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 Then, the amount of heat being put into the account becomes greater than the amount being withdrawn.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.
If I remember correctly from college classes, one thing that helps account for the difference is the fact that the ground is a heat sink. Heat is stored during the day, and radiates away at night, and the effect causes a lag in actual temperature. This is the same reason why the hottest part of the day is actually a couple hours after noon, not at noon.
RC: Indeed.. and the coldest part of the night is just before the dawn, not right after the sun goes down.
Of course, factors like wind and cloud cover play big roles too.
Rick C: The ground as a "heat bank" doesn't make any more sense than the air as such, and for the same reasons. As you said, the surface of the ground cools and heats over the course of hours, not months.
Mark: I don't have any trouble understanding why temperature lags sunlight by hours, but why does it lag by months?
Well, take it up with the textbook makers, then, Michael, because that's what they say.
MW: What makes a lot more sense to me is that the amount of sunlight isn't as significant of a factor. What about a day or even a week (maybe even two weeks) before or after the shortest day of the year? I doubt the difference in the length of the day over that span of time is anything of significance.
During the time that we in the northern hemisphere experience "summer", we get a lot more *direct* sunlight (as close to direct as you can get for being away from the equator) and, over time, more heat either stored in the atmosphere or absorbed/radiated by the land/water. The mitigating factors in how all of that direct sunlight affects our temperatures are, among many things, the warm and cool air masses that circulate around the planet. The shortest day of the year could happen to occur when a warmer air mass is over you, resulting in a warmer day than a day a few weeks ago when a cooler air mass was influencing your weather.
I think it's because even after the shortest day, the nights are still sufficiently longer than the days for average temperature to be dropping. Think of it this way: if the rate of heat loss at night equals the rate of heat gain during the day (not true, but a simple case to consider), then it would only start getting warmer again at spring equinox. Also, it would only start getting colder after summer at the vernal equinox.
Day/Night fluctuations won't effect the overall climate or jetstream because they are averaging out on trends. I would suspect that the polar regions tend to build up during the winter and affect the macro weather patterns as the region in the north and south get colder or warmer. Since this change isn't happening as fast as daily changes, it produces a lag on the warming trends so to us, hotter months July, August, Sept lag behind the longest Day - June 20th or 21st while the cold does the same in the winter. Just a thought.
What you have to bear in mind is that the air/ground never gets so hot that, in continued sunlight, it wouldn't get hotter! Even in high summer, temperature is limited by the fact that, save for the poles, we don't get continues sunlight. Until sometime in February, the temperature at dawn continues to get lower, and that is noticeable even though temperature rises through the day.
I don't believe a heat sink is necessary for this to work.
If you need one though, there are the oceans. They are a very effective heat sink, the vast majority of them are stuck at 4 deg C.
What are the statistically warmest and coldest days of the year? What dates and are those dates both equally distant from the longest and shortest days of the year?
Michael, ground temperature varies dramatically with depth. The first inch of soil would have a heating and cooling cycle tied almost completely to the sun rising and setting. But go down a foot or two and it's very different. Go down twenty feet, and the ground temperature doesn't vary at all with air temperature.
It's similar to varying depths in a lake. In the summer the shallow water is warm, but it gets much colder as you go deeper. And in winter the surface freezes first, but it takes a long time for the cold to work its way down far enough to freeze a thick sheet of ice. And at a certain depth the water will never freeze, because winter isn't long enough for the cold to travel that far down.