3.1.1 - biosphere  (Page 3/4)

The troposphere is the atmospheric layer closest to the earth's surface. It extends about 8 - 16 kilometers from the earth's surface. The thickness of thelayer varies a few km according to latitude and the season of the year. It is thicker near the equator and during the summer, and thinner near the poles andduring the winter. The troposphere contains the largest percentage of the mass of the atmosphere relative to the other layers. It also contains some 99percent of the total water vapor of the atmosphere.

The temperature of the troposphere is warm (roughly 17º C) near the surface of the earth. This is due to the absorption of infrared radiation from thesurface by water vapor and other greenhouse gases (e.g. carbon dioxide, nitrous oxide and methane) in the troposphere. The concentration of these gasesdecreases with altitude, and therefore, the heating effect is greatest near the surface. The temperature in the troposphere decreases at a rate of roughly6.5º C per kilometer of altitude. The temperature at its upper boundary is very cold (roughly -60º C).

Because hot air rises and cold air falls, there is a constant convective overturn of material in the troposphere. Indeed, the name troposphere means“region of mixing.” For this reason, all weather phenomena occur in the troposphere. Water vapor evaporated from the earth's surface condenses inthe cooler upper regions of the troposphere and falls back to the surface as rain. Dust and pollutants injected into the troposphere become well mixed inthe layer, but are eventually washed out by rainfall. The troposphere is therefore self cleaning.

A narrow zone at the top of the troposphere is called the tropopause. It effectively separates the underlying troposphere and the overlyingstratosphere. The temperature in the tropopause is relatively constant. Strong eastward winds, known as the jet stream, also occur here.

The stratosphere is the next major atmospheric layer. This layer extends from the tropopause (roughly 12 kilometers) to roughly 50 kilometers above theearth's surface. The temperature profile of the stratosphere is quite different from that of the troposphere. The temperature remains relatively constant up toroughly 25 kilometers and then gradually increases up to the upper boundary of the layer. The amount of water vapor in the stratosphere is very low, so it isnot an important factor in the temperature regulation of the layer. Instead, it is ozone (O3) that causes the observed temperature inversion.

The third layer in the earth's atmosphere is called the mesosphere. It extends from the stratopause (about 50 kilometers) to roughly 85 kilometers above theearth's surface. Because the mesosphere has negligible amounts of water vapor and ozone for generating heat, the temperature drops across this layer. It iswarmed from the bottom by the stratosphere. The air is very thin in this region with a density about 1/1000 that of the surface. With increasing altitude thislayer becomes increasingly dominated by lighter gases, and in the outer reaches, the remaining gases become stratified by molecular weight.