Light and the Atmosphere

The key to understanding Earth's atmosphere lies in interactions between atmospheric gases and energy from the Sun. Although the Sun also emits ultraviolet light and x-rays, most light coming from the Sun is visible light. Most visible sunlight reaches the ground and warms the surface, but the small amount that is scattered by gases in the atmosphere has two important effects. First, scattering makes the daytime sky bright, which is why we can't see stars in the daytime. Without scattering, sunlight would travel only in perfectly straight lines, which means we'd see the Sun against an otherwise black sky, just as it appears on the Moon. Scattering also prevents shadows on Earth from being pitch black. On the Moon, shadows receive little scattered sunlight and are extremely cold and dark.

Second, scattering explains why our sky is blue. Gas molecules scatter blue light (higher energy) much more effectively than red light (lower energy). The difference in scattering is so great that, for practical purposes, we can imagine that only the blue light gets scattered. When the Sun is overhead, this scattered blue light reaches our eyes from all directions and the sky appears blue. At sunset or sunrise, the sunlight must pass through a greater amount of atmosphere on its way to us. Most of the blue light is scattered away, leaving only red light to color the sky.

The ground returns the energy it absorbs by radiating it away in the infrared (light with even less energy than red light). Greenhouse gases like carbon dioxide absorb this infrared light and warm the troposphere - the lowest layer in the atmosphere. Because the infrared light comes from the surface, more is absorbed closer to the ground than at higher altitudes, which is why the temperature drops with altitude in the troposphere. (The relatively small amount of infrared light coming from the Sun does not have a significant effect on the atmosphere.) The drop in temperature with altitude, combined with the relatively high density of air in the troposphere, explains why the troposphere is the only layer of the atmosphere with storms. The primary cause of storms is the churning of air by convection, in which warm air rises and cool air falls. Convection occurs only when there is strong heating from below; in the troposphere, the heating from the ground can drive convection. In fact, the troposphere gets its name from convection; tropos is Greek for "turning."

Above the troposphere, the air density is too low for greenhouse gases to have much effect, so infrared light from below can travel unhindered through higher layers of the atmosphere and into space. Heating from below therefore has little effect on the stratosphere, the second lowest layer in the atmosphere. Instead, the primary source of heating in the stratosphere is the absorption of solar ultraviolet light by the gas ozone. Most of this ultraviolet absorption and heating occurs at moderately high altitudes in the stratosphere, which is why temperature tends to increase with altitude as we go upward from the base of the stratosphere. This temperature structure prevents convection in the lower stratosphere, because heat cannot rise if the air above is hotter. The lack of convection makes the air relatively stagnant and stratified (layered), with layers of warm air overlying cooler air; this stratification explains the name stratosphere. The lack of convection also means that the stratosphere has essentially no weather and no rain. Pollutants that reach the stratosphere, including the ozone-destroying chemicals known as chlorofluorocarbons (CFCs), remain there for decades.