The atmosphere's layers

The atmosphere is held to the Earth by gravity. It is mostly nitrogen (78%) and oxygen (21%), with small amounts of argon, carbon dioxide and water vapour.

It has layers. The two that matter most for us:

The troposphere is tallest over the equator and lowest over the poles. Above the stratosphere are the mesosphere, thermosphere and exosphere. Outer space begins at about 100 km.

The atmosphere protects life in two ways:

• it absorbs part of the incoming radiation: ozone blocks harmful UV, and clouds absorb some sunlight;
• it traps outgoing heat: greenhouse gases hold back some of the infrared the ground gives off.

Without an atmosphere, the Earth would be too cold for life. But too much carbon dioxide traps too much heat: global warming.

Venus is hotter than Mercury, even though Mercury is closer to the Sun. Venus has a thick atmosphere with a runaway greenhouse effect.

An Indian scientist on the mountain. In 1934, K. R. Ramanathan climbed to 18,000 feet in the Himalayas to measure ozone, and found less than expected. His work helped explain how UV absorption changes with height and pollution. He later led early monsoon forecasting.

The ozone hole. In the late 20th century, man-made chemicals called CFCs, used in old refrigerators and spray cans, destroyed ozone faster than it formed, making an "ozone hole" over Antarctica. Countries signed the Montreal Protocol to cut CFCs, and the ozone layer is now slowly recovering: proof that the world can solve problems by working together.