Enhanced Snow and Precipitation Monitoring within Minnesota Portions of Watersheds Draining to Lake Superior
Effects from air masses influenced by Lake Superior as well as from adiabatic cooling due to elevation changes can be seen in the data. Elevation ranges from just over 600 near the lake to over 1900 feet at Isabella. The particular behavior along the transect can vary rapidly in time such as shown in the graph just below of transect temperatures at two times exactly 24 hours apart. On May 12, 2004 the prevailing wind direction was from the northeast, transporting lake-cooled air inland and cooling the near-shore locales. A changing weather pattern on May 13, 2004 brought the wind around to the northwest. The 20 degree drop in temperature was moderated at the near-shore locations by the relatively warm lake surface temperatures.
Graphs that show the temperature behavior along the transect at 2am and 2pm CST were prepared.
Those hours will show much of the diurnal range (as shown in the June diurnal graphs mentioned above)
but avoid hours that may have excess solar influence. The devices were first deployed on Feb 6 and were last
read (at the time of this writing) on June 26.
feb mar apr may jun
Those same data can be 'normalized' by subtracting the average temperature along the whole transect from the
values at each thermochron. The resulting graphs make it easier to compare the behavior along the
transect in different months and at different times.
feb mar apr may jun
A comparison of the afternoon (2pm CST) behavior between February and June clearly shows the cooling influence of Lake Superior on sites close to the lake in June and a more modest warming influence in February. ('Fast' ice along the Lake Superior shore near the transect for a large part of February would have the effect of increasing the distance to open water and a source of warm air.) During the day in summer one might expect that afternoon land temperatures in June would be almost always warmer than Lake Superior surface water temperatures. Such a situation would tend to cause a 'sea breeze' where air flows onshore from cool water to warm land. On winter afternoons the situation is much less obvious and as hinted above can be somewhat dependant on the state of the ice pack on Lake Superior which can vary strongly from year to year and even significantly from month to month within a given season. Air can also be 'forced' on or off shore by 'prevailing winds'; that is, the winds of weather systems that traverse the state have their own wind patterns which will often be more obvious than a 'sea breeze'.
At night when land surfaces can cool more than the liquid surface of a nearby large water body the reverse of a 'sea breeze', namely a 'land breeze', can occur. When air flows from the land to the water, the water temperature no longer directly influences temperatures onshore. In that case the differences in temperature between monitoring sites will be mostly due to differences in the sites themselves rather than be a function of how far from water they are. In the graphs of 2a CST temperatures (just below) the temperature tends to drop away from the shore both in February and in June. However, elevation above sea level is increasing fairly regularly away from the shore. The rate at which a given parcel of air would cool as it changes elevation (due to decompression as pressure decreases with altitude) is known as the 'adiabatic lapse rate'. In the 2am CST graph, lines which show the 'dry adiabatic' and 'moist adiabatic' lapse rates are also shown. Except for the apparent 'frost pocket' at EckBeck Campground the behavior looks very strongly adiabatic. That is, if you know the temperature at one site at 2am then using the elevation of other sites in the neighborhood you could make a pretty good guess of their temperatures!
A common variation would be that the snowboard is covered by some snow. In that case the thermochron would be ‘insulated’ from relatively quick changes in the nearby air temperatures. Early results suggest that that effect typically causes the board to be somewhat warmer than the air temperature.
The example below shows the effect of a clear cold night with no new insulating snow cover on the boards. In a case like this heat radiates away from the surface so readily that it gets colder than the air itself. The difference of up to 8.5 degrees in this example is pretty dramatic but probably not uncommon on such clear, cold nights.