STRUCTURE OF THE ATMOSPHERE

Troposphere
Begins at the surface and extends to between 7 km (at the poles) and 20 km (at the equator)
Temperature in the troposphere decreases with altitude i.e. the lowest parts are the warmest
The troposphere contains roughly 75% of the mass of the atmosphere and 99% of its water vapour
The lowest part of the troposphere, where friction with the Earth’s surface influences air flow is called the planetary boundary layer. Usually extends from a few hundred metres to about 2 km
The tropopause is the boundary between the troposphere and the stratosphere


Stratosphere

Extends from the troposphere to about 51 km
Temperature increases with height
Restricts turbulence and mixing
Commercial airliners usually fly within the stratosphere (10 km) to optimize jet fuel burn and to avoid atmospheric turbulence
The stratopause is the boundary between the stratosphere and the mesosphere


Mesosphere

Extends from stratosphere to about 80 km
Upon entering the earth’s atmosphere, most meteors burn up in the mesosphere
Temperature decreases with height
The mesopause, the end of the mesosphere, is the coldest place on Earth with an average temperature of -100 C


Thermosphere

Biggest layer of the atmosphere
Extends from the mesosphere to about 500-1000 km
Thermopause is a temperature boundary contained within the thermosphere
Temperature increases up to the thermopause, then remains constant
The temperature can reach 1500 C. However, despite the high temperature one would not feel warm because the atmospheric density is too low to enable heat transfer
The International Space Station orbits in the thermosphere (320 – 380 km)
The ionosphere is formed in this layer as a result of ionization caused by ultraviolet radiation
The boundary between the thermosphere and the exosphere is called exobase


Exosphere

Uppermost layer of the atmosphere
It is a transitional zone between the Earth’s atmosphere and interplanetary space and does not fully fall within the atmosphere
Extends to about 190,000 km. This is half the distance to the Moon, at which the influence of solar radiation becomes greater than the Earth’s gravitational pull
The density is so low that molecules can travel hundreds of km without colliding with each other
Composed mainly of the lightest gases such as hydrogen and some helium

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OTHER LAYERS AND BOUNDARIES OF THE ATMOSPHERE

*Ozone layer
It is contained within the stratosphere at about 10 – 50 km above the Earth’s surface
About 90% of the ozone layer is present in the stratosphere
The ozone layer absorbs 93-99% of harmful ultraviolet light
Ozone is formed when UV light strikes oxygen in the stratosphere to split the oxygen atoms, which then reform as ozone
The ozone layer was discovered by the French physicists Charles Fabry and Henri Buisson in 1913
British meteorologist GMB Dobson established a worldwide network of ozone monitoring stations between 1928 and 1958 that continues to operate today. He also developed a spectrophotometer (called the Dobsonmeter) to measure stratospheric oxygen from the ground. The Dobson unit, a measure of ozone density is named in his honour


*Ionosphere
Stretches from the thermosphere to the exosphere (100 km – 700 km)
This is caused due to ionization by solar UV radiation
Responsible for radio propagation by reflecting radio waves back to the Earth’s surface thereby enabling long-distance communication
Plays an important part in atmospheric electricity (like lightning)
Responsible for auroras


*Homosphere and Heterosphere
Homosphere is the part of the atmosphere where gases are well mixed due to turbulence
This includes the troposphere, stratosphere and mesosphere
Heterosphere is the part of the atmosphere where gases are not well mixed
This usually happens above the turbopause (100 km) where distance between particles is large due to low density
This causes the atmosphere to stratify with heavier gases like oxygen and nitrogen present in the lower layers and lighter gases like hydrogen and helium in the upper layers


*Planetary boundary layer
Part of the troposphere closest to the Earth’s surface and most influenced by it
Friction with the earth’s surface causes turbulent diffusion
Ranges from 100 m to about 2 km


*Magnetosphere
A mix of free ions and electrons from solar wind and the Earth’s atmosphere
It is non-spherical and extends to more than 70,000 km
It protects the Earth from harmful solar winds
Mars is thought to have lost most of its former oceans and atmosphere to space due to the direct impact of solar winds. Similarly Venus is thought to have lost its water due to solar winds as well


*Karman line
Defines the boundary between the Earth’s atmosphere and outer space
Lies at an altitude of 100 km above mean sea level
At this altitude the atmosphere becomes too thin for aeronautical purposes
However, there is no legal demarcation between a country’s air space and outer space


*Van Allen Belt
It is a region of energetic charged particles (plasma) around the Earth held in place by the Earth’s magnetic field
Extends from about 200 km to 1000 km
Has important implications for space travel because it causes radiation damage to solar cells, integrated circuits, sensors and other electronics