Late summer Passumpsic fog

Late August is a time of thick nightime fog in the Passumpsic valley. Traces of temperature and dewpoint at the LSC weather station show how this fog evolves overnight. The fog is present when temperature is equal to the dewpoint between midnight and about 9 AM. At that point the sun begins rapidly warming the surface of the earth and the fog quickly evaporates. This can be confirmed in the time lapse video of the LSC baseball field below.

It looks like the fog will not set in as rapidly in the evening and start sometime around 1 AM the following night.

Figure 1: Top frame shows evolution of temperature, dewpoint and relative humidity starting at 00:00 and ending 23:55 for Tuesday, 26 August 2014. 

Cold spring

It has been a cold spring in Vermont, even by Northeast Kingdom standards. Yesterday's climate report for Burlington shows a high temperature of 32 F and a low of 22 F compared with normals of 44 and 27 F for this date. It has been like this for a week.

The reason for this is a large-scale high pressure system stalled over the middle of North America. Because wind around high pressure centers blow  clockwise and outward from highs, it means we have been getting northwesterly winds across the northeastern U.S. for the entire week. These northwesterly winds have been advecting cold arctic air over us non-stop. The springtime sun is no match for this cold arctic air.

Overview of warm and cold front passage

Tuesday and Wednesday 27 and 28 April, 2009 saw temperatures fluctuate wildly as a warm and cold front passed over Vermont.The images at right document that passage (see Unisys archive for more maps).

At 0Z 28 April (Fig. 1) a warm front was slowly approaching from the west as soutwesterly winds brought warm, dry air from the southwestern desert states across New York State and into Canada.

Overnight, the warm front would slowly move across Vermont, and by 12 Z was located on the Maine-New Hampshire border (fig. 2). A cold front associated with the same Low pressure system over Quebec approached New England from the west.

This cold front would pass over Vermont within 6 hours, ushering in a much cooler air mass. By 0Z 29 April (Tuesday evening), the cold front had swept over Maine into the Atlantic (Fig. 3).

More links to graphics for this case can be found here.