and frequent lightning. These can produce flooding and moderate to
severe tornadoes. They may last for hours and travel across multiple states.
More
information about types of thunderstorms can be found at the University of
Illinois Atmospheric Sciences Dept.
- Frazil Ice
- This is an interesting term not frequently used. It refers to ice crystals or
needle-like spicules which form in supercooled water of river or stream currents which
move too fast for surface ice sheets to form. In salt water is is called lolly ice. The
term frazil is from the French fraisil which means cinders. These ice crystals form under
the surface in pools or along channel edges and often build up into masses which extend to
the stream bottom. The mass of ice may become so large that it effectively dams the flow
of the river or stream and causes local flooding. A similar situation can develop from
anchor ice, which develops on the rocky bottom of some rivers and builds up toward the
surface.
- Freezing level
- This is a term used in meteorology to refer to the lowest altitude in the atmosphere
over a given location at which the air temperature is 32 degrees F (0 degrees C). In other
words, the height of the 32 degree temperature surface. It is highly variable and changes
markedly with the seasons in Minnesota. In summer it might be as high as 10,000 ft, while
in winter it comes right down to the ground at times. Average height of the freezing level
at MSP airport during the first week of November is about 3200 ft, but by the end of the
month it is about 1200 ft. This change in average freezing level during the month of
November is associated with a number of other significant changes in climate during the
month including: over a 1 hour reduction in daylength (over 10 hrs to just over 9 hrs); an
18 degree decline in daily mean temperature (from 40 degrees to 22 degrees); an increase
in cloudiness; and a increase in the occurrence of freezing precipitation (freezing rain,
sleet, snow).
- Freshet and Coulee
- "Freshet" and "coulee," old and rarely used words, have a gentler
connotation associated with the surplus flows of water across a landscape than the words
"flood" or "crest."
"Freshet" is derived from Scottish
and Middle English terms and has three
meanings: (1) a running stream of fresh water which empties into salt water (as in Shakespeare's "He shall
drink naught but brine; for I'll not show him where the quick freshets are"); (2)
in cold climates, the annual spring rise in streambeds which occurs with snowmelt runoff;
(3) a sudden great rise in a stream when it overflows its banks due to heavy rain or
snowmelt runoff, causing a local scale flood (much of this has occurred in Minnesota this
spring).
"Coulee" is taken from the French word for flow, and may refer to channeled
flow or sheet flow off a landscape into a lowland area or basin. "Coulee"
sometimes refers to a steep- sloped valley such as the Grand Coulee of the Columbia
River basin in the western U.S. Coulees feed into Devils Lake, ND, which has no natural
outlet and therefore has been growing in size due to abnormally wet years recently. With
abundant snowmelt runoff this year, Devils Lake is expected to grow and perhaps surpass
its maximum estimated historical size which occurred in 1830.
- Frost Heaving
- "Frost heaving" is the lifting of a surface by the internal action of frost.
It generally occurs as a result of freeze-thaw cycles. It can also occur, to a lesser
extent, by sublimation (ice forming from water vapor at high humidity).
During a thaw period, water droplets can fill the pore spaces in soils or road pavements.
This is especially true for macropores or the larger cracks and channels. Subsequent
freezing produces lenticular masses of ice or "ice lenses," which by expansion
force the overlying material upward. Because moisture is not evenly distributed in the
soil or pavement, this expansion is irregular and may produce considerable bumpiness in
the surface. This is good for agricultural soils (helping with drainage and aeration), but
in pavements and roadbeds this process is very destructive.
- Fulgurite
- This word is used by both geologists and meteorologists. Derived from the Latin root
word "fulgur", meaning lightning, this is a term used for the glassy, rootlike
tube that is formed when lightning strikes a sandy soil. The intense heat causes soil
moisture to vaporize, and the remaining molten material fuses into a tube-like sturcture
which may be an inch or two in diameter and inches to several feet in length. The
wall-like material holding them together is very thin and fragile, so they typically
crumble and fall apart when dug up. Recently University of Florida lightning researchers
reported finding a fulurite with three branches, one of which extended 16 feet into the
soil. This was noted as a world record size for a fulurite. More on this topic can be
found in the July/August issue of Weatherwise Magazine.
- GOOS
- This is another acronym that's kind of fun to pronounce. It stands for Global Ocean
Observing System, which is part of a global climate monitoring system initiated by the World Meteorological Organization, the International
Oceanographic Commission, the United Nations
Environment Programme and the International
Council of Scientific Unions. Besides monitoring winds, temperature and moisture for
climate assessment and prediction, this program will monitor and assess marine living
resources, coastal zone environmental changes, carbon fluxes, sea ice and the general
health of the oceans. Research in recent years has consistently pointed to the
oceanic-atmospheric coupled processes that regulate the climate of Earth. Most historical
climate data and research has been associated with land-based studies, but through this
program much greater attention will be given to ocean processes in future years.
- Garcia Method
- No, this does not refer to a style of guitar play or to a method of fishing, even though
the Garcia name is known in both these areas! It is a method for forecasting the amount of
snowfall expected from a given winter storm system. Named for Chris Garcia, a lead
forecaster with the National Weather Service in Milwaukee, WI, this method was published
in 1994 and has become a favorite tool of National Weather Service. The method is
emperical and considers a number of parameters (ingredients) which cause snowfall,
including vertical velocity (upward lift which carries air aloft to a condensation level)
and mixing ratio (the mass of water vapor per mass of dry air expressed as g/kg). The
method also considers the wind speed aloft because it is important to the advection
(transport) of water vapor into the storm system (increasing the mixing ratio) and also
because it dictates how fast the winter storm moves over a given area. The level of the
atmosphere where these processes are considered important for winter storms ranges from
7,000 to 10,000 feet. A number of studies in recent years have shown that the Garcia
Method is superior to many others in forecasting the amount of snow to expect over a 12
hour forecast period. In fact, thanks to the new computing and display power provided by
the Advanced Weather Interactive Processing System (AWIPS) installed at the National
Weather Service Forecast Offices, the Garcia Method has become a favority tool of their
meteorologists in forecasting snowfall amounts 6 to 18 hours ahead.
- Gnomonists
- Originally the Greek word gnomon meant an interpreter, judge or knowledgeable person.
The center post of of a sundial is called a gnomon, because it casts a shadow to show what
time it is. Thus gnomonists are people in the know who tell time and direction by reading
the size and shapes of shadows. The shadow is not always cast upon a sundial. Historically
other objects that cast shadows, such as rock formations (Stonehenge), buildings or
monuments have been used as well. The space probe named Mars Surveyor carries a sundial of
sorts. A gnomon has been added to a color target that is used to calibrate the landers
camera. So when the space probe lands on Mars in 2002, an image will be transmitted back
to Earth which, in effect, will convey what time it is on Mars.
- Gorge
- The term "gorge" is used by many Americans to describe a Thanksgiving Holiday
habit, but that is not the definition I intend. The term is used by hydrologists to
describe a blockage in a stream or river. This may be in the form of ice, debris such as
old logs, or a composite of both. A gorge can be particularly destructive during the onset
of winter conditions by blocking the stream or river and allowing surroundings lowlands to
flood, or the shoreline to be scoured away by moving ice or depris.
- Green Flash
- "Green flash" refers to what meteorologists call a twilight or optical
phenomenon. It is a brilliant green coloration, often in the form of a dish or disk,
located over the upper rim of the sun's apparent disk just before or after it crosses the
distant horizon. There is an excellent article about the green flash by Mark Coco in the
January (1997) issue of Weatherwise magazine
The green flash results from two characteristics of our Earth's atmosphere. It is produced
by atmospheric refraction, causing sunlight to be bent as it passes through a prism.
Longer wavelengths which produce red light are refracted or bent less than shorter
wavelengths which produce blue or green light. Secondly, when the atmosphere has
distinctly different temperature or density structure in the vertical, it acts as a lens,
producing a large mirage-type image, and therefore magnifies the size of the green flash.
Because the sun sets and rises more rapidly with respect to the horizon in the low
latitudes, the green flash is a fleeting image (lasting only seconds) and difficult to
view or capture on film. In higher latitudes, because the suns movement with respect to
the horizon is slower, the green flash may last for several minutes. It was observed to do
so by Admiral Byrd at the South Pole. One needs to have a clear, unobstructed view of the
horizon to observe the green flash.
While vacationing on the Gulf Coast of Florida this spring, I hope to view the green flash
at sunset.
- Gustiness factor
- Not much used anymore, this term was used to describe the variability in the wind
conditions, based on short temporal measurements of wind gusts. It was more precisely
defined as the ratio of the range in wind gusts (maximum minus minimum) divided by the
mean wind speed. For example, the wind gusts during the 3 pm hour on Wednesday of this
week in the Twin Cities ranged from a 30 mph maximum speed down to 6 mph minimum speed and
the mean wind speed for the hour was 24 mph. The ratio was 24/24 which equals a gustiness
factor of 100 percent. This is not uncommon in the spring. This measurement used to be
routinely reported from airports in order to alert pilots about significant wind variation
that might affect their approach and take-off.
- Haugull
- This is not a type of seabird, but a Scottish term used to describe a cold, damp wind
blowing from the sea. This type of wind often brings either fog, rain or mist. The literal
meaning of this word is a "gray coastal meadow." The term is also used in
Norway. It could apply equally to the type of weather experienced in Duluth and along the
northshore earlier this week (especially Monday), when a cool east wind brought fog, rain
and mist inland over the hills and meadows of the Superior National Forest.
- Heat-burst
- Like a downburst from a thunderstorm which brings destructive winds, a heat-burst
sometimes occurs as a result of sinking motion within the downdrafts of thunderstorms. It
is not destructive, but it does bring a rise in temperature as a result of compressional
heating (rising air cools, falling air warms). This can produce some remarkable effects as
it did last Friday at Sioux City, IA. At 10pm, well after sunset, a heat burst occurred
which temporarily raised the surface temperature from 84 degrees F to 93 degrees F, a
value which tied the all-time record high for that date (Sept 25th). As the thunderstorm
cell passed by, the temperature dropped to 82 degrees F in less than one hour.
- Heat Index
- The National Weather Service provides public advisories and warnings when the
combination of temperature and humidity becomes high enough to pose a health risk. The
Heat Index is used somewhat interchangeably with the term Comfort Index, or
Temperature-Humidity Index, to evaluate the combined effects of temperature and humidity
on the body's ability to cool itself. An air temperature of 85 degrees F with a relative
humidity of 60 percent feels the same as a temperature of 90 degrees F with a humidity of
30 percent according to the Heat Index. For nighttime values of 80 F or above and daytime
values 105 F or more, the National Weather Service usually issues a heat advisory. These
conditions can cause fatigue, heat cramps, sunstroke or heat exhaustion in some people.
Recently some deaths in the northeastern United States were blamed on a persistent heat
wave that produced Heat Index Values of 105 to 115 F.
- Heat Lightning
- This term was derived from observations of lightning under clear skies during warm
summer evenings. It was misconstrued that the lightning was produced by an excessively
heated atmosphere.
Heat lightning is really the luminosity of the sky overhead produced by distant lightning
flashes off the horizon and too far away to be seen. There was some heat lightning in the
Twin Cities area on Tuesday evening of this week.
Technically all lightning produces heat, since a single stroke can heat the surrounding
air to over 50,000 degrees F, causing sound waves due to the rapid expansion of air, which
we later hear as thunder. Sound travels approximately a mile every 5 seconds, so you can
gage the distance of the lightning flashes by counting how many seconds pass between the
flash and the resulting thunder (approximately 1/5 of a mile for every second). Thus a 15
second interval between observed lightning and the sound of thunder indicates that the
flash occurred about 3 miles away. Lightning strokes from over 10 miles away are rarely
heard as thunder.
- Heliotropic Plants
- Some plants exhibit a character known as heliotropism, taken from the Latin root words
"helio" for "sun" and "tropos" meaning "to turn."
The daily orientation of these plants actually changes with the position of the sun in the
sky. For this reason these plants are called "sun trackers." Agronomic crops
like sunflowers and some species of cotton are heliotropic, facing east to greet the sun
in the morning, and west to say goodbye to the setting sun in the evening. It has been
estimated that the sunflower receives up to 40 percent more sunlight on its leaves than it
would if it were in a fixed orientation all day. Some desert plants exhibit heliotropic
behavior but only during the winter months when the daylength is shorter and the sun's
elevation angle is lower.
- Horn Card
- A couple of weeks ago, we discussed veering winds (wind shifts in a clockwise direction)
and backing winds (wind shifts in a counterclockwise direction). Many years ago, sailors
aboard trade ships used horn cards to remind them of wind conditions associated with
tropical cyclones (storms). Horn cards were transparent disks with diagrams etched on each
side to show average wind directions associated with pressure falls in a tropical cyclone.
One side showed average wind directions for northern latitudes and the other side for
southern latitudes. Sailors could use these cards to determine where their ships were
located with respect to the storm field and to anticipate wind shifts as they moved
through it.
- Hostile Ridge
- This term, used earlier in the week by forecaster Byron Paulson of the National Weather
Service Forecast Office in Chanhassen, MN can be safely called a meterological
colloquialism. But among friends and colleagues of the Weather Service it might be
referred to as a Paulsonism. Ridge used by itself refers to an area of high pressure,
characterized by low dewpoints, sunny skies, and light winds. However, when combined with
the word hostile it has a somewhat different meaning. A relatively strong high pressure
ridge will deflect or weaken approaching low pressure systems, diminishing their potential
to deliver any significant precipitation. This is the character of a hostile ridge noted
by forecasters who might otherwise forecast significant snowfall from an approaching low
pressure center in the high plains. It is thus appropriate that those who might be hoping
for more snowfall should call this particular weather feature hostile. Similarly, a
hostile ridge may be the undoing of an approaching thunderstorm complex in the summer
severe weather season. In this context, this weather feature might be more appropriately
referred to as a protective ridge or a ridge shield.
- Hurricane and Typhoon Seasons
- Severe tropical cyclones (wind speeds greater than 74 mph) are called hurricanes in the
central and eastern Pacific, Gulf of Mexico and north Atlantic Oceans, typhoons in the
western Pacific and Indian Oceans, and willy-willies off the west coast of Australia. The
portion of the year having the highest relative frequency of these types of storms is
called the hurricane or typhoon season. For the north Atlantic and Gulf of Mexico region
this season is June 1 to November 30, while for the eastern Pacific it is May 15 to
November 30. In the southwestern Pacific and Indian Oceans the typhoon season runs from
November to April, while in the northwestern Pacific it runs from April to October. The
typhoon season for Hong Kong is July through September.
Currently Typhoon Maggie is tracking through the Philippine Sea in the northwestern
Pacific, but is not presently a threat to make landfall soon.
- Hydrogenesis
- "Hydrogenesis" is an old word used to describe the process in which natural
condensation occurs within the surface cracks and pore spaces of rocks and soils. When the
air drops below the dew point within these spaces and cavities, condensation can occur.
The liquid water can absorb soluble constituents on the rock or soil and sometimes even
percolate to deeper layers.
- Hydropolitics
- Used by resource managers, hydrologists, politicians, and to a limited extent by
climatologists, this highly contemporary term refers to political negotiation and
confrontation over riparian rights which may be associated with both water use and water
quality. Two areas of the world where much attention is being given to hydropolitics are
the Colorado River Basin in the southwestern United States and northern Mexico, and the
Nile, Jordan, and Tigris-Euphrates river systems of the Middle East. The rapid growth in
water consumption as more and more land is settled and developed, along with the relative
scarcity of alternative water resources in these regions have contributed to somewhat of a
hydropolitical crisis. In order to insure an adequate and stable water supply in the face
of even normal climatic fluctuations and extrapolated population growth curves, nations
and states are negotiating water-sharing treaties and agreements with more vigor than
ever. We will undoubtedly be hearing much more use of the term hydropolitics in our
remaining lifetime.
- Hythergraph
- Rarely used anymore, but in the old glossary of meteorology, "hythergraph"
referred to a climate diagram which showed temperature along one axis and some form of
moisture, such as humidity or precipitation, along the other axis. For example, certain
climate zones could be characterized by the shape of a hythergraph using mean monthly
values. Another form of hythergraph is the comfort chart which shows values of temperature
versus values of humidity. For indoor environments in the winter, our comfort zone is most
tolerable from 68 to 70 degrees if the indoor humidity remains between 35 and 60 percent.
If humidities are lower than this range, we tend to feel too cool; if higher than this
range, we tend to feel too warm.
- Ice Shove
- The word "shove," taken from the old Middle English word "shouven,"
is most often used as a verb, meaning to push or force away. In the term "ice
shove," however, it is a noun, referring to the slabs of ice pushed upon a shoreline
as a result of thermal expansion of lake, sea, or river ice cover, or as a result of
strong winds. These sometimes large, flat slabs of ice, called pans, may pile up along the
shore into odd shaped mounds and towers.
Ice shoves become more evident along the shorelines of larger lakes in Minnesota during
late winter and early spring as the temperatures warm up and the winds tend to increase in
strength. Some good pictures of ice shoves are online at the U.S. Army Corps of Engineers
Web site, which can be found at the following URL:
- Ice Spar
- The term "ice spar" is used to refer to a stout beam, raft, or spear-like
protrusion of ice extending out from a glacier or an iceberg. Spars may be entirely
visible or partially submerged in water. Such a feature is suspected to have been the
cause of rupture in the sides of the ocean liner Titanic; the vessel avoided a head-on
collision with an iceberg, but skirted along one side of it.
- Indoex
- This acronym stands for the Indian Ocean Experiment, a collaborate effort among
scientists from Asia, Europe and the United States. One of the primary objectives during
the 1999 field campaign was to utilize ships, balloon soundings, aircraft measurements and
satellite imagery to assess how much pollution (dust, soot, fly ash, sulfates, nitrates,
etc) was in the atmosphere over the Indian Ocean and how much was precipitating out in
convective storms. During the northern hemisphere winter, a number of pollution plumes
originating over India and the Asian continent can be tracked for hundreds of kilometers
out over the Indian Ocean. Scientists think that these plumes may be having some impact on
the temperature and water vapor patterns detected over that region. Given the severity of
convective storm development in recent months (cyclones in the Indian Ocean which have
struck Madagascar and Mozambique), this experiment may provide some valuable insight into
why this region has seen an increased frequency and intensity of storms.
More
information about this experiment is available on their web site:
http://www-indoex.ucsd.edu/
- Interception and interceptometer
- These words are taken from the disciplines of agricultural and forest meteorology.
Interception refers to the manner by which trees, brush, and crop canopies prevent all
precipitation from reaching the soil. The amount of precipitation interception among these
species varies from 40 to 100 percent depending on the rate that it is falling, the wind
conditions, and the size and shape of the vegetative canopy. Interception helps prevent
soil erosion by reducing both the amount of moisture and the kinetic energy which reaches
the soil surface.
The interceptometer is the old name given to a rain gage or
collection device that is placed underneath the vegetative canopy and allowed to catch the
drip from the leaves and stems, or the throughfall which is not intercepted. This catch is
compared to that of a rain gage left in an unshielded open setting in order to estimate
the amount of rainfall being intercepted.
- The Intertropical Convergence Zone (ITCZ)
- Highly visible on satellite imagery of the Earth's equatorial regions, this is the
somewhat narrow discontinuous belt along either side of the equator where convective
clouds and thunderstorms often dominate. The easterly trade winds (NE in the northern
hemisphere and SE in the southern hemisphere) converge here, forcing air to rise which
triggers convective storms and squalls. The storminess is often more prevalent over the
oceans than over the equatorial land surfaces.
- Isophane or Isophene
- "Isophane" (or "isophene") is a word derived from Greek terms,
"iso" meaning "equal" and "phainein" meaning "to
show." Either word is used to refer to a line drawn through geographical points on a
map where a given seasonal phenological event occurs at the same time. For example, the
dates of flowering for crab apple trees or the blooming of lilacs in the spring might be
depicted as isophanes on a map. Certainly the maps which commonly appear this time of year
in our local newspapers showing where the fall colors are beginning, peaking, or ending
might be considered a depiction of isophanes as well.
- Isoerodent
- This is not a type of rodent, nor is it a meteorological term. It is a term from soil
science and means a line connecting points on a map which have an equal erosivity index.
The average erosivity of soils (loss of soil in sediment runoff) is computed from the
Universal Soil Loss Equation partially based on the long term historical rainfall and
rainfall intensity. Many other soil characteristics are considered as well. The relative
differences in the erosivity index across a landscape can be compared by mapping these
values using isoerodents (lines of equal value).
- Katabatic and Anabatic Winds
- Katabatic winds (taken from the Greek word "katabatikos," meaning "to go
down") are sometimes called "gravity winds," "drainage winds,"
"mountain winds," or "glacier winds." They result when air flows
downward from higher positions in the landscape. The air may be channeled through canyons
as it flows to lower elevations. This will tend to accelerate the air flow and produce
strong winds. Some regional winds such as the foehn (German and Austrian Alps), Chinook
(Rocky Mountains), and Santa Ana (Southern California) are dry, warm katabatic winds. On
the other hand, glacier winds, which flow downslope as well, are very cold winds, some of
which produce the world's worst windchill conditions. An example would be the Cape
Denison-Commonwealth Bay region of Antarctica, where winds flowing downward from the
interior of the continent to the coast may reach 100 to 200 mph, producing windchill
conditions well below -100 degrees F.
Anabatic winds (taken from the Greek word "anabatikos," meaning "to
mount") are ascending or upslope winds, often the result of heating along valley
slopes. These winds are prevalent in many landscapes with pronounced topography,
especially during the daylight hours. Balloonists and pilots of sailplanes often use these
winds to maintain or gain altitude. There is even a type of sailplane called an Anabat.
- Keras-meltem and Karpooz-meltem
- The eastern Mediterranean is famous for many climate characteristics well documented by
the ancient Greeks and Romans. One of these climate characteristics is the etesian
(periodic) wind of the summer months. This is a moderate to strong northerly wind which
blows from mid May to mid October, peaking during July and August. The winds stir up the
Aegean and Ionian Seas so much that many still refer to this time as "the season of
large boats" because travel becomes too dangerous in smaller craft.
When the winds
first start during the increasing daylength up to the summer solstice, they are modest
(10-20 mph) and intermittent, usually occurring in the early part of the day, then
diminishing in the afternoon and evening. These are called the keras-meltem by the Turkish
people because they occur when the cherries (keras) are ripening and being picked. After
the summer solstice, the northerly winds become stronger and more persistent, sometimes
blowing all day at 25 to 35 mph. These winds are called the karpooz-meltem by the Turkish
people because they occur when watermelons (karpooz) are being harvested.
- Khamsin
- This is the Arabic name given to a class of winds which occur in Egypt and over the Red
Sea. The word also means 50 and implies that the frequency of these winds is greatest
during the 50 days following the Shem el Nassim holiday in April. But, most commonly these
winds occur anywhere from February through June. These hot, dry winds are generally from
the southeast, south, or southwest and may range from 30 to 40 mph, carrying a good deal
of dust and sand. They result from low pressure systems moving along the Mediterranean
near the coast of Egypt.
Historians and Egyptologists have associated these winds with the drifting sands that have
buried many Egyptian structures and artifacts. They have also linked these winds to the
highly eroded and weathered surfaces of Egyptian monuments, obelisks, and pyramids. Some
have claimed that these winds may have assisted in the Exodus of the Israelites from Egypt
across the Red Sea, by blowing strong enough to push back the waters.
- Lamb Storm, Lamb-showers, or Lamb-blasts
- Our April snow showers, not uncommon in northern Minnesota, do not have a colloquial
name associated with them. But they certainly do in England and Scotland. "Lamb
storm," "lamb-showers," and "lamb-blasts" are used to refer to
nuisance storms which produce a light falling of snow in the spring when new lambs are
born, most often during March or early April. More severe snow storms or squalls during
lambing can be lethal to the newborn lambs, so the U.K. Meteorological Office provides
special forecasts to sheep producers during the spring season to help them avoid, or at
least anticipate, any weather-related difficulties.
- The Laplace Formula
- Pierre Simon Laplace was born in Normandy in 1749. Residing in Paris for much of his
life, he was a mathematical genius and published many pioneering works in fluid mechanics,
calculus and probability theory. His best known work is Mecanique celeste in which he
describes a formula for the determination of height from barometric pressure. The fall of
air pressure with elevation had been observed for many years, but few had attempted to
develop an equation to describe it. This is one of the most important formulas in
meteorology and has been used for generations to standardize barometric pressure readings
taken from all kinds of landscape elevations to mean sea level values. Aircraft alitmeter
settings are based on the Laplace formula.
- Laurence
- This is a term used in meteorology to describe a shimmering or terrestrial scintillation
observed over a hot surface on a calm, cloudless summer day. In Minnesota this shimmering
may be seen on a hot cloudless summer afternoon over a paved road or a stubble field where
small grains have been harvested. The shimmering is caused by the unequal refractions of
light caused by numerous convective air columns (some only inches in diameter) which
differ in temperature and density.
The term laurence comes from St Laurence, the partron saint of cooks, who lived in Roman
times (mid 3rd century AD). Laurence was a deacon in the Church of Rome. Only days after
the death of Pope St Sixtus, a Roman magistrate demanded that Laurence bring him the
riches of the church. Laurence gathered the poor people of Rome and brought them before
the magistrate, claiming that they were the riches of the church. The magistrate was so
offended that he order Laurence to be roasted alive on a gridiron (basically barbecued).
He was later canonized as a saint. In fact tomorrow (August 10) is the traditional St
Laurence Day or Feast of St Laurence.
- Leaching and Denitrification
- "Leaching" and "denitrification" are terms used more often by soil
scientists and farmers discussing fertilizer options than climatologists. Leaching refers
to the movement or washing out of soluble constituents (chloride, bromide, sulfate or
nitrate) within the soil by percolation of water. Moisture moves through successive layers
of soil by gravity. In many agricultural soils the moisture is stored until it is removed
by plant roots during the growing season. However with some soils, deeper percolation
occurs, depositing these soluble materials into acquifers which may be sources of drinking
water or water for irrigation. Potential leaching losses are governed by soil moisture,
soil texture, and rainfall frequency and intensity.
Denitrification is the biological process in the soil where nitrate nitrogen is converted
into a gas and lost through the soil surface to the atmosphere. This process occurs more
rapidly under warm and moist conditions and in fine textured soils. Unlike leaching which
represents an environmental concern with respect to ground water quality, denitrification
losses are not an environmental threat instead can represent an economic loss with respect
to a farmers fertilizer program.
- Little Brother or Little Sister
- These terms are sometimes used by meteorologists to refer to a subsidiary (smaller
scale) storm which follows a major one. For example, a tropical cyclone, typhoon or
hurricane may be accompanied by a weaker low pressure system which trails along its path.
Even in our continental climate here in Minnesota, an occasional little sister or little
brother will trail along behind a major winter or spring storm system and bring some
addition precipitation or wind following a significant snowfall.
- Long Range Forecasting (Outlooks)
- "Long range forecasting" refers to the creation of weather statements which
forecast for the next 10 days or longer. These are sometimes referred to as
"outlooks," since they provide information about expected departures from normal
temperatures and precipitation. Long range forecasts are released once per month, on or
about the 15th.
- Mandatory Levels
- Twice daily, at 12 hour intervals, the weather services around the world probe the
atmosphere with balloon launched instrumentation, called radiosondes. These measurements
of temperature, pressure, humidity and wind throughout the vertical profile of the
atmosphere provide the input to a number of numerical forecast models. The design of the
forecast models mandates numerical input from certain constant pressure levels in the
atmosphere (e.g. 1000 mb, 850 mb, 700 mb, 500 mb, and others). These are called mandatory
levels because they are required to make the forecast models work. Radiosondes are
generally designed to sample the atmosphere up to an elevation of 19 miles, at which point
the balloon may burst and the instrument package released will parachute back to the
Earth's surface. The 19 mile elevation includes nearly 99 percent of the Earth's
atmospheric mass. The radiosonde balloons are designed to ascend at a nearly constant rate
of 300 meters/minute. Numerous precautions are taken to insure that high quality data are
collected to run the forecast models. For example if the balloon does not ascend to at
least the 400 mb level (approximately 4.5 miles), then a second attempt is made with
another balloon. Even if the balloon ascends properly to the 19 mile elevation or beyond,
if for any reason it fails to transmit data for an interval of 10 minutes or longer, a
second balloon is launched to try again. This reflects on the importance of maintaining a
consistent and comprehensive data flow to run the operational forecast models.
- Medium Range Forecasting
- "Medium range forecasting" refers to the creation of weather statements which
forecast for the next 3 to 10 days. Models used to make this forecast are run once per day
and depict the weather conditions for either 12 hour or 24 hour time steps. The medium
range forecast in Minnesota is often updated each evening using the European Center for
Medium Range Weather Forecast model (ECMWF) run out to 6 days, while in the morning the
medium range forecast is updated using the Medium Range Forecast (MRF) Model of the National Weather Service run out to 7 days.
- Mercury on the chute
- This is an expression rarely used anymore, but in the first 50 years of the National
Weather Service it was used to describe the onset of a cold wave, when temperatures fall
rapidly and reach levels that are a threat to agriculture and commerce. Often times when
the mercury in the thermometer was dropping rapidly, meteorologists would report that the
"mercury was on the chute." We have already had two episodes of mercury on the
chute this month, the 8th to the 9th and the 18th to the 19th when the temperatures fell
by more than 30 degrees F.
- Mesoscale Convective System (MCS)
- This expression, often used by the National Weather Service, refers to a cluster of
thunderstorms which is larger in scale than any individual cumulonimbus cloud, but smaller
in scale than a frontal system. These systems appear on satellite imagery as circular or
linear cloud forms with very bright tops (indicating cold air). Often times severe weather
including, hail, damaging winds, heavy rainfall and tornadoes are associated with an MCS.
We have already recorded a number of these over the state this June. Many have produced
hail, ranging in size from 0.5 to 3.0 inches in diameter.
- METAR
- This is an acronym for the meteorological aviation reports which specifically refer to
the international coded reports issued hourly from airport stations. Tailored to the needs
of aviation, these reports describe current conditions such as air temperature, dew point,
humidity, wind direction, wind speed, altimeter setting, visibility, and cloud ceiling. In
coded form they are difficult for the general public to read and interpret. However, in
recent years many sites on the Internet have made them available in decoded form, and in
real-time. Some of these web sites are:
National Weather Service Home Page
Minnesota Interactive Weather
Information Network
National Weather Service Forecast Office
in Chanhassen, MN
- Meteograms
- Meteograms are charts which depict various weather elements (meteorological parameters)
for a given location over time based on National Weather Service forecast models. They are
available in a variety of forms, but only for major cities in the United States. Typically
they show the expected pattern for air temperature, dew point, pressure, wind, sky
condition, and precipitation. The graphic for each of these parameters may be plotted hour
by hour, or in 3, 6, 12 or 24 hour time steps.
Both the National Weather
Service Forecast Office in Chanhassen, MN and the Weather Processor at Purdue University
have websites which display meteograms.
- Michael-riggs
- "Michael-riggs" is an old term relating to Michaelmas, an English name for the
fall season. Celebrated for St Michael, the patron saint of healing, Michaelmas Day falls
on September 29 and is commonly marked by a harvest festival in many towns. Symbols of the
festival include a glove, representing openhanded generosity, a cooked goose as the
centerpiece of the harvest dinner, and gingerbread or gingerbeer because ginger is
recognized as a healing spice.
"Michael-riggs" is the term given to the occasional strong gales which blow over
England during this season. "Rig" by itself is an old English term for a gale,
derived from the observation of how quickly a strong wind can fill the sailing rigs of
ships.
- Moby Dick Balloon
- Most instrumented balloons are released and tracked by weather services around the world
every 12 hours for the purpose of taking profile measurements of the atmospheric
conditions aloft (pressure, temperature, humidity and wind). However, on occasion weather
services have used an instrumented constant-level balloon for long duration flights (over
24 hours) to characterize atmospheric conditions over ocean or land transects. These
balloons often reach elevations of 40,000 ft. or higher and maintain a constant altitude
as they move with upper level winds. With such low air pressure at these altitudes, these
plastic balloons, initially inflated to 6 to 8 ft. diameters at ground launching sites,
expand to considerable dimensions (perhaps 20 or 30 ft. diameters) and look perhaps like
floating whales in the sky. Thus the nickname, "Moby Dick balloon."
- MOS
- This is an acronym for Model Output Statistics, a tool used by meteorologists to make
local forecasts. It refers specifically to a statistical method of relating the output
parameters of a numerical weather prediction model (usually scaled for the entire United
States or northern hemisphere) to local weather elements such as temperature and humidity.
Using the historical relationship between climate records of a given location and past
forecasts, the numerical model values are adjusted for local scale effects or biases. The
National Weather Service provides forecasters with updated MOS data every three hours
which they can use to revise forecasts throughout the day and night if warranted.
- Nephoscope
- This term is derived from the Greek words nepho, meaning "cloud," and scope,
meaning "to view." A Nephoscope is an instrument for viewing clouds and
determining their motion. There are two types of Nephoscopes: one with a direct view and
one with a mirror view of the sky. Each instrument allows the observer to determine the
compass direction in which clouds are moving. Cloud motions may vary with elevation; that
is, low clouds may be moving in one direction and high clouds moving in another.
- Nimbostratus
- We have already seen this type of cloud several times this month. This term is derived
from the original Latin based cloud classification system proposed by Englishman Luke
Howard in 1803. The cloud form is gray, with a low, ill-defined ceiling. Generally, it
completely obscures the sun. Nimbostratus clouds bring rain, snow or sleet. On satellite
imagery, they show up as darker clouds when compared to the higher, colder cloud tops.
- Niphablepsia
- Here's a word for the master Scrabble player to use. A medical term derived from two
Greek words: "nipha" meaning snow; and "blepsia" (after
"blepharon" for eyelid) meaning affliction of the eye. Those who don't speak
Classical Greek or medical jargon use the more common term "snow blindness" to
describe this condition, which is caused by the high reflection of sunlight from snow
cover. This intense reflection is so bright that it can cause impaired vision or even
temporary blindness. It is most common on bright, sunny winter days in areas where snow
has drifted into a relatively uniform surface with little roughness to it. I am sure
numerous citizens in Minnesota have been exposed to this condition this winter.
- Niphometrology
- Adopted by participants at the Western Snow Conference in 1942,
"niphometrology" was used to refer to the science of snow measurement. It is a
composite formed from the Greek root words "nipho," meaning "snow,"
and "metron," meaning "to measure." This term never caught on in the
scientific community, perhaps because it sounded a bit too academic for a process as
simple as sticking a yardstick in the snow.
- Noctilucent clouds
- These are rarely seen wavy, thin bluish or silvery clouds. They appear in shapes and
patterns similar to cirrus clouds, but they are much higher, 45 to 55 miles above the
Earth's surface in the mesosphere, where temperatures range below -100 degrees F. Most
often seen at high latitudes just before sunrise or just after sunset, noctilucent clouds
are thought to be composed of ice that is deposited on ejected volanic depris or the dust
particles from meteorites.
- NOGAPS
- A meteorological acronym pertaining to weather prediction, this term stands for Navy
Operational Global Atmospheric Prediction System. This global scale weather prediction
model operates on a gridded resolution of about 12 miles and calculates conditions at 18
different vertical layers in the atmosphere. It is one of the few models which produce a
wind field analysis on a global scale, showing streamlines aloft and at the surface, over
oceans and over land masses. In addition to the normal radiosonde data, the NOGAPS
assimilates ship observations, buoy observations and satellite observations from the
Defense Meteorological Satellite Program system of polar orbiters. The web site to access
NOGAPS model output is: http://www.fnmoc.navy.mil/
- Nordenskjold Line
- The "Nordenskjold line" is named for the early 20th century Norwegian
geologist and polar explorer, Otto Nordenskjold. He documented expeditions to Antarctica,
Greenland, the Yukon, Tierra del Fuego and southern Chile. The Nordenskjold line is used
to estimate the arctic tree line, which denotes the boundary between the Boreal forest and
tundra. This is done by drawing a line through geographic points where the mean
temperature for the warmest month (usually July) is equal to 51.4 - 0.1k, where k is equal
to the mean temperature of the coldest month (usually January). This equation shows a
climate warm enough to produce tree growth in the warmest month, but not cold enough in
winter to kill all tree species. The Nordenskjold line is rarely used anymore, since the
Boreal forest and tundra boundaries are now more precisely discriminated by satellite
imagery.
- Nowcasting
- "Nowcasting" refers to the creation of weather statements which highlight
current conditions and forecast for the next few minutes to hours. For example, radar and
satellite data are used to project when skies will clear or severe weather will end.
- Omega High
- An omega high is a ridge of high pressure which disrupts the normal westerly flow
pattern across North America. On a surface or upper air map of the pressure pattern it
shows up as a feature which looks like the Greek letter omega. Persistent dry, fair
weather under the high pressure ridge is often the case, lingering for days or sometimes
weeks on end. This feature is also called a "blocking high" because it prevents
the normal progression of weather systems and fronts from west to east. Omega highs are
more prevalent in the spring, summer, and fall than they are during the winter months.
- Operational Weather Limits
- Most commonly this refers to the limiting values of cloud ceiling, visibility and winds
which allow for safe operation of aircraft, particularly in takeoff and landing. They are
typically different for daytime and nighttime operations and are also tailored to the
specific airport environments, which account for navigational aids and types of aircraft
used. There are also operational weather limits for other endeavors such as snowplowing,
well drilling, shipping, painting, and just about any other outdoor activity you can think
of.
- The Ozone Season
- During the warm season in North America, stronger sunlight and heat combine to convert
industrial and transportation emissions into smog (ground-level ozone). This is
particularly true for major metropolitan areas. The Environmental Protection Agency (EPA)
and others have monitoring sites in over 1300 locations, covering more than 70
metropolitan areas and 30 states. National Weather Service forecast models along with data
from these monitoring sites are used to produce daily ozone forecasts during the
"ozone season", defined for North America as May 10 to September 30 for most
places, except in California where it extends to October 31. The season is defined based
on the historical frequency of ozone levels (sometimes called the Air Quality Index) that
may be unhealthy.
Daily ozone forecast maps and health risks for major cities in the
United States can be found on the Internet at the EPA's Office of Air and Radiation web
site:
http://www.epa.gov/airnow/
Additional information about ozone and health risks can be found on the Internet at the
National Oceanic and Atmospheric Administraton Public Affairs Office web site:
http://www.publicaffairs.noaa.gov/grounders/ozo1.html
- PAOBS
- Yes, another scientific acronym, courtesy of the Australian Meteorological Services. The
acronym stands for Australian Surface Pressure Bogus Data for the Southern Hemisphere).
These are estimated surface pressure observations for the southern hemisphere derived from
satellite observations, interpolated from conventional data (ship, buoy, or land surface
barometer readings), or extrapolated over time from radiosonde (balloon) measurements.
These data are important to an international data assimilation project, called Long Range
Reanalysis, which is an attempt to reconstruct all of the surface and upper air
meteorological data taken over the last 40 years. The data, originally taken from a
variety of measurements and analyzed by a variety of methods, will be quality controlled,
and reconstructed in a similar format for researchers to examine large scale trends and
patterns in the behavior of the Earth's atmosphere. The PAOBS are important because the
southern hemisphere of the Earth is dominated by oceans and has far fewer meteorological
data points than the northern hemisphere. The Australian Meteorological Services attempts
to routinely characterize the pressure pattern in the southern hemisphere by making PAOB
estimates. Though far from perfect, PAOBs are nevertheless a consistent source of data
over the decades and valued by the Reanalysis* research community. Both climatologists and
meteorologists should gain a better understanding of the behavior in large scale pressure
patterns of the Earth from this effort.
*Reanalysis uses available observations and the equations for conservation of energy and
momentum that govern the atmosphere to interpolate in time and space meteorological
conditions for areas where no observations exist. This is superior to a simple graphical
interpolation because the resulting pressure fields are consistent with the physical
equations that describe atmospheric fluid dynamics.
- Percent Possible Sunshine
- Percent possible sunshine is a standard climate measurement made at National Weather Service Offices, but it is often
misunderstood as a indicator of solar energy. It is the ratio of the actual duration of
bright sunshine (unobstructed sunlight measured by a sunshine recorder) in hours and
minutes compared to the astronomically possible duration of sunshine in hours and minutes,
from sunrise to sunset, for the local station. Thus, in the winter, when only 8 hours of
sunshine are possible for the Twin Cities, 7 hours of actual sunshine would equal 87.5
percent possible sunshine; while in summer when 15 hours of sunshine are possible, 7 hours
of actual sunshine would only equal about 47 percent possible sunshine.
Incidentally, long term climate averages show that it is this time of year that we see the
maximum percent possible sunshine in Minnesota (typically the last few days of September
and first few days of October). Indeed, this climatology seems to be holding true this
year!
- Phreatophytes
- Derived from the Greek word "phreatos" meaning well or underground water, this
term refers to types of plants with extensive root systems that utilize water from the
underlying water table. These plants generally have adapted to more arid climates in
establishing powerful root systems which can lift water from great depths. The root
systems of some phreatophytes have been traced to the depths of 45 feet. Greasewood,
mesquite, willow, and alfalfa, among others, are considered phreatophytes.
- Pirry, Parry, or Perry
- These are not all MPR news anchors! "Pirry," "parry," or
"perry" are Scottish and English terms used to describe a sudden squall, or
heavy rainfall. Technically, they sometimes refer to squalls that approximate a "half
gale" on the Beaufort wind scale (20-22 mph). Some of the brief storms that have
occurred this July (1997) might be described as a "perry."
- Pleion and Antipleion
- In the old days, before meteorologists and climatologists referred to significant
climate departures as "anomalies", they used the terms "pleion" and
"antipleion" to describe areas with abnormal and persistent positive and
negative departures in the elements of climate. These terms were introduced by the famous
Polish meteorologist, oceanographer, and geologist Henryk Arctowski (1871-1958), who was
the first to keep an entire year of meteorological observations in the Antarctic.
Like the word "anomaly," these words imply a trend in the long term climate,
such as a prolonged warm spell lasting many months. The word "pleion," taken
from the Greek, means greater than average, while "antipleion" means less than.
Thus, pleion signifies an area of positive departure in the pattern of temperature,
pressure, or precipitation, while antipleion signifies an area of negative departure.
Using pleionic terminology for the current winter, Minnesota would be described as a
pleion with respect to snowfall and snow cover, and an antipleion with respect to
temperature.
- Plimsoll's Mark or Plimsoll's Line
- This is the name given to the conspicous marks painted on the sides of merchant ships
that indicate the limit of submergence allowed by law. It is named for Samuel Plimsoll,
who championed the law before the British Parliament to prevent captains from overloading
their merchant ships. The British merchant ships used this system for years (since at
least 1899) before the United States adopted a similar system in 1930.
Load limits or
lines were designated as FW (fresh water), S (summer), W (winter) and WNA (winter in the
North Atlantic). For merchants carrying goods to and from India, there was an additional
Plimsoll Mark, IS (Indian Summer), but this actually a misnomer. It was meant for the
October to April period in the Indian Ocean when the summer monsoon season had ended and
seas were relatively calm. Loads could be greater during this time.
In fact, expected or prevailing weather is an important factor in designating load
limits on merchant ships both on inland waters and in open seas. Since its inception, the
Plimsoll Mark has been partially based upon the seas traveled, the time of year, and the
prevailing weather. Where the weather historically produced rougher seas with larger
swells, Plimsoll Marks designated lighter load limits for ships. If these lines or marks
were not rigidly observed, shipping companies could be fined and their insurance policies
could be cancelled.
- Pluviometric Coefficient
- Taken from the Latin root words "pluvio," meaning "rain," and
"metre," meaning "to measure," the pluviometric coefficient is a
manner of expressing rainfall as a ratio of the average monthly value to one-twelfth of
the normal annual amount. Sometimes called an "isomer," it is a way for
climatologists to express each month's "share" of the annual precipitation for a
particular location. Values less than one represent less than an equal share of the annual
precipitation (a relatively dry month), while values greater than one represent a greater
than equal share of the annual precipitation. Taking MSP average monthly precipitation
values as an example shows that February's 0.88 inch average represents less than a 0.4
share (pluviometric coefficient), while June's 4.05 inch value represents a 1.7 share
(pluviometric coefficient). This variability in the pluviometric coefficient is typical of
mid-latitude continental climates, while equatorial and marine type climates often have
values that are uniformly close to one.
- Portmanteau words
- This term, meaning "to carry a cloak or mantle," is derived from two french
words, porter and manteau. Portmanteau words are formed from partial combinations
of two or more words, with parts of each word suppressed.
These are common in meteorological jargon. For example, in upper air measurements and
observations "rabals" (radiosonde balloons), "pibals" (pilot
balloons), and "pireps" (pilot reports) are used to determine the strength and
direction of winds aloft. In air quality terminology, smog is a portmanteau word formed
from smoke and fog.
- Prognostic Chart and Agnostic Chart
- A prognostic chart, often referred to by meteorologists as a "prog," depicts
the expected pressure pattern or height pattern of a given synoptic chart (typically
scaled to a country, a continent, or an entire hemisphere) for some specified future time,
perhaps 24 hours or even 196 hours ahead. Positions of weather fronts and cloud formations
are often shown on these charts, which assist forecasters in determining the areal
coverage of different weather types. Several times each day, the National Weather Service produces various progs using
a variety of models.
An "agnostic chart" is the tongue-in-cheek term used by forecasters to refer to
a prog which no one believes. This may be due to observed differences in the local weather
conditions, errors in the forecast model, or bad initial measurements to set up the model
run. I suppose economists must have a similar term for their bad forecasts.
- Psychrometric Tables
- These are tables used by meteorologists to determine measures of atmospheric water
content (dewpoint, vapor pressure, or relative humidity) from the observed dry-bulb and
wet-bulb temperature values taken with a psychrometer. Today's modern electronic
instrumentation is often programmed to give these values. The term is composed of the
Greek word "psychros" meaning cooling, and meter meaning to measure. The cooling
power of the air is related to the water vapor content, the drier the air the more rapidly
evaporation, or cooling will occur.
- PUFF and VAFTAD
- These acronyms refer to models that are used by various meteorological services around
the world to forecast the movement of volcanic ash plumes in the atmosphere. PUFF (short
for ash puff), a real-time tracer model named by its developer H. Tanaka, uses atmospheric
profile data including winds aloft, along with hypothetical particle size distributions of
ash to forecast the plume trajectory, dispersion and settling rate of particles. The model
forecast is updated and corrected using satellite observations of the ash plume, when it
is large enough to detect.
VAFTAD stands for Volcanic Ash Forecast Transport and
Dispersion Model and is used in conjunction with the numerical forecast models of the
National Weather Service to predict the track and dispersion of ash plumes. The chief
purpose of this mo |