On Wednesday morning, we will officially enter fall as the autumnal equinox occurs at 3:20 a.m.
On Wednesday, in common with every location on Earth, we will enjoy exactly 12 hours of daylight and 12 hours of night.
Of course, 12 hours of daylight in Madison (latitude 43N) is substantially different from 12 hours of daylight at the North Pole (latitude 90N) where the sun will barely be visible above the horizon for the 12 hours of “daylight.” On the very next day, the sun will not appear above the horizon at the Pole and will not come back for six months. As the days march on, the same fate will gradually overtake other latitudes in the Northern Hemisphere until by Dec. 21, the sun will not rise even at 66.5N.
The primary means by which air is warmed is by its contact with the surface of the Earth that, when the sun shines, can absorb radiation and heat up.
A leading consequence of the shortening days, especially at high latitudes, is that air masses can begin to get really cold again. Even as we enjoy one the warmest Septembers in quite some time, the areal extent of the cold wintertime air is growing at high latitudes.
As it encroaches southward from the Pole, the polar jet stream, always on the warm edge of the coldest arctic air, begins to take up residence at lower latitudes bringing with it the powerful storms of late autumn and winter.
So, enjoy these beautiful early fall days we are having in Madison because the cosmic deck is stacked against us.