What air is made of

We eat a few times a day and drink more often, but we breathe every few seconds: people can live a while without food and water, but only minutes without air. The atmosphere is a mixture of gases that envelops the earth. It holds oxygen for people and animals and carbon dioxide for plants. It is colourless and odourless, and 99 per cent of its mass lies within 32 km of the surface.

Composition: the atmosphere is made of gases, water vapour and dust particles. By volume, near the surface, it is mostly nitrogen (about 78 per cent) and oxygen (about 21 per cent), with argon (just under 1 per cent) and small amounts of carbon dioxide and other gases. Higher up the mix changes: oxygen is almost negligible at 120 km, and carbon dioxide and water vapour are found only up to 90 km.

Gases with special roles:

• Carbon dioxide is transparent to incoming sunlight but opaque to outgoing heat from the earth. It absorbs part of that heat and sends some back down, so it is largely responsible for the greenhouse effect. Other gases stay constant in volume, but carbon dioxide has been rising for decades, mainly from burning fossil fuels, and air temperatures have risen with it.
• Ozone, found 10-50 km above the surface, acts as a filter, absorbing the sun's ultra-violet rays before they reach the ground.

Water vapour is a variable gas that decreases with height and from the equator to the poles. In the warm, wet tropics it can be up to 4 per cent of the air by volume; in dry, cold deserts and polar regions, less than 1 per cent. It absorbs some sunlight and keeps in the earth's heat, and it affects whether air is stable or unstable.

Dust particles come from sea salt, fine soil, smoke and soot, ash, pollen and broken-up meteors. They are concentrated in the lower layers, though convection can lift them high. There is more dust in subtropical and temperate regions (dry winds) than at the equator or poles. Dust and salt particles act as hygroscopic nuclei: water vapour condenses around them to form clouds.

Example (new): on a winter morning after a dry, dusty week, fog forms more easily near a busy road than over a clean field: the smoke and dust give the moisture more particles to condense on.