Temperature and layers
The temperature of the Earth’s atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies among five different atmospheric layers (ordered highest to lowest, the ionosphere is part of the thermosphere):
· Exosphere: from 500 – 1000 km up to 10,000 km, free-moving particles that may migrate into and out of the magnetosphere or the solar wind.
exobase boundary
· Ionosphere: the part of the atmosphere that is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the thermosphere and is responsible for auroras.
thermopause boundary
· Thermosphere: from 80 – 85 km to 640+ km, temperature increasing with height. mesopause boundary
· Mesosphere: From the Greek word “μέσος” meaning middle. The mesosphere extends from about 50 km to the range of 80 to 85 km, temperature decreasing with height. This is also where most meteors burn up when entering the atmosphere.
stratopause boundary
· Stratosphere: From the Latin word “stratus” meaning a spreading out. The stratosphere extends from the troposphere’s 7 to 17 km range to about 50 km. Temperature increases with height. The stratosphere contains the ozone layer, the part of the Earth’s atmosphere which contains relatively high concentrations of ozone. “Relatively high” means a few parts per million—much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 to 35 km above Earth’s surface, though the thickness varies seasonally and geographically.
tropopause boundary
· Troposphere: From the Greek word “τρέπω” meaning to turn or change. The troposphere is the lowest layer of the atmosphere; it begins at the surface and extends to between 7 km at the poles and 17 km at the equator, with some variation due to weather factors. The troposphere has a great deal of vertical mixing because of solar heating at the surface. This heating warms air masses, which makes them less dense so they rise. When an air mass rises, it does work against gravity. This work changes some of the thermal (kinetic) energy into gravitational potential energy, so the temperature of the air mass decreases. As the temperature decreases, water vapor in the air mass may condense or solidify, releasing latent heat that further uplifts the air mass. This process determines the maximum rate of decline of temperature with height, called the adiabatic lapse rate. The troposphere contains roughly 80% of the total mass of the atmosphere. Fifty percent of the total mass of the atmosphere is located in the lower 5 km of the troposphere.
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The average temperature of the atmosphere at the surface of Earth is 15 °C (59 °F).
Heterosphere
Below the turbopause at an altitude of about 100 km (not far from the mesopause), the Earth’s atmosphere has a more-or-less uniform composition (apart from water vapor) as described above; this constitutes the homosphere. However, above about 100 km, the Earth’s atmosphere begins to have a composition which varies with altitude. This is essentially because, in the absence of mixing, the density of a gas falls off exponentially with increasing altitude but at a rate which depends on the molar mass. Thus higher mass constituents, such as oxygen and nitrogen, fall off more quickly than lighter constituents such as helium, molecular hydrogen, and atomic hydrogen. Thus there is a layer, called the heterosphere, in which the earth’s atmosphere has varying composition. As the altitude increases, the atmosphere is dominated successively by helium, molecular hydrogen, and atomic hydrogen. The precise altitude of the heterosphere and the layers it contains varies significantly with temperature.
In pre-history, the Sun’s radiation caused a loss of the hydrogen, helium and other hydrogen-containing gases from early Earth, and Earth was devoid of an atmosphere. The first atmosphere was formed by outgassing of gases trapped in the interior of the early Earth, which still goes on today in volcanoes.
Density and mass
Temperature and mass density against
altitude from the NRLMSISE-00
standard atmosphere model.
The density of air at sea level is about 1.2 kg/m³(1.2 g/L). Natural variations of the barometric pressure occur at any one altitude as a consequence of weather. This variation is relatively small for inhabited altitudes but much more pronounced in the outer atmosphere and space because of variable solar radiation.
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The atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used by meteorologists and space agencies to predict weather and orbital decay of satellites.
The average mass of the atmosphere is about 5 quadrillion metric tons or 1/1,200,000 the mass of Earth. According to the National Center for Atmospheric Research, “The total mean mass of the atmosphere is 5.1480×1018 kg with an annual range due to water vapor of 1.2 or 1.5×1015 kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27×1016 kg and the dry air mass as 5.1352 ±0.0003×1018 kg.”
Opacity
The atmosphere has “windows” of low opacity, allowing the transmission of electromagnetic radiation. The optical window runs from around 300 nanometers (ultraviolet-C) at the short end up into the range the eye can use, the visible spectrum at roughly 400-700 nm, and continues up through the visual infrared to around 1100 nm, which is thermal infrared. There are also infrared and radio windows that transmit some infrared and radio waves. The radio window runs from about one centimeter to about eleven-meter waves.
III. Answer the questions:
1. What is the Earth’s atmosphere?
2. What are the main elements of the atmosphere?
3. How does the atmosphere protect life on the Earth?
4. Is there definite boundary between the atmosphere and outer space?
5. What does temperature of the atmosphere depend on?
6. How many atmospheric layers do you know? Make a list of them.
7. What layer is responsible for auroras?
8. Where do the most meteors burn up?
9. Where is ozone layer situated?
10. What is adiabatic lapse rate?
11. Where are 80% of the total mass of the atmosphere situated?
12. What is heterosphere?
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13. How was the first atmosphere formed?
14. What do you know about density and mass of the atmosphere?
15. What is opacity?
IV. Do you agree or disagree with the following statements? Prove your opinion.
1. The Earth’s atmosphere is a layer of gases surrounding the planet Earth that is retained by the Earth’s gravity.
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