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.
The most 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. Nothing else. The material which makes up 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.
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:
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).
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).
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.
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