Layers of the atmosphere

The atmosphere has layers of different density and temperature. Density is highest at the surface and falls with height. By temperature, there are five layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere.

Troposphere (the lowest layer):

• Average height about 13 km: roughly 8 km near the poles and 18 km at the equator.
• It holds the dust and water vapour, and all weather and climate changes happen here; it is the most important layer for life.
• Temperature falls with height, by about 1°C for every 165 m (the book's figure).
• The boundary above it is the tropopause, about -80°C over the equator and -45°C over the poles, where temperature stays nearly constant.

Stratosphere: above the tropopause, up to 50 km. It contains the ozone layer, which absorbs ultra-violet radiation and shields life on earth. Because ozone absorbs this energy, temperature here no longer falls with height.

Mesosphere: up to 80 km. Temperature falls with height again, reaching about -100°C at 80 km. Its upper limit is the mesopause.

Ionosphere (thermosphere): from 80 to 400 km. It contains electrically charged particles (ions) and reflects radio waves sent from the earth back down. Temperature rises with height here.

Exosphere: the highest layer, about which little is known. Its contents are extremely rarefied, and it merges gradually with outer space.

Geographers are mainly concerned with the first two layers.

Example (new): a passenger jet usually cruises at about 10-12 km. Over Delhi in winter that is near the top of the troposphere, above most clouds and bumpy weather, which is why flights are smoother once they reach cruising height.