The atmosphere is divided into four layers because each layer has a distinctive temperature gradient. The four layers of the atmosphere are the troposphere, the stratosphere, the mesosphere and the thermosphere. Sometimes, the troposphere is termed the lower atmosphere, and the thermosphere is known as the upper atmosphere, with the stratosphere and mesosphere grouped together as the middle atmosphere.
The troposphere is the layer of the atmosphere in which people live, with breathable air and generally comfortable temperatures, due to the reflection of the sun's heat off the earth. This layer, which is where weather occurs, begins at the earth's surface and extends out about 4 miles at the poles and 11 miles at the equator. The stratosphere extends out about 30 to 35 miles above the surface of the earth; this is the layer that airplanes generally fly through. The stratosphere includes the ozone layer, which absorbs much of the harmful ultraviolet radiation in sunlight. In the mesosphere, temperatures drop drastically, reaching a point as low as -120 degrees Fahrenheit. The mesosphere extends from about 35 to about 50 miles above the earth. The thermosphere reaches out as far as 400 miles above the earth's surface; while it technically does contain air molecules, they are so far apart that there is no breathable air. This is the layer in which the International Space Station operates.