 Okay, well let's talk a little bit about Earth's atmosphere. Here we're viewing Earth from outer space and we can see there's this faint blue shell encasing the planet and that faint blue shell is indeed Earth's atmosphere. So this gives us a sense of how relatively thin Earth's atmosphere actually is as it turns out the lower 80 kilometers, that is the first 80 kilometers above the Earth's surface contains just about 99% of the atmosphere. So the atmosphere really is this thin blue shell around the Earth. Now we can further decompose the atmosphere into various layers. The lowest of these layers is what we call the troposphere. Depending on the latitude it's somewhere between the first 10 and 14 kilometers. And it's the area of the atmosphere within which we reside, within which most of Earth's surface resides, in fact all of Earth's surface, even the tallest mountains. It's also the layer within which weather takes place. Now this plot here shows temperature as a function of altitude and we can see that within the troposphere temperatures decrease. It turns out they decrease at about a rate of about six and a half degrees Celsius per kilometer and they continue to do so until we reach this boundary here between the troposphere and the next layer up the stratosphere where that temperature trend reverses and temperatures in fact start to increase as we go up further in the atmosphere. The boundary between these two layers is what we call the tropopause. Now why do temperatures increase as we get into the stratosphere? Well it has to do with the chemistry of the atmosphere and in fact the existence of ozone within the stratosphere. It's the photo dissociation of ozone by solar radiation and the heat given off during that photo dissociation process that actually heats the stratosphere and leads to the increasing temperatures as we go up in the atmosphere. Now primarily in this course we are going to focus on the troposphere and to some extent the stratosphere. Those are the main parts of the atmosphere that we're going to be interested in.