Madison’s overnight low temperature of minus 3 on Dec. 28 was a relatively rare event, all things considered.
As we all know, the temperature dipping below zero in winter is not unusual. But it doesn’t usually happen without snow on the ground, and there are good reasons for that.
Snow on the ground works double duty to keep the air near the ground very cold. First, since the ground is a vast storehouse of heat, the presence of snow insulates the air above from that heat.
The fact that snow is an excellent absorber and emitter of infrared energy — the kind of radiation that nearly everything on Earth emits, including the ground — gives this insulation effect an added radiative dimension. With a couple of inches or more of snow cover, the infrared energy emitted by the ground is absorbed by the overlying snow.
The snow then emits that energy in all directions, some of which are back toward the underlying ground. Thus, much less than half of the energy that would otherwise be emitted from the ground to the overlying atmosphere never gets there.
Secondly, since the top surface of the snow is emitting infrared exceptionally well, the air in contact with that snow surface can get really, really cold. The result is extremely cold surface air.
In our last cold spell, the coldness was imported from locations where this radiative cooling had been in operation — places to our north. This coming week, our low temperatures will be partly “home-brewed” by the snow we just received.
