Snow cover in the winter plays a large role in the temperatures observed, even more so in February when the sun angle gets progressively higher. The reasons are two-fold. First, the sub-freezing snow surface acts to chill the air above it, similar to putting a room temperature object in the refrigerator or freezer. Secondly, when the sun melts the snow, the energy used actually results in a net cooling effect on the air mass above it. Without snow cover, air masses, even Arctic air masses, can undergo substantial modification, as has been seen much of this very mild winter.
A sunny, dry day like today (Sunday) provides an ideal situation to see the difference snow cover can make on air temperatures.
The image below is a visible satellite snapshot taken around noon, overlaid with airport observations. You can readily see the deep snow cover laid down by the heavy lake effect event on Friday night across northwest and north central Indiana and far southwest lower Michigan, but it has been highlighted with a white dashed line for clarity. In contrast, the much lighter snow cover produced by the Arctic front on Friday was bascially completely gone. With strong high pressure centered to our southwest, a very dry air mass enveloped the region. Dew point temperatures were in the single digits and low teens. The combination of the higher sun angle this time of year and lack of moisture in the air allowed temperatures to rapidly rise to the lower and in some cases middle 30s by the early afternoon over the no snow cover area. But notice the portions of northern Indiana still with a deep snow cover had only risen to the mid and upper 20s by the same time. More specifically, while temperatures were around 30 at noon at both O'Hare and Midway Airports in Chicago, the temperature at Valparaiso Airport in Porter County, about 30 miles away as the crow flies, was only 25 degrees at the same time!
- Richard Castro 2/12/12