The earth can be conveniently divided into four layers: the troposphere, the stratosphere, the mesosphere, the thermosphere. The region outside the thermosphere is sometimes referred to as the exosphere.
The troposphere is the lowest layer and the layer in which we live. By mass it accounts for 80% of the atmosphere. It is the increase of carbon dioxide in the troposphere that is cause of global warming. The height of the troposphere varies by latitude and other effects. It is thinnest at the poles and thickest at the equator and can vary from about 7-17 km in thickness.
Generally speaking, temperature decreases with height through the troposphere. The surface temperature of the earth is about 288 K, and it decreases about 6.5 K per km in altitude. There are thin regions within the troposphere in which the temperature increases with altitude. These temperature inversions inhibit mixing and can exacerbate smog by inhibiting dilution.
At the top of the troposphere, the decrease in temperature stops in a region called the tropopause. the temperature in this region averages about 218 K.
The stratosphere extends from the tropopause to about 50 km in altitude. Within the stratosphere temperature increases with height, slowly at first and more drastically at higher altitude. At the top of the stratosphere, the stratopause, the temperature is about 270 K.
The stratosphere is very dry compared to the troposphere. For the same reason that temperature inversions inhibit mixing, the stratosphere also inhibits mixing.
The stratosphere has a high concentration of ozone. The ozone layer absorbs ultraviolet light. Chlorinated fluorocarbons (CFCs) have been implicated in the depletion of the the ozone layer and are being phased out under a treaty called the Montreal Protocol. CFCs are long lived in the atmosphere and are able to migrate to the stratosphere. In the stratosphere, chlorine atoms are liberated and participate in heterogeneous reactions catalyzed by polar stratospheric clouds (PSCs) that deplete ozone.
The media often confuse the issue of ozone depletion with global warming, but they are really two different processes that occur for two different reasons. Ozone depletion occurs because of chemical reactions within the stratosphere involving chlorine from CFCs, whereas global warming occurs because of the increased absorption of infrared radiation in the troposphere by increasing amounts of carbon dioxide emitted from burning fossil fuels. Although there are potential feedback mechanisms that may cause these phenomena to interact with one another, to first order, they are separate issues.
One effect of global warming is that the stratosphere is actually cooling! (
At the top of the stratosphere, in stratopause, the increase in temperature stops.
The Mesosphere and Thermosphere
The Mesosphere extends from the top of the stratopause at about 50 km to about 85 km. The temperature decreases with altitude from about 270 K to about 180 K. The Thermosphere extends to the exobase, which varies significantly from about 350-800 km.
Weather and Geography
Of course the temperature varies according to location on the earth as well as local and temporal weather patterns. The values of temperature (as well as pressure) cited here are approximate mid-latitude mean values. Accounting for variation, of course, makes the model more complex.
Pressure and Density
As altitude increases through the atmosphere, pressure decreases. As pressure decreases there are fewer absorbers in the same volume of air. Additionally, the frequency of collisions between molecules decreases and thereby changes the infrared spectra.
Pressure, p, through the atmosphere decreases approximately exponentially. If p0 is the pressure at sea level (or other reference point), then:
p = p0*exp (-z/H)
where z is altitude in meters and H is a quantity called the scale height. Scale height is a function of temperature at 288 K, it is about 8435 m. Density decreases with altitude with the same relationship.
The following table shows the composition of the atmosphere by volume, relative to a dry atmosphere. Infrared active species are noted.
The next logical step to take in building a radiative-transfer model of the troposphere is to expand the three-layer model to a multi-layer model, in which I account for the temperature and pressure variation. The next post in this series builds such a model for carbon dioxide alone. After that step, I can start to discuss more sophisticated models of the atmosphere. It is perhaps worth mentioning the natural tension between keeping the model understandable to the lay person, and including more detail into the model. By going step-by-step, I hope to at least make some of the more sophisticated models understandable, but first I extend the simple model.