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. In the United States, persons who travel above an altitude of 50 miles (80 kilometers) are designated as astronauts. 400,000 feet (75 miles or 120 kilometers) marks the boundary where atmospheric effects become noticeable during re-entry. The altitude of 100 kilometers or 62 miles is also frequently used as the boundary between atmosphere and space.
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2 Pressure 3 Density and mass 4 Various atmospheric regions 5 The "Evolution" of the Earth's atmosphere 6 Global warming |
Temperature and the atmospheric layers
The temperature of the Earth's atmosphere varies with altitude; the relationship between temperature and altitude varies between the different atmospheric layers:
The average temperature of the atmosphere at the surface of earth is 14 °C.
Pressure
Gravity "pulls" the atmosphere towards the Earth's surface. 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% for every 5.5 km altitude increase. The average atmospheric pressure, at sea level, is about 101.3 kilopascals (about 14.7 pounds per square inch).
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 atmopshere.
About 3.5 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes releasing 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. 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 couple billion years, water vapor condensed to form rain and oceans, which started to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. Photosynthesizing plants would evolve and convert 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 oxygen atoms) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere".
The IPCC concluded, in their Climate Change 2001 Report, that "most of the observed warming, over the last 50 years, is likely to have been due to the increase in greenhouse gas concentrations".\n
Density and mass
The density of air at sea level is about 1.2 kilograms per cubic meter. This density decreases at higher altitudes at approximately the same rate that pressure decreases (but not quite as fast). The total mass of the atmosphere is about 5.1 × 1018 kg, a tiny fraction of the earth's total mass.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 is only poorly constrained prior 1 billion years ago, but the following presents a plausible sequence of events. This remains an active area of research.Global warming
In the past hundred years, the average temperature of the troposphere has increased by about one degree Celsius. While there have historically been comparable variations in the global temperature, this increase is notable for how quickly it has occurred. It is possible that this temperature change is related to an increase in atmospheric carbon dioxide, although half the warming was before 80% of burning of fossil fuels took place. Water vapor and carbon dioxide are greenhouse gases; this means that they trap heat in the atmosphere instead of allowing it to escape into space in the form of infrared radiation. Significant warming of the atmosphere is a concern because it could contribute to extreme weather and rising sea levels from melting polar ice caps.