SCISAT-1

Fact Sheet 3

Atmospheric Temperature Profile

Troposphere

This is the lowest layer of the atmosphere and varies in height in different parts of the world from roughly 8 km above sea level at the poles, to 16 km at the equator. Within the troposphere the pressure, density and temperature all decrease rapidly with height. Most of the "weather" occurs in the troposphere because of the presence of water vapour and strong vertical currents produced by the radiation of the sun's rays from the Earth's surface. In the upper regions of the troposphere, very strong, fast-moving and complex winds occur called jet streams. The top layer of the troposphere is known as the tropopause. Within the troposphere the temperature drops to a low of -56ºC which marks the beginning of the tropopause. Through the tropopause, the temperature reverses and begins to increase. The height of the tropopause varies, as already stated, from the poles to the equator, but also from the summer to winter.

Stratosphere

For a distance of about 18 km above the tropopause, there is a layer known as the stratosphere in which the pressure continues to decrease but in which the temperature continues to increase gradually to 0ºC. This layer also varies in thickness, being quite deep over the poles and thinner over the equator. Water vapour is almost non-existent and air currents are minimal. A layer of ozone is present in the stratosphere which absorbs the sun's ultraviolet rays and, hence, creates the rise in temperature. The top layer of the stratosphere is called the stratopause wherein the temperature, once again, beings to fall.

Mesosphere

The mesosphere is characterized by a marked decrease in temperature that is carried through from the stratopause. It is in the mesosphere that meteorites typically burn up as they enter the atmosphere. In the top layer of the mesosphere, called the mesopause, the temperature bottoms out at a low of about -100ºC at 80 km above the Earth, then begins to rise again with greater altitude.

Thermosphere

Temperature continues to rise in the thermosphere and beyond, increasing for an indefinite distance into space. Air molecules are few in this layer which extends upwards from a point 80-100 kilometres above the earth's surface.

Exosphere

Beyond the thermosphere lies a layer where pressure drops to little more than a vacuum. Under these conditions the concept of temperature has little meaning and is usually replaced by definitions of the energy states of individual molecules. The spectacular auroras form in the regions of the exosphere, the bottom of which is found at 500 kilometres above the Earth's surface.