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atmosphere Definitions. APES 2013-2014. Troposphere.

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  • Where weather forms. Storms take place in the troposphere, which contains about 75% of the atmosphere (by mass), as well as the majority of the water vapor and dust particles found in the atmosphere. The water cycle (including clouds) occurs in the troposphere.

As altitude increases, temperature decreases—an exception occurs during temperature inversions, in which case temperature increases with altitude. An inversion can lead to pollution such as smog being trapped close to the ground, with possible adverse effects on health. An inversion can also suppress convection by acting as a "cap". If this cap is broken for any of several reasons, convection of any moisture present can then erupt into violent thunderstorms. Temperature inversion can notoriously result in freezing rain in cold climates.


Ozone has harmful effects in the troposphere, including chest pain, coughing, throat irritation, and congestion. It can also worsen bronchitis, emphysema, and asthma, reduce lung function, and may permanently scar lung tissue.

  • The “barrier” between the troposphere and stratosphere. The tropopause allows some movement between these two layers.
  • At this point, air ceases to cool with height, and becomes almost completely dry.
  • Most commercial aircraft are flown below the tropopause if at all possible to take advantage of the troposphere's temperature lapse rate. Jet engines are more efficient at lower temperatures.
  • Where the protective ozone layer floats. Stratospheric ozone (sometimes referred to as "good ozone") plays a beneficial role by absorbing the majority of incident UV-B and nearly all incident UV-C, allowing only a small amount to reach the Earth's surface. This ultraviolet light can cause biological damage like skin cancer, tissue damage to eyes and plant tissue damage. Note that the ozone in the stratosphere does not block out UVA.

Human-produced chemicals are responsible for the observed depletions of the ozone layer. The ozone-depleting compounds contain various combinations of the chemical elements chlorine, fluorine, bromine, carbon, and hydrogen and are often described by the general term halocarbons.


The compounds that contain only chlorine, fluorine, and carbon are called chlorofluorocarbons, usually abbreviated as CFCs. CFCs, carbon tetrachloride, and methyl chloroform are important human-produced ozone-depleting gases that have been used in many applications including refrigeration, air conditioning, foam blowing, cleaning of electronics components, and as solvents.


The stratosphere’s temperature increases as altitude increases. This is due to heat created from the absorption of ultraviolet radiation by ozone in this layer.

  • The stratosphere is “stratified” or separated into poorly-mixed layers (as opposed to the well-mixed troposphere, which is under the influence of convection currents).
  • The boundary between the stratosphere and mesosphere. At this point, air begins to cool with height again.
  • Space debris begins to burn as it enters the mesosphere. The mesosphere’s temperature decreases as altitude increases.
  • The mesosphere is odd
    • Noctilucent clouds can form in the mesosphere, far above the region where clouds normally form, the troposphere.
    • Site of odd types of lighting called “sprites” and “ELVES”
    • The mesosphere is hard to study. Weather balloons and jet planes cannot fly high enough to reach the mesosphere. The orbits of satellites are above the mesosphere. We don't have many ways to get scientific instruments to the mesosphere to take measurements there. We do get some measurements using sounding rockets (these rockets make short flights that don't go into orbit).
  • The boundary between the mesosphere and thermosphere. At this point, air begins to warm with height again.
  • The thermosphere’s temperature increases as altitude increases. The thermosphere reaches high temperatures (over 200C) due to absorption of solar radiation (largely x-ray and ultraviolet). This radiation (particularly UV) also causes ionization in the thermosphere. Solar activity strongly influences temperature in the thermosphere.

Contains two layers: the ionosphere (charged particles that can interfere with radio broadcasts) and the magnetosphere (charged particles that can interact with the Earth’s magnetic field to produce Aurora Borealis and Aurora Australis).

    • Charged particles (electrons, protons, and other ions) from space collide with atoms and molecules in the thermosphere at high latitudes, exciting them into higher energy states. Those atoms and molecules shed this excess energy by emitting photons of light, which we see as colorful auroral displays.

Although the thermosphere is considered part of Earth's atmosphere, the air density is so low in this layer that most of the thermosphere is what we normally think of as outer space. In fact, the most common definition says that space begins at an altitude of 100 km, slightly above the mesopause at the bottom of the thermosphere.

thermopause not shown in diagram
Thermopause (not shown in diagram)
  • The boundary between the thermosphere and exosphere. The exact altitude of the thermosphere varies by the energy inputs of location, time of day, solar flux, season, etc. and can be between 500–1000 km high at a given place and time because of these.
exosphere not shown in diagram
Exosphere (not shown in diagram)
  • The atmosphere dwindles into nothing as molecules drift into space. As explained in the definition for the thermosphere, outer space actually “starts” in the thermosphere.