Lesson 3 part 2: Nighttime Cooling

By: Shaun Tanner , 11:03 PM GMT on March 27, 2012

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Okay, so it has been awhile. It is funny where the weather takes you if you let it. The weather has been quite busy lately, so I have been dashing around between tornadoes, West Coast storms, heat waves, and such. But, I am getting back to my lessons of teaching you all how the atmosphere works. My goal is to make you all experts by the time I am finished so you can make educated guesses about the atmosphere and climate change, rather than listen to experts on either side of the fence. If you are coming across this blog and want to get caught up, go ahead and look back at some of my old blogs from the end of 2011. They aren't the best writing you will ever read, but they are something.

Last time, we tackled the topic of daytime heating. The ground receives radiation from the Sun and then, in turn, it radiates energy into the atmosphere. The big note is that the Sun DOES NOT warm the atmosphere. The ground receives radiation from the Sun after passing through the atmosphere. Next, we will discuss nighttime cooling today.

Nighttime Cooling

Here's a big surprise...the atmosphere cools at night. Okay, I guess we are done with this part of the lesson. Everybody can go to sleep now.

For those of you who want to learn more, read on.

I am going to tell you something very simple, yet very complicated. It is a very important sentence that you have to remember throughout this lesson:

The temperature of an object will decrease as long as its emission of radiation is greater than its absorption of radiation.

There are some words in there you might not understand, so I will bring up the analogy of a bank account once again. Let's say your bank account balance goes up. That is the equivalent of your temperature increases. If your bank account balance goes down, your temperature decreases. When you put money into your bank account, that is the equivalent of you receiving radiation. If you withdraw money, then that is the equivalent of emitting radiation. With these ideas in mind, it is easy to see that if you withdraw (emit) more money than you deposit (absorb), then your back account will go down (temperature decreases). The opposite is also true.

So, why does temperature decrease at night? Don't think too hard about this one...THE SUN IS DOWN! Now, which part of the above absorption/emission problem is taken away when the sun sets? Absorption! So if you take away absorption, you are left with the surface only emitting radiation. If this is the only part of the equation, then of course temperature will decrease as long as the Sun is away from the surface.

The process at which the ground cools at night is called radiational cooling. It basically means that the ground cools because it radiates its heat away and into the atmosphere. But because the ground is a much, much better radiator of heat than the atmosphere, it loses its heat much quicker than the air. Thus, by late evening/early morning, the coldest part of the atmosphere is most likely to be the few feet closest to the ground. This is exactly why you should always build a survival shelter off the ground if at all possible. Even if you build the shelter a few inches off the ground you will be warmer than if you were to sleep directly on the ground.

So, if you think about it, a weird temperature profile sets up during the nighttime. When you have cold temperatures near the surface and warmer temperatures higher up in the atmosphere, this is called an inversion. There can be strong inversions in which a very cold surfaces transitions very quickly to warmer temperatures higher in the atmosphere, or there can be weak inversions that have a much slower change.

This brings up a very important topic. Inversions are very important to the atmosphere because they limit vertical movement. If you remember from an earlier discussion, cold air is heavier than warm air. That means cold air always wants to be below arm air. So, if you have an inversion with cold air below warm air already, that is the natural state of this. If you try to pull the cold air above the warm air, it will want to sink back down the surface. This, this inversion is inhibiting vertical movement. Because of this characteristic, inversions are often easy to spot even if you don't have a thermometer and a hot air balloon. Because of this inhibition of vertical movement, our normal tellings of vertical movement are not there. If you see large, billowy, vertical clouds, then you have great vertical movement in the atmosphere. However, if you see something like this:



Then this is telling of an inversion. Do you see how the clouds have a definite flat top? The clouds are being held down by the inversion.

Here is another example, but with pollution:



Very bad pollution episodes are usually accompanied by a strong inversion that prohibits the pollution from mixing high into the atmosphere.

So what nights do we get the strongest inversions?

Well, if you think about it, that is pretty simple. Since the temperature of the surface will cool as long as the Sun is away from warming it, then it would be logical that the coolest nights of the year occur in the Winter, with the nights are the longest. This is also when we experience the strongest inversions.

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About shauntanner

Shaun Tanner has been a meteorologist at Weather Underground since 2004.

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