Though the first really nasty cold snap of the year is already underway, by the middle of this week we should be in the depths of a deep freeze, flirting with a daily high temperature no higher than 20 degrees Fahrenheit.
Such cold maximum temperatures, though not unprecedented for early December, are also not common in Madison. In fact, only 18 of the last 40 Decembers in Madison have had at least one daily maximum temperature of 10 degrees Fahrenheit or colder.
Upon dividing the month into early December (Dec. 1-10), mid-December (Dec. 11-20) , and late December (Dec. 21-31), we find that seven of these years had at least one such cold day in early December (1976, 1977, 1978, 1985, 1991, 1995, and 2009). Six of the years had the first such day in mid-December (1983, 1989, 1999, 2000, 2004, and 2008) while the remaining 5 didn’t get that cold until late December (1973, 1980, 1990, 1993, and 1996).
So about every third December we can expect to get this cold before Dec. 21, and we might just do that again this week. Our chances will be enhanced thanks to the several inches of snow that blanketed the area on Sunday.
With a fresh snowcover on the ground, a number of effects that can deepen a cold spell come immediately into play. First, the snowcover is an excellent insulator and, if as deep as 3 inches, it effectively eliminates the vast amount of heat stored in the ground from helping to warm the air.
Second, snow is very good at cooling overnight, and that helps to lower the surface air temperature. Third, the snow is very good at reflecting sunlight during the day, which helps to minimize the daytime heating of the air near the ground.