On Dec. 3 the high temperature of 65 degrees F was the all-time highest December temperature ever recorded in Madison. Among the interesting aspects of this record high was the fact that the entire day was cloudy so local sunshine had no role in achieving this record. This prompts an interesting question — what processes can contribute to changing the temperature at a location?
The answer is that there are basically two. Everyone knows that on a sunny, windless day, the fact that the sun is out always contributes to warming the air temperature. At night, in the absence of sunshine, the air cools. These changes are a result of radiative transfer, one of the two mechanisms.
The other mechanism is called advection and it involves the importation of air from remote locations by the action of the horizontal wind. Last Monday, the winds in Madison were blowing strongly from the southwest all day. It was much warmer, as is usually the case, at locations to our southwest. Therefore, the southwesterly winds were dragging that warmer air toward us all day and we reached our daily high temperature at 5:25 p.m., more than an hour after sunset.
The interaction of these mechanisms becomes a high-profile topic as we approach winter. The long nights of arctic winter provide a perfect environment for air to cool nearly continuously, by radiation, building huge masses of really frigid air to our north. Weather systems can tap that cold air and send it southward on northerly winds during periods of strong cold air advection that can drive the temperatures down even on sunny winter days.