Atmospheric Temperature

The reason that temperature is distributed as shown in the figures is mostly due to the sun and the way solar energy is deposited in the atmosphere. Most of the solar radiation is emitted as visible light, with smaller portions at shorter wavelengths (ultraviolet radiation) Figure 1. The temperature of Earth's atmosphere is broadly determined by the deposition of solar energy and by the absorption of infrared (heat) radiation by "greenhouse" gases. Concern has recently been growing about increases in greenhouse warming due to human activities. This possible global warming is the subject of active research. Illustration by Hans & Cassidy. Courtesy of Gale Group. and longer wavelengths (infrared radiation, or heat). Little of the visible light is absorbed by the atmosphere (although some is reflected back into space by clouds), so most of this energy is absorbed by the Earth's surface. Earth is warmed in the process and radiates heat (infrared radiation) back upward. This warms the atmosphere, and just as one will be warmer when standing closer to a fire, the layers of air closest to the surface are the warmest.

According to this explanation, the temperature should continually drop as one goes higher in atmosphere. Yet Figure 1 shows that temperature rises with height in the stratosphere. The reason for this apparent contradiction is another case of solar energy deposition in the atmosphere. The stratosphere contains nearly all the atmosphere's ozone. Ozone (O₃) and molecular oxygen (O₂) absorb most of the sun's short wavelength ultraviolet radiation. In the process they are broken apart and reform again and again. The net result is that the ozone molecules transform the ultraviolet radiation to heat energy, heating up the layer and causing the increasing temperature profile observed in the stratosphere.

The mesosphere resumes the temperature drop with height. The thermosphere however is subject to very high energy, short wavelength ultraviolet and x-ray solar radiation. As the atoms or molecules present at this level absorb some of this energy, they are ionized (have an electron knocked off) or dissociated (molecules are split into their component atoms). The gas layer is strongly heated by this energy bombardment, especially during periods when the sun is active, that is, emitting elevated amounts of short wavelength radiation.