We have made reference a few times in this column to the areal extent of cold air over the Northern Hemisphere as a measure of wintertime severity, that is, the geographic reach of air of a certain temperature.
Specifically, we have reported on the 23-degree air at 1 mile above the ground (where the atmospheric pressure is just 85 percent of its near-surface value). By mid-July it is impossible to find air that cold at that elevation in the Northern Hemisphere.
In other words, the cold air that in mid-winter covers over 26.2 million square miles disappears entirely in midsummer.
This is vivid testimony to the power of the increased sun angle and the longer days that characterize our summer versus the winter.
As we move past the summer solstice toward the autumnal equinox, both the sun angle and the length of the day begin to decline, with the length of day shortening dramatically faster at high latitudes than it does here in Madison.
The shorter day means, of course, a longer night, which allows more time for the ground to radiate energy away to space. This energy loss eventually and inevitably leads to the production of cold air near the surface of the planet.
As we head further into the late summer and early fall, high-latitude cold air production continues to benefit from the short day and low sun angle. The cold air builds up to the point where it is exported regularly from high latitudes to lower latitudes and the areal coverage of the 23-degree air grows, on average, at a uniform rate from Sept. 1 to Dec. 1. It reaches its peak near late January and then shrinks uniformly from around April 1 to late June.
Our longest day of the year has already come and gone. In the next week or two we will begin to see the inevitable return of cold air to the Northern Hemisphere, a harbinger of winter.