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Last edited 23 Aug 2022
The atmosphere is the gas and aerosol envelope of the Earth ( as well as other planets) that extends from the ocean, land, and ice-covered surface outward into space. The density of the atmosphere decreases with distance from the surface due to the gravitational attraction of the planet that pulls gases and aerosols (microscopic suspended particles of dust, soot, smoke, or other chemicals) inward. It is often described as one of the four elements that describe the Earth, with the other three other being:
The troposphere is the lowest layer of the Earth's atmosphere, it contains the majority of mass of the atmosphere around 75-80%. The air in the Troposphere is made up of 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, trace gases, and varying amounts of water vapour.
Temperature and atmospheric pressure decrease with altitude, whilst humidity decreases with altitude because water vapour condenses at lower temperatures. If the air is at the saturation vapour pressure, then the rate temperature decreases with altitude is called the saturated adiabatic lapse rate, whilst the actual rate temperature decreases with altitude is the environmental lapse rate.
Hence this is the layer where most weather activities occurs and where clouds are formed. The term used to describe this shifting movement of air, moisture, pressure and temperature globally is called Atmospheric Circulation. Thermal energy gets distributed and re-distributed across the earths surface latitudinally by three features called the Hadley, Ferrel and Polar cells and Longitudinally by features known as the walker cell and El Niño. Similar effects of temperture re-distribution also occur in the global oceans often called ocean circulation.
The edge of the Troposphere, is called the tropopause and can be located because temperature reduction with altitude slows at a certain band, becomes constant and starts to invert in the Stratosphere. This is because in the Stratosphere the Ozone layer acts to absorb and retain ultraviolet (UV) radiation from the Sun, thus creating a different relationship between temperature and altitude.
The stratosphere is so called because it is made up of stratified temperature layers, which increase with altitude, this makes this layer very stable, unlike the troposphere with only some disturbances through significant changes in the troposphere such as volcanoes.
Its most prominent feature is the Ozone layer which sits approximately 10-25 miles above the Earth's surface, in the stratosphere. It is made up of Ozone which is a molecule in turn made up of three oxygen atoms, referred to as O3. It is formed when heat and sunlight cause chemical reactions between oxides of nitrogen (NOX) and Volatile Organic Compounds (VOC's), also known as Hydrocarbons.
The Ozone layer absorbs a portion of the sun's radiation, preventing it from reaching the earth's surface. In particular it absorbs a portion of UV light called UVB which can be harmful. Some compounds release chlorine or bromine when they are exposed to intense UV light, called ozone-depleting substances (ODS) this is because when chlorine and bromine atoms come into contact with ozone they destroy ozone molecules. ODS substances such as chlorofluorocarbons (CFC's), hydro-chlorofluorocarbons (HCFC's), carbon tetrachloride, and methyl chloroform have been banned because although they take up two five years to reach the ozone once they do a single chlorine atom can destroy over 100,000 ozone molecules, creating what is know as the hole in the ozone layer.
The top edge of the stratosphere is called the stratopause (previously named the mesopeak) and is the boundary with the mesosphere. Here temperatures in the stratosphere reach their maximum of around -2.5 °C, it is located about 30 miles above sea level.
The mesosphere is directly above the stratosphere and extends from about 31 to 53 miles above the surface of the earth. Temperatures decreases with height throughout the mesosphere reaching -90°C at is boundary with the Thermosphere, which is called the Mesopause.
The Thermosphere is directly above the stratosphere and extends from about 311 to 621 miles above the surface of the earth. Its main feature is the dramatic temperature differentials, climbing to between 500°C and 2,000°C at the upper edges and with differences of up to 500°C depending on day, night and the suns activity. Air density is very low in the thermosphere and it is considered by many to be outer space, its boundary with the exosphere is called the thermopause.
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