Earth's Atmosphere

Earth's atmosphere is divided into four regions. The lowest of these regions is the troposphere. Here exists turbulence and atmospheric mixing due to convection currents (weather, for example). The atmospheric density shows a rapid decrease with height. The dominant source of energy in this lowest region is conductive heating near the ground, and the dominant energy losses are radiation and convective mixing. The net result is a rapid temperature decrease with altitude. At a greater height is the stratosphere, where there exist strong horizontal wind currents (the jet stream) that affect ground level weather. Going higher, the solar ultraviolet radiation that oxygen and the ozone absorb is the dominant source of energy gain, and radiation and convective mixing are the dominant means of energy loss. The balance between energy gain and loss results in a local temperature maximum at a height of 50 kilometers at the base of a region termed the mesosphere. Even higher is the thermosphere, where absorption of very short wave solar radiation is balanced by thermal radiation. Because there is very little material at this height to absorb this solar energy, the result is a high temperature. The lowest layer of the thermosphere (also called the ionosphere, 80 kilometers to 400 kilometers high) reflects radio waves because of ionization, and it is here that meteors burn up. Farther out, the extremely tenuous outermost part of the thermosphere (or exosphere) fades away until it is indistinguishable from the interplanetary material. (See Figure 1.)




Figure 1

Temperature structure of Earth's atmosphere.


Global atmospheric conditions, such as weather, are also affected by the Coriolis Effect. Earth is a solid globe, and hence every latitude moves once around the rotational axis in the same period of time. The distance of travel around the rotational axis, however, depends on the latitude, with equatorial distance being the largest. The eastward velocity of Earth's surface, therefore, is greatest at the equator and least at the poles. Air masses moving north or south, however, share the eastward motion of that part of Earth's surface from where the air began moving and hence drift eastward or westward relative to Earth's surface. Air moving from all directions into a region of low pressure shows, relative to the surface, a counterclockwise motion in the Northern Hemisphere (hurricanes are the extreme examples), but clockwise in the Southern Hemisphere. Air moving in all directions outward from high‐pressure regions circulates clockwise in the Northern Hemisphere.

The atmosphere further produces a thermal moderation of temperature over the whole Earth (resulting in less extreme temperatures both geographically and seasonally), shields the surface from life‐destroying ultraviolet, and is the source of necessary gases for life.

The magnetosphere

The magnetosphere is that region surrounding Earth that is influenced by the planet's magnetic field (see Figure 2). Within it are two doughnut‐shaped regions, the Van Allen belts, high above the equator, in which are trapped charged particles from the solar wind. These particles drift north and south along the magnetic field lines, enter the atmosphere near the magnetic poles, and produce the aurora, or the luminous irregular or streamer‐like phenomena visible at night in a zone surrounding the magnetic poles. The solar wind streaming by the magnetosphere distorts it into a long tail pointing opposite the Sun. Other planets with magnetic fields also have similar magnetospheres.




Figure 2

Earth's magnetosphere.

The greenhouse effect

Earth is warmer than expected from the global equilibrium between thermal radiation loss and absorption of sunlight because its atmosphere traps part of the thermal radiation that is trying to escape. This re‐absorption of radiated thermal energy, or greenhouse effect, forces an increase in temperature in order to achieve a balance between solar heating and radiative energy loss (see Figure 3). Atmospheric gases that contribute to energy absorption are carbon dioxide (CO 2), water (H 2O), methane (CH 4), and chlorofluorohydrocarbons (freons). Consequently, Earth's surface temperature is closer to 300 K (80°F) than the expected 250 K. Venus, with its extremely dense CO 2 atmosphere, is the planet most dramatically affected by a greenhouse effect.



Figure 3
The greenhouse effect.