So we talked about this E shaped temperature profile. And what I want to do now is look at why this temperature profile exists. It's actually relatively simple to explain. And the approach that I want to take to explain it, is simply to say there are three areas in this temperature profile that are warm. And why is that region of high temperature present in each one of these locations. And so what I've done here is broken down what the cause of that heating is. Let's start at the bottom in the troposphere. Within the troposphere, we see relatively warm air near the ground and the temperature decreasing with height until we get to the top of the troposphere. This heating comes from the greenhouse effect. Solar radiation comes down, is absorbed by the ground. The ground having a high temperature then emits infrared radiation into the lower part of the atmosphere. Which the atmosphere then absorbs. So. In the bottom, we have the greenhouse effect. Heating from the ground, and we'll talk more about this in a later lecture. Why is it warm here at the stratopause, the boundary between the stratosphere and the mesosphere? It's warm here because of the presence of ozone. We have ultraviolet radiation that's coming down, coming from the sun. It encounters this ozone, and it breaks apart the ozone into molecular oxygen plus atomic oxygen, so O2 + O. This process of breaking the bonds that construct the ozone leads to the release of heat. And although the ozone is concentrated mostly in the stratosphere, this ultraviolet radiation encounters it in the upper part of the stratosphere and lower mesosphere. So that's why it's warm in this region here around 50 kilometers. Why is it warm in the thermosphere? Well much more, a few more reactions going on here. When we get up high in the atmosphere, we start to encounter more intense radiation coming from the sun. We have x-rays and extreme short wave ultra-violet radiation, radiation that would be damaging to cells had it made it all the way down to the ground. Both the oxygen and the nitrogen absorb this radiation and it causes them to break apart into their atomic components, oxygen and nitrogen. Then this radiation interacts with the oxygen and nitrogen and in it's atomic state and kicks out electrons from these atoms leaving ions. And, you may also hear this area being referred to as the ionosphere, this is where the aurora occur. These regions are releasing heat because of the ionization of these atoms, and the result is a relatively warm thermosphere. So, the way I like to think about the thermo structure of the atmosphere is one that would be relatively uniform with height, except there are three areas of warmth. One, in the troposphere, due to the greenhouse effect. One near the stratopause, due to the heating of ozone and the breakup of ozone by ultraviolet light. And then one in the thermosphere that is the result of the ionization of the gases in the atmosphere. So, what I want to do now is summarize this lecture in a way that illustrates why this temperature profile is important and why I'm making a big deal out of it. Here, we have this inverted W, this tilted W shape. To the earth's temperature profile. And we know this area here in the middle, in the stratopause, why it's warm. It's due to the breakup of ozone, O3. Well obviously, if there wasn't any oxygen in the earth's atmosphere, then there wouldn't be any ozone, either and this warming in the upper part of the stratosphere and lower mesosphere would not be occurring. And indeed, if we look at other planets that don't have a lot of molecular oxygen in them, such as Mars and Venus, then we see temperature profiles that look more like this. They are warming near the surface. Because of whatever greenhouse effect they have. Venus has a very strong one. Mars has a very weak one. But they both have warming near the ground. Both planets have warming in the upper part of the atmosphere due to the ionization of the gasses in the atmosphere. But neither of these planets have a strong stratopause. And we know that because there's no oxygen in these atmospheres.
