Online Astronomy eText: The Planets
The Structure of the Earth's Atmosphere

Definitions of the Terms in the Diagram Below
     Similar definitions are given in the Glossary, and will replace this list, when it is replaced by a detailed discussion of the various regions in the Earth's atmosphere.

Troposphere -- "region of mixing" -- nearest the surface, 5 to 10 miles high depending upon location/weather (higher near equator, and in summer, lower near poles, and in winter). Temperature decreases with altitude (avg gradient 6C/km, or 18F/mi). When temperature decreases more rapidly than usual (e.g., cold front comes in over a warm front), becomes unstable against vertical mixing (convection), so relatively well mixed. Heat source = absorption of light by the surface of the Earth, vertical mixing of gases.

Tropopause = boundary between troposphere (below) and stratosphere (above). By definition, temperature constant with height, because neither decreasing, as in troposphere, nor increasing, as in stratosphere.

Stratosphere -- next highest layer, about 20 miles thick depending upon location/weather. Temperature increases with altitude. Because an inversion layer, vertical mixing impossible, hence various layers are not mixed (= stratified, hence the name of the layer). Heat source = absorption of UVB (240-320 nm radiation) by the ozone layer.

Stratopause = boundary between stratosphere (below) and mesosphere (above).

Troposphere and Stratosphere = "lower atmosphere".

Ozone layer -- region where the abundance of ozone is relatively high, produces a substantial increase in temperature, compared to other planetary atmospheres. Extends from the middle of the stratosphere to the lower mesosphere, depending upon how the amount of ozone is measured (total abundance, or percentage -- see discussion near bottom of page).

Mesosphere -- "middle atmosphere", ends about 50 miles above surface, depending upon solar activity. Temperature decreases with height.

Mesopause = boundary between mesosphere (below) and thermosphere (above).

Thermosphere -- "heat sphere", goes out into space (about 50 to 300 miles from surface). Temperature increases rapidly to around 1200 to 1500 Kelvins (approx. 1700 to 2200 Fahrenheit degrees), and can be even higher if solar activity is higher than usual. Heat source = absorption of high energy photons (extreme UV and X-rays), and collisions with high energy particles in the Van Allen radiation belts (these particle collisions are responsible for the aurorae).

Exosphere -- literally, "outer atmosphere". Originally, everything above the mesosphere, but now, sometimes only the outermost part of the thermosphere, where the average distance moved by particles between collisions is greater than the scale height, so that uncharged particles which are moving upwards at more than escape velocity have a substantial chance of escaping into space. Extends from about 300 to 600 miles above surface.

Ionosphere = "ionized region". Region encompassing the outer and middle atmosphere, in which collisions with high energy particles or absorption of high-energy photons ionize atoms and molecules at a substantial rate. In the upper ionosphere, more than half the particles are ionized at any given time, but in the lower reaches of the ionosphere, recombination of ionized particles is so rapid that the percentage of ionized particles is relatively low. During the day, solar UV ionizes particles at the base of the ionosphere more rapidly than they can recombine, and the base of the ionosphere moves downwards, but at night, ions recombine without being reionized, and the base of the ionosphere moves upwards, and some ionospheric "layers" may completely disappear.

The structure of the atmosphere.

     Most of the layers in our atmosphere are defined by temperature changes (scale at bottom). Height (in km) and pressure (in millibars) are shown on the left and right. Within each layer, temperature either goes up (in the stratosphere and thermosphere), or down (in the troposphere and mesosphere). Boundaries ("pauses") between layers are defined by where temperature stays about the same with height. (This diagram, based on an apparently extinct Web diagram, only shows the sixty miles closest to the surface of the Earth. It will be replaced by one which shows the upper atmosphere as well, later this summer.)

      The heat source at the base of the atmosphere is the absorption of sunlight by the surface of the Earth, and the transfer of that heat to the lower atmosphere by conduction. Heat is then transported upwards by convection (especially during thunderstorms), which requires a decreasing temperature gradient, with height. This heat source exists for all planets, and so, in the lower atmosphere of all planets, temperature is relatively high at the surface, and decreases upwards.
      The heat source at the top of the atmosphere is primarily due to the absorption of extremely high-energy photons (extreme UV and X-radiation) from the Sun; and, to a lesser extent, varying considerably with latitude, collisions with high-energy particles trapped in the Earth's magnetic field, and energized by compression of the magnetosphere by the solar wind. Some of these heat sources are more efficient at higher altitudes; but there is also a considerable movement of heat from the lower thermosphere to the upper thermosphere as a result of differing diffusion rates for atoms and molecules of different masses and kinetic energies. Again, although there are differences in the details, this is true for all planetary atmospheres.
      The heat source in the lower mesosphere and upper stratosphere is the absorption of ultraviolet radiation. For most gases, only wavelengths shorter than 240nm (2400 Angstroms), or UVC, can be absorbed -- in the process, breaking down the molecules (photodissociation). There isn't much of this radiation in sunlight, which peaks in the visible, and rapidly decreases as you go into the ultraviolet, so the heating is only a few tens of degrees at best, and if this is the only heat source, the middle atmosphere is relatively cold (the so-called "cold trap" in the atmospheres of most planets). However, for the Earth, the high abundance of oxygen allows the formation of ozone, which readily absorbs wavelengths as long as 320nm (3200 Angstroms), or UVB. There is far more of this radiation, since it is closer to the visible, so the heating of the atmosphere is much greater than in the atmospheres of the other planets, causing the high temperatures in the lower mesosphere and upper stratosphere. (There is a similar effect in the atmosphere of Titan, caused by methane, but Titan is not a planet, so the Earth is still unique among the planets, in this respect.)
      Ozone: The percentage of ozone peaks near the top of the stratosphere, and that, in combination with the greater amount of UV at height (less reaches lower altitudes), produces the greatest heating of the middle atmosphere at the stratopause. However, since the amount of air increases rapidly as you go downwards, even though the percentage of ozone is less at lower altitudes, the peak abundance, in terms of actual numbers of molecules, is greatest in the middle of the stratosphere. As a result, a graph showing the ozone concentration in the atmosphere will peak at the stratopause, if it refers to heating by ozone, or ozone concentration; whereas a graph showing the actual amount of ozone will peak in the middle stratosphere, because the actual amount of ozone must decrease at higher altitudes, if only because there is less and less air of all sorts at higher altitudes.