I've long wondered why the shortest day of the year is not also the coldest, and USA Today offers an explanation that sounds wrong to me.

We could think of the air in such a place as being like a bank account. If you add money to a bank account, it grows. If you add heat to air it warms up. The Earth is always losing heat, like a bank account that you're always taking some money from. If the amount of heat arriving from the sun is exactly equal to the amount leaving, the temperature stays the same.

As days grow longer in spring and early meteorological summer, the balance tips to more heat arriving than leaving. This is like adding money to the account faster than you are withdrawing it. The air grows warmer and warmer. On the longest day, the amount of heat arriving is greatest. But, even after the days begin growing shorter, the amount of heat arriving is more than the amount leaving. It's like continuing to add more money to the bank account than you're taking out, even though you are adding less than you were before.

Sometime in the late summer or during the fall, depending on how far north of the equator you are, the heat "account" is in balance. From then on, more heat is leaving than arriving and the days grow colder. Now, you're taking out more than you're adding to the account.

In December, when days are shortest, the "withdrawals" from the heat account are greatest. But even after the days start growing longer, more heat is leaving than arriving. The heat account is growing smaller, even though less heat is leaving. Eventually, however, you arrive at a day when the amounts of heat leaving and arriving are in balance. Then, the amount of heat being put into the account becomes greater than the amount being withdrawn. The air begins warming up.

Why does this explanation strike me as wrong? Because everyone knows that air doesn't take months to warm or cool. Every day the air gets warmer, and every night it gets cooler, often swinging by tens of degrees in a few hours. It doesn't make any sense to claim that air can warm from 32 degrees at 6am to 65 degrees at 1pm, but that the daily high takes months to migrate from 40 degrees to 60 degrees.

The temperature at any given moment appears to be directly tied to how long the sun has been up... but still, the coldest day of the year isn't the shortest, nor is the hottest the longest. There must be more going on than I'm aware of, and I don't think the "bank account" explanation covers it.



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