Atmosphere of Earth

"Atmosphere" redirects here. For the unit of measure, see Atmosphere (unit).

The Earth's atmosphere is the layer of gasses around the Earth. It is held in place by Earth's gravity. It is today made up mainly of nitrogen (78.1%). It also has plentiful oxygen (20.9%) and 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.

Solid particulates, including ash, dust, volcanic ash, etc. are small parts of atmosphere. They are important in making clouds and fog.

The atmosphere does not end at a specific place. The higher above the Earth, the thinner the atmosphere. There is no clear border between the atmosphere and outer space, though the Kármán line is sometimes treated as a border. Even higher, for some purposes the edge of the magnetosphere is treated as a border. 75% of the atmosphere is within 11 kilometres (6.8 miles) of the Earth's surface.

History of Earth's atmosphere

An important thing to remember about the Earth and other planets is that they do not come from the Sun. They or their materials were picked up by the Sun's gravitation as it moved through space. The Sun is just composed of hydrogen, with a little bit of helium. The material which makes up rocky planets and their satellites is almost entirely heavier elements whose origin was in earlier supernovae explosions. The planets do give off small quantities of hydrogen and helium: this comes from the decay of larger radioactive molecules whose origin is also ancient supernovae.

Originally, the Earth's atmosphere had almost no free oxygen.

1. The first atmosphere consisted of gases in the solar nebula, mainly hydrogen. There might also have been simple hydrides such as those now found in the gas giants (Jupiter and Saturn): water vapor, methane and ammonia.
2. The atmosphere gradually changed to mostly carbon dioxide and nitrogen. The lighter gases, like hydrogen and helium, cannot be held by the Earth's gravity, and would escape. For a long time (say two billion years or more), the atmosphere was dominated by carbon dioxide.
3. In the Great Oxygenation Event the atmosphere changed to the kind we have now, with oxygen replacing the carbon dioxide. Our atmosphere is still mostly nitrogen, but most living organisms interact more with oxygen than with nitrogen. Oxygenation began with cyanobacteria making free oxygen by photosynthesis. Most organisms today need oxygen for their respiration: only a few anaerobic organisms can grow without oxygen.

Temperature and the atmospheric layers

Temperature at various altitudes

Some parts of the atmosphere are hot or cold, depending on height. Starting from the surface and climbing straight up, the air gets colder in the troposphere, but then it becomes hotter, higher in the stratosphere. These changes of temperature are divided into layers. These are like layers of an onion. The difference between the layers is the way the temperature changes.

These are the layers of the atmosphere, starting from the ground:

• Troposphere - Starts at the ground. Ends somewhere between 0 to 18 kilometres (0 to 11 miles). The higher, the colder. Weather in this layer affects our daily life.
• Stratosphere - Starts at 18 kilometres (11 miles). Ends at 50 kilometres (31 miles). The higher, the hotter. The heat comes from the Ozone layer at the top of the stratosphere. 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 some radio waves.
• Exosphere - Above the thermosphere. This is the top layer, and merges into interplanetary space.

Regions 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 boundaries.

The average temperature of the atmosphere at the surface of Earth is 14 °C (57 °F).

Pressure

The atmosphere has pressure. This is because even though air is a gas, it has weight. The average atmospheric pressure at sea level is about 101.4 kilopascals (14.71 psi).

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.

Atmosphere Of Earth Media

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  View of the crescent moon through the top of the Earth's atmosphere. Photographed above 21.5°N, 113.3°E by International Space Station crew Expedition 13 over the South China Sea, just south of Maca

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  Composition of Earth's atmosphere by molecular count, excluding water vapor. Lower pie represents trace gases that together compose about 0.0434% of the atmosphere (0.0442% at August 2021 concentrations). Numbers are mainly from 2000, with Template:CO2 and methane from 2019, and do not represent any single source.

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  Earth's atmosphere. Lower four layers of the atmosphere in three dimensions as seen diagonally from above the exobase. Layers drawn to scale, objects within the layers are not to scale. Aurorae shown at the bottom of the thermosphere can form at any altitude within this layer.

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  Afterglow of the troposphere (orange), the stratosphere (blue) and the mesosphere (dark) at which atmospheric entry begins, leaving smoke trails, such as in this case of a spacecraft reentry

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  A picture of Earth's troposphere, with different cloud types at low and high altitudes casting shadows. Sunlight, filtered into a reddish hue by passing through much of the troposphere at sunset, is reflected off the ocean. The above-lying stratosphere can be seen at the horizon as a band of its characteristic glow of blue scattered sunlight.

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  Temperature trends in two thick layers of the atmosphere as measured between January 1979 and December 2005 by microwave sounding units and advanced microwave sounding units on NOAA weather satellites. The instruments record microwaves emitted from oxygen molecules in the atmosphere. Source:

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  Temperature and mass density against altitude from the NRLMSISE-00 standard atmosphere model (the eight dotted lines in each "decade" are at the eight cubes 8, 27, 64, ..., 729)

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  Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light

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  Distortive effect of atmospheric refraction upon the shape of the sun at the horizon

• Animation shows the buildup of tropospheric Template:CO2 in the Northern Hemisphere with a maximum around May. The maximum in the vegetation cycle follows in the late summer. Following the peak in vegetation, the drawdown of atmospheric Template:CO2 due to photosynthesis is apparent, particularly over the boreal forests.

Related pages

• Air
• Life timeline

Other websites

• The Atmosphere