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Atmosphere of Earth
Earth's atmosphere is the layer of gases around the Earth. The atmosphere is held in place by Earth's gravity. It is made up of nitrogen (78.1%) and oxygen (20.9%), with small amounts of argon (0.9%), carbon dioxide (~ 0.035%), water vapor, and other gases. The atmosphere protects life on Earth by absorbing (taking) ultraviolet rays from the sun. It makes our days cooler and our nights warmer.
The atmosphere does not end at a specific place. The higher above the Earth something is, the thinner the atmosphere around it is. There is no clear line between the atmosphere and outer space. 75% of the atmosphere is within 11 kilometres (6.8 miles) of the Earth's surface.
History of Earth's atmosphere
Originally, the Earth's atmosphere had almost no free oxygen. It gradually changed to what it is today, over a very long time (see Great Oxygenation Event). The process began with cyanobacteria. They were the first organisms to make free oxygen by photosynthesis. Most organisms today need oxygen for their metabolism; only a few can use other sources for respiration.
Temperature and the atmospheric layers
Some parts of the atmosphere are hot or cold, depending on height. If something climbed straight up, it would get colder, but then it would get hotter as the object climbed higher. These changes of temperature are divided into layers. These are like layers of an onion. However, no difference can be seen between each layer. They are just places where the temperature changes differently.
These are the layers of the atmosphere, starting from the ground:
- Troposphere - Starts at the ground. Ends somewhere between 7 to 14 kilometres (4.3 to 8.7 miles). The higher, the colder. Weather in this layer affects our daily life.
- Stratosphere - Starts at 7 to 14 kilometres (4.3 to 8.7 miles). Ends at 50 kilometres (31 miles). The higher, the hotter. There is little water vapor and other substances in this layer. Airplanes fly in this layer because it is usually stable and air resistance is small.
- Mesosphere - Starts at 50 kilometres (31 miles). Ends at 80 or 85 kilometres (50 or 53 miles). The higher, the colder. Winds in this layer are strong, so the temperature is not stable.
- Thermosphere - Starts at 80 or 85 kilometres (50 or 53 miles). Ends at 640 kilometres (400 miles) or higher. The higher, the hotter. This layer is very important in radio communication because it helps to reflect AM radio waves.
Where one layer changes to the next have been named "-pauses." So the tropopause is where the troposphere ends (7 to 14 kilometres (4.3 to 8.7 miles) high). The stratopause is at the end of the stratosphere. The mesopause is at the end of the mesosphere. These are called boundaries.
The average temperature of the atmosphere at the surface of earth is 14 °C (57 °F).
Density and mass
The density of air at sea level is about 1.2 kilograms per cubic meter. This density becomes less at higher altitudes at the same rate that pressure becomes less. The total mass of the atmosphere is about 5.1 × 1018 kg, which is only a very small part of the Earth's total mass.
- Heinrich D. Holland: The oxygenation of the atmosphere and oceans. In: Phil. Trans. R. Soc. B, vol. 361, 2006, p. 903–915 http://rstb.royalsocietypublishing.org/content/361/1470/903.full.pdf
- Knoll, Andrew H. 2004. Life on a young planet: the first three billion years of evolution on Earth. Princeton, N.J. ISBN 0-691-12029-3