104 Years of Twin Cities Dew Point Temperature Records:
1902-2006
The dew point temperature is the temperature to which the air must be cooled at constant pressure for it to
become saturated. The higher the dew point is, the more uncomfortable people feel. This is because people
cool themselves by sweating and if the dew point is at a high temperature, then it becomes more difficult
for sweat to evaporate off the skin.
High summertime dew point temperature readings were frequent news stories during the sweltering
summer months in the late 1990's to mid 2000's. Are summertime Twin Cities dew points increasing?
Here is a look at part of the Twin Cities dew point record that may
answer some questions and bring up new ones.
Dew point measurements began in September 1902 at the Minneapolis Weather Bureau in downtown
Minneapolis. As far as can be determined the dew point was measured at the top of the
old U.S. Court House. Dew point readings were only measured once per day at
1800 hours. Later, a Noon observation was added. At 1:00 pm April 9, 1938, the official
record was transferred to the Wold-Chamberlain terminal building at what would become
the International Airport. The measurements were made on the roof of the terminal building
until 1960 when they were moved to five feet above the ground on the airport grounds.
On June 1, 1996 an ASOS (Automated Surface Observing System) was installed and took
over as the official measurement at the International Airport.
The season looked at in this study was "meteorological summer" which are
the months June, July and August. For June-August,
the minimum dew point temperature typically occurs at sunrise. There are two maxima,
one just before noon, and another near sunset. Thus the early evening dew point is
typically not the maximum for the day, but has a tendency to be close.
One look at the graph below shows some interesting results. The first of which is
the minima beginning in 1924 and lasting until 1937. This stretch of lower dew
points matches well with the dust bowl era when precipitation was also at a
minimum. The period 1938 to 1945 corresponds with a period of higher precipitation
that immediately followed the dust bowl. Note the similarity with the 1990's.
The high dew point period of the 1990’s to the early 2000’s also reflects an era
of higher precipitation. What does stand out beginning in the 1990’s is the lack of
dry dew point years. 1992 was a different case showing a lower dew point because
it was one of the coldest summers on record. Both 2003 and 2004 were somewhat
drier in the Twin Cities and the dew point values reflect that accordingly. 2005 saw
a return to average summer dewpoint temperatures that were above the long term mean. 2006
fell below the long term mean.
A trend line drawn through the data indicate a slight increase of about one degree per century.
It is impossible to say if the Twin Cities will continue the trend of higher dew points.
The monthly dew point temperature data used to construct this graph can be seen here.
80 degree dew points are rare in the Twin Cities
historical record. Since 1945, there have been only twenty hours of 80 degree dew
points recorded. Ten of those twenty hours came in a ten hour period on July 12
and 13, 1995. The highest dew point temperature ever
recorded in the Twin Cities was 81 degrees at 11:00 am on July 30, 1999.
The highest single hour dew point temperature in 2003 was 77 degrees.
In 2004 75 degrees was the max, 2005 it was 81 degrees and in 2006 it was 76 degrees F
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