Atmosphere

The Earth and all the living things on it are protected from the Sun by a huge ‘blanket’ or gaseous envelope called the atmosphere. This blanket, which is fairly uniform throughout its lower layers, consists largely of nitrogen (78%), oxygen (21%) and small amounts of other gases (about 1%), although this is dependent on altitude. It stretches about 1,000km up and is made up of a number of different layers:

• Troposphere: this layer stretches up 20km from the Equator and 10km at the Poles and is the only layer where living things can breathe normally. It is also where the Earth’s weather
• Stratosphere: this layer contains the gas ozone that absorbs harmful ultraviolet rays from the Sun. It can stretch up to 50km above the ground but varies in thickness. Pollutants can cause thinning of the ozone layer, thus reducing its capacity to absorb harmful ultraviolet radiation. The temperatures in the stratosphere vary from about –60^oC at the bottom to just above 0^oC at the top.
• Mesosphere: the top of the Mesosphere is about 80km above the ground and has temperatures as low as –100^o
• Thermosphere: this is the hottest layer since there are very few air molecules to absorb the radiation from the Sun. Temperatures at the top, 450km from the Earth, can reach as high as 2,000^o The thermosphere includes:
  • the Ionosphere: contains many ions (atoms which have lost some electrons) and free electrons and is able to reflect radio waves;
  • the Exosphere: here, the air is very thin. Gas molecules exit from here into space giving this layer its name.