First 463 words of the document:
Structure of the atmosphere
The atmosphere is made up of four
layers, the Troposphere, the
Stratosphere, the Mesosphere and
Troposphere: The troposphere is the
lowest layer of the Earth's
atmosphere and is also the layer in
which clouds form. The average depth
of the troposphere is around 8km
deep at the poles and 17km deep
within the tropics. There is a decrease
in temperature as you ascend within
the troposphere at approximately
6.5C per km. At the top of this
boundary there is the Tropopause.
This is the layer between the
troposphere and stratosphere that
acts as a temperature inversion which
forms a 'ceiling' for the Earth's weather system containing it at this level. Within the
troposphere vertical convection currents disturb the atmosphere and the air masses flow
horizontally from one latitude to another.
Stratosphere: The stratosphere extends to approximately 50km above the Earth's surface.
Within this layer there is an increase in temperature as height increases. The stratosphere is
free of clouds and dust, and here is where ozone absorbs and filters out ultraviolet radiation.
This warming is greater over the Polar Regions and the temperature differences between
the tropics and the Polar Regions cause strong horizontal air movements at great heights.
The transition boundary which separates the stratosphere from the mesosphere is called the
Mesosphere: The mesosphere extends from the Stratopause to about 85 km above the
earth. The gases continue to become thinner and thinner with height. As such, the effect of
the warming by ultraviolet radiation also becomes less, leading to a decrease in temperature
with height. On average, temperature decreases is from about -15°C to as low as -120°C at
the Mesopause. The gases in the mesosphere are still thick enough to slow down
meteorites hurtling into the atmosphere, where they burn up, leaving fiery trails in the night
sky. The transition zone between the mesosphere and the thermosphere is called the
Thermosphere: The thermosphere extends from the Mesopause to 690 km above the earth.
The gases of the thermosphere are thinner than in the mesosphere. This means incoming
high energy ultraviolet and x-ray radiation from the sun, absorbed by the molecules in this
layer, causes a large temperature increase. Because of this absorption, the temperature
increases with height and can reach as high as 2,000°C near the top of this layer; however,
despite the high temperature, this layer of the atmosphere would still feel very cold to our
skin because of the extremely thin air.