Temperatures dropped quickly Monday evening thanks to clear skies and calm conditions. Our saving grace was ground temperatures which are still warm for this time of the year and the lack of snow cover, which prevented temperatures from dropping even further. Snow cover is a game changer when it comes to the surface energy balance during the day and night time. It acts to mostly seperate the exchange of energy from the sun to the soil and back to the air. The featured chart presents the frequency of having snow cover at a given daily low temperature (blue line) and the overall frequency of that temperature (red line). The main point is to show that the coldest temperatures are increasing associated with the presence of snow cover. For example, a low temperature of zero degrees also had snow cover present 90% of the time. The chart also had an interesting (but unexplained here) aspect of having the lines cross at the 50% probabilities for both y-axes around 18 degrees. It is difficult for near surface soil temperatures to get cold enough to support very cold air temperatures, so snow cover is increasing necessary for the coldest temperatures.

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Tags:   lows   snowcover