About the Composition of the Atmosphere

Exosphere (1) The exosphere merges with the interplanetary medium and although there is no definite boundary with the ionosphere it starts at a height of about 600 kms. The rarified air mainly consists of a small amount of atomic oxygen up to 600 kms and equal proportions of hydrogen and helium with hydrogen predominating above 2 400 kms. Ionosphere (2) Air particles of the ionosphere are electrically charged by the Sun's radiation and congregate in four main layers, D, E, F1 and F2, which can reflect radio waves. Aurorae, caused by charged particles deflected by the Earth's magnetic field towards the poles, occur between 65 and 965 kms above the Earth. It is mainly in the lower ionosphere that meteors from outer space burn up as they meet increased air resistance. Stratosphere (3) A thin layer of ozone contained within the stratosphere absorbs ultra-violet light and in the process gives off heat. The temperature ranges from about -55°C at the tropopause to about -60°C in the upper part, known as the mesosphere, with a rise to about 2 °C just above the ozone layer. This portion of the atmosphere is separated from the lower layer by the tropopause. Troposphere (4) The Earth's weather conditions are limited to this layer which is relatively thin, extending upwards to about 8 kms at the poles and 15 kms at the equator. It contains about 85% of the total atmospheric mass and almost all the water vapour. Air temperature falls steadily with increased height at about 1°C for every 100 meters above sea level.

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