|MadSci Network: Earth Sciences|
The difference between layers of the atmosphere is based on how temperature changes with elevation in each layer. In the troposphere, temperature decreases as elevation increases. In the stratosphere, temperature increases as elevation increases. The boundaries between each layer, called the tropopause, stratopause, etc. are zones where the temperature reaches a maximum or minimum to effect the transition. A good atmospheric physics textbook could probably describe this in mathematical detail, but some of the more readable descriptions I've found can be located in Hewitt, et. al's Conceptual Physical Science, in the chapter on atmosphere and the oceans. The height of the tropopause above the earth's surface varies with latitude so it isn't a constant, but the way temperature changes with elevation in the layer is distinctly different for each layer.
There is a diagram of the layers at the Enchanted Learning site, which is a middle or high school appropriate site for a number of topics in physical and earth science (as well as other content.)
Why is this useful? Study of the atmosphere requires special instruments and special techniques. I worked with an atmospheric research group that launched balloons which ascended to 100,000 feet [ Admin: 30 kilometres, or 18 miles ], and sent "real time" data back to our computers on particle size in the troposphere, and quantity of ozone in the stratosphere. Each kind of data we received needed a specific instrument to do the measurement. Knowing the layers of the atmosphere we were researching gave more information for our analysis.
Try the links in the MadSci Library for more information on Earth Sciences.