Earth's atmosphere is the layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains about four-fifths nitrogen and one-fifth oxygen, with trace amounts of other gases. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.
As the atmosphere has no abrupt cut-off, but rather thins gradually with increasing altitude, there is no definite boundary between the atmosphere and outer space. Three-fourths of the atmosphere is within 11 km of the planetary surface. In the United States, persons who travel above an altitude of 50.0 miles (80.5 km) are designated as astronauts. An altitude of 120 km (75 mi or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The Karman line, at 100 km (62 mi), is also frequently used as the boundary between atmosphere and space.
Temperature and the atmospheric layers
The temperature of the Earth's atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies between the different atmospheric layers:
troposphere: from the surface to 7 km to 17 km, depending on latitude and weather factors, temperature decreasing with height.
stratosphere: from that 7–17 km range to about 50 km, temperature increasing with height.
mesosphere: from about 50 km to the range of 80 km to 85 km, temperature decreasing with height.
thermosphere: from 80–85 km to 640+ km, temperature increasing with height.
The boundaries between these regions are named the tropopause, stratopause and mesopause.
The average temperature of the atmosphere at the surface of earth is 14 °C.
- Main article: Atmospheric pressure
Atmospheric pressure is a direct result of the weight of the air. This means that air pressure varies with location and time because the amount (and weight) of air above the earth varies with location and time. Atmospheric pressure drops by ~50% at an altitude of about 5 km (equivalently, about 50% of the total atmospheric mass is within the lowest 5 km). The average atmospheric pressure, at sea level, is about 101.3 kilopascals (about 14.7 pounds per square inch).
Carbon dioxide and methane updated (to 1998) by IPCC TAR table 6.1 
Minor components of air not listed above include: Nitrous Oxide (0.5 ppmv), Hydrogen (0.5 ppmv), Xenon (0.09 ppmv), Ozone (0.0 to 0.07 ppmv, 0.0 to 0.02 ppmv in Winter), Nitrogen Dioxide (0.02 ppmv), Iodine (0.01 ppmv), Carbon Monoxide (0.0 to trace), and Ammonia (0.0 to trace).
The mean molecular mass of air is 28.97 g/mol.
Below an altitude of about 100 km, the Earth's atmosphere has a more-or-less uniform composition (apart from water vapor) as described above. However, above about 100 km, the Earth's atmosphere begins to have a composition which varies with altitude. This is essentially because, in the absence of mixing, the density of a gas falls off exponentially with increasing altitude, but at a rate which depends on the molecular weight. Thus heavy constituents, such as oxygen and nitrogen, fall off more quickly than lighter constituents such as helium, molecular hydrogen, and atomic hydrogen. Thus there is a layer, called the heterosphere, in which the earth's atmosphere has varying composition. As the altitude increases, the atmosphere is dominated successively by helium, molecular hydrogen, and atomic hydrogen. The precise altitude of the heterosphere and the layers it contains varies significantly with temperature.
Density and mass
The density of air at sea level is about 1.2 kg/m3. Natural variations of the barometric pressure occur at any one altitude as a consequence of weather. This variation is relatively small for inhabited altitudes but much more pronounced in the outer atmosphere and space due to variable solar radiation
The atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used by meteorologists and space agencies to predict weather and orbital decay of satellites.
The total mass of the atmosphere is about 5.1 × 1018 kg, or about 0.9 ppm of the Earth's total mass.
The above composition percentages are done by volume. Assuming that the gases act like ideal gases, we can add the percentages p multiplied by their molar masses m, to get a total t = sum (p·m). Any element's percent by mass is then p·m/t. When we do this to the above percentages, we get that, by mass, the composition of the atmosphere is 75.523% N2, 23.133% O2, 1.288% Ar, 0.053% CO2, 0.001267% Ne, 0.00029% CH4, 0.00033% Kr, 0.000724% He, and 0.0000038 % H2.
This graph is from the NRLMSISE-00 atmosphere model, which has as inputs: latitude, longitude, date, time of day, altitude, solar flux, and the earth's magnetic field daily index.
Various atmospheric regions
Atmospheric regions are also named in other ways:
The evolution of the Earth's atmosphere
The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but the following presents a plausible sequence of events. This remains an active area of research.
The modern atmosphere is sometimes referred to as its "third atmosphere", in order to distinguish the current chemical composition from two notably different compositions. The original atmosphere was primarily helium and hydrogen; heat (from the still molten crust, and the sun) dissipated this atmosphere.
About 3.5 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the "second atmosphere"; which was, primarily, carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. Simulations run at the University of Waterloo and University of Colorado in 2005 suggested that it may have had up to 40% hydrogen. This second atmosphere had ~100 times as much gas as the current atmosphere. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from freezing.
During the next few billion years, water vapor condensed to form rain and oceans, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. One of the earliest types of bacteria are the cyanobacteria. Fossil evidence indicates that these bacteria existed approximately 3.3 billion years ago and were the first oxygen producing evolving phototropic organisms. They are responsible for the initial conversion of the earth’s atmosphere from an anoxic (state without oxygen) to an oxic (with oxygen) state. Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen playing a major role in oxygenating the atmosphere.
Photosynthesizing plants would evolve and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, bacteria would also convert ammonia into nitrogen.
As more plants appeared, the levels of oxygen increased significantly (while carbon dioxide levels dropped). At first it combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere — resulting in mass extinctions and further evolution. With the appearance of an ozone layer (a compound of three oxygen atoms) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere".
Last updated: 06-02-2005 00:12:41