Hi, my name is David Schultz. Welcome to Our Earth, Its Climate, History, and Processes. In this lecture, I want to talk about one of the components of the Earth's climate system, the atmosphere. And we're going to start with the chemical composition of the atmosphere and then its structure. As you may know, the atmosphere is composed primarily of three gases. We have the molecular form of nitrogen, N2, which comprises 78% of the volume of the atmosphere. We have oxygen in its molecular state comprising 21% of the Earth's atmosphere. And then we have argon, which is 1% of the Earth's atmosphere. A minor constituent, but quite an important one, is carbon dioxide, now presently running about 400 parts per million in the Earth's atmosphere. And besides these fixed components of the atmosphere, we also have variable components as well. Water vapor could have variable percentages, anywhere from near 0% in the driest environments to around 4% of the Earth's atmosphere in very humid tropical environments. Ozone varies tremendously with altitude in the atmosphere. Near the surface, we can expect relatively small values, even in urban environments, of around 0.01 parts per million at the surface. But as we go up into the ozone layer, we'll see values as high as ten parts per million at a height of about 25 kilometers. And then we have solid and liquid constituents. We have water droplets and ice that comprise the clouds and precipitation. And then we have dust and salt particles, what we call aerosols. These are also a component of the Earth's envir, Earth's atmosphere. Now I want to talk about the structure of the atmosphere, both the pressure and the temperature profile with height. Let's start with the pressure. Here you can see the graph of pressure with height is an exponential decreasing with height. The value of pressure average across the Earth is 1,013 millibars, and you can see with height, it decreases. At a height of around ten kilometers, we have 250 millibars of pressure. Around 50 kilometers in altitude, we have one millibar of pressure. So, what does this mean? If you think of pressure as the weight of the atmosphere pressing down on us, then here, this is roughly 1,000 millibars. So, 75% of the Earth's weight lies below ten kilometers. If we go up to 50 kilometers, then 99.1% of the Earth's atmosphere lies below this altitude. So, keep that in mind. Not only does the pressure decrease with height, but it decreases quite rapidly with height. Let's look at the temperature now. Here we can see the temperature profile and it's got a very different shape to it. I like to think of it as a W on its side, or if you prefer, the capital letter E. Here we can see that the temperature through about ten kilometers decreases with height. Above that, the temperature increases with height. Below that, above that, it decreases with height again. And then it continues to increase with height as we go up above 85 kilometers. So now let's look at the temperature structure and how that's related to the pressure structure with height. And we're going to use the temperature structure to define different layers of the atmosphere. Let's start with the lowest layer here, below about ten kilometers. We call this layer the troposphere, and tropo, in this case, means changing. As we saw before, about 75% of the mass of the atmosphere lies within the troposphere. And this is where almost all of the weather that we experience on a day-to-day basis is generated. This is a region of a lot of activity. All the rain hurricanes, tornadoes, everything occurs within this layer. Eventually, as you go up with height, the temperature stops decreasing and it begins to increase. We call this layer of the atmosphere where the temperature increases with height the stratosphere. The stratosphere is a region that's very stable, as the name suggests. Remember, we talked about stratigraphy, which represented layers in the rock. Strato means layers, and in this case, the layering of the atmosphere makes it very stable in the stratosphere. So, this temperature increasing with height is where the stratosphere occurs. And as you may have heard from the news, this is also the layer where we expect to see a lot of ozone. At some point, we reach a temperature about freezing, and then the temperature decreases above about 50 kilometers. We call this layer the mesosphere. We go up higher to about 85 kilometers, the temperature reaches another minimum, and then it rises again above that. And we call this layer the thermosphere. The thermosphere is a result of the intense radiation coming down into this layer, breaking up the constituent atoms into ions and causing heating. And thus, the very high temperatures in the thermosphere as the name suggests, are a result of the breaking apart of these atoms.