Auroras are shimmering curtains of light seen at night in the polar regions. They are known as the Northern Lights in the Arctic, and as the Southern Lights in the Antarctic. These spectacular displays are caused by charged particles from the Sun striking the upper atmosphere above the poles.

The troposphere is the lowest layer of the atmosphere and contains 75 per cent of all its gases. Above is the stratosphere, which includes the ozone layer. Higher still is the thin air of the mesosphere, where meteors burn up. The thermosphere contains an electrically charged layer that radio waves bounce off. The exosphere is the top layer, fading off into space.

Just two gases, nitrogen and oxygen, make up 99 per cent of the mixture of gases in the atmosphere. Nitrogen contributes 78 per cent and oxygen 21 per cent. The last 1 per cent is mainly argon (0.93 per cent), plus 0.03 per cent carbon dioxide and traces of other gases, including helium, neon, ozone, methane, and hydrogen.

Ozone is a form of oxygen that gathers in the stratosphere to form a layer. This layer screens out harmful ultraviolet (UV) rays from the Sun, which can cause skin cancer. In the 1980s, scientists discovered that thin areas, or holes, were appearing in the ozone layer over the polar regions each spring. Ozone loss is caused by chemicals called chlorofluorocarbons (CFCs).

Pressure systems in the atmosphere are masses of moving air that create winds and the weather. The air is set in motion by changes in temperature and air pressure (the pushing force of air from all directions). Air pressure is greatest at sea level, because there is a larger weight of air pushing down. The higher in the atmosphere you go, the less air and air pressure there is.