How Clouds Form

Many people think clouds are made of water vapour, but water vapour is an invisible gas. It is all around you right now. A cloud is what you see when some of that vapour has condensed into tiny drops of liquid water, or frozen into tiny ice crystals. Every cloud is a huge crowd of these particles, held up in the air.

So the real question is: what makes invisible vapour turn into visible droplets? The answer is cooling, and the usual way air cools is by rising.

Air pressure is highest near the ground and falls as you go higher. When a block of air (meteorologists call it a parcel) rises, the pressure around it drops, so it expands. Expanding air cools, even though no heat has been taken away. This is called adiabatic cooling.

Until it becomes saturated, rising air cools by about 9.8 °C for every kilometre it climbs. NOAA gives an example: air at 30 °C near the ground, with a dew point of 18 °C, cools to saturation at about 1,200 metres (4,000 feet). Above that height, condensation can begin.

The dew point is the temperature to which air must be cooled to become saturated. Saturated air holds as much water vapour as it can at that temperature, so any more cooling makes some of the vapour condense.

You can see the same thing at ground level. On a humid day, a glass of iced water quickly becomes wet on the outside. The glass cools the air touching it below its dew point, and vapour condenses on the cold surface. Fog, dew on grass in the morning and misty bathroom mirrors all form in the same way.

Water molecules are far too small to clump together on their own. They need a surface to condense on. The air is full of microscopic particles that do this job, called cloud condensation nuclei: sea salt from ocean spray, dust, wind-blown soil, pollen and smoke from fires or volcanoes. Many of them attract water.

A nucleus is about one hundredth of the width of the droplet that forms on it, so a cloud droplet is still almost pure water. But every droplet has a speck of salt, dust or dirt at its heart. Droplets are constantly forming and evaporating at the edges of a cloud, which is why clouds keep changing shape.

A typical cloud droplet is only about 0.01 to 0.02 millimetres across. It falls so slowly that rising air keeps it up, which is why clouds seem to float. A typical raindrop is about 2 millimetres across, which holds around a million times more water than a 0.02 mm droplet. Droplets grow in two main ways:

• Collision and coalescence: in warm clouds, larger droplets fall a little faster, bump into smaller ones and merge. This is the main way rain forms in the tropics.
• The ice crystal process: in cold clouds, droplets can stay liquid well below 0 °C (they are supercooled). Where ice crystals form among them, the crystals grow by taking up vapour at the droplets' expense. They fall as snow, which melts into rain if the air below is warm enough.

When a drop or crystal becomes too heavy for the rising air to support, it falls. That is precipitation, the subject of Part 4.