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Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres.

The atmosphere has a mass of about 5.15 three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface.

The concentration of water vapor (a greenhouse gas) varies significantly from around 10 ppm by volume in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air (without water vapor).

among which are the greenhouse gases, principally carbon dioxide, methane, nitrous oxide, and ozone.

The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.

The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space.

The exosphere contains most of the satellites orbiting Earth.

The thermosphere is the second-highest layer of Earth's atmosphere.

Because the general pattern of the temperature/altitude profile is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers.

In this way, Earth's atmosphere can be divided (called atmospheric stratification) into five main layers.

In general, air pressure and density decrease with altitude in the atmosphere.

However, temperature has a more complicated profile with altitude, and may remain relatively constant or even increase with altitude in some regions (see the temperature section, below).

This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase.

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