The Earth's Atmosphere

Our atmosphere is unique in the solar system. At sea level it has a pressure of about 1 x 105 pascals (Pa). The total mass is 5.3 x 1018 km which is about 1 millionth of the total mass of the planet (5.97 x 1024 kg). The atmospheric structure is stratified by gravity and characterized by the complex photochemical interactions between sunlight and the various molecular species present.

Typical composition of dry air at sea level:

Species Percent by Volume

N2 78.08

O2 20.95

Ar 0.93

CO2 0.031

Ne 0.0018

He 0.00052

Kr 0.00011

Xe 0.0000087

H2 0.00005

CH4 0.0002

NO 0.00005

O3 0.000007 (summer)

0.000002 (winter)

Note that only nitrogen, oxygen and argon are present in any appreciable amounts. Water vapor varies from nearly zero up to about 4% (100% humidity at 120F, 1 atm) and averages about 0.8%.

Composition of the atmosphere varies little from this with altitude except ozone, water vapor, and carbon dioxide. Water vapor is almost totally absent above about 8 km.

Unlike water vapor, most ozone is produced at high altitude. Therefore its concentration increases at high altitude.

Carbon dioxide concentration has increased greatly in the last two centuries. Living matter, combustion of fossil fuels and volcanic eruptions, produce it. Large volcanic eruptions can significantly change the relative composition of the upper atmosphere.

Temperature varies with altitude due to a complex balance between pressure, radiation, and photochemical processes.

It is important to remember that temperature is a measure of the average kinetic energy of the particles of the air. Thus the average free path between collisions and the nature of the collisions (elastic or reactive) directly affect the temperature as well as the energy flow.

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The atmosphere can be thought of as a series of roughly spherical regions or shells.

Troposphere. (0 km to 10 km) The lowest shell is the region of water vapor, clouds, and all of our weather. It extends from sea level up to about 18 km at the equator or about 8 km at the poles. This is a zone of turbulence and rapid mixing. Localized surface heating, topographic rises, and water phase changes, all cause significant changes in atmospheric temperature and pressure called weather. However, the most significant effects are primarily due to gravity. Because of this, the temperature, pressure, and density generally decrease with altitude.

Stratosphere. (10 km to 50 km) This is the layer above almost all water vapor where the temperature rapidly increases with altitude. Here the density is low enough that direct heating by light from the sun is much more significant than conduction or convection from below as it was for air in the troposphere. This makes it vertically stable and "stratified", hence the name. This is the zone where most of our protective ozone layer resides. This stratosphere extends up to about 50 km.

Mesosphere. (50 km to 80 km) This is the "middle sphere". Here the temperature again decrease with altitude This is the zone where most of our protective ozone layer is produced by ionizing solar and cosmic radiation. The mesosphere extends up to about 80 km.

Ionosphere. Above about 80 km, most of the particles are ionized by high-energy light from the Sun as well as cosmic rays. This vast region is usually broken thermosphere, exosphere and magnetosphere.

Thermosphere. (80 km to 700 km) The thermosphere is characterized by large temperature variations (> 300C). There are very few molecules and so heat retention is very low. This is the region of the Low Earth Orbit, where the Space Shuttle, the Hubble Telescope, and many earth observing satellites reside. By human standards, this is almost a perfect vacuum. However, the few atoms and molecules that exist here are easily ionized by sunlight and cosmic radiation. This ionization leads to the production of the hauntingly beautiful Aurora Borealis and Aurora Australis as well as the so-called D, E, F1, & F2 layers that make long distance radio communication possible by reflecting radio waves back to earth.

Exosphere. (700 km to 5000 km) The exosphere is the zone from which atoms and molecules are continuously escaping into space.

Magnetosphere. (5000 km to >> 60,000 km) The outermost shell is enormous and is strongly influenced by the interaction of Earth's magnetic field and the solar wind. It contains the Van Allen radiation belts where high energy charged particles are trapped and concentrated. This is the region occupied by Geosynchronous Satellites.