Clouds are formed when advancing cold air forces warm air upwards. Water vapour is an invisible gas and there is one essential rule about how much water vapour air can carry. The warmer the air, the more water vapour it can hold. When warm, moist air cools, it sometimes reaches a point, know as dew point, where it can no longer hold all its water vapour. Some condenses out into tiny water droplets, which form clouds, mist or fog.
You can see temporary clouds form when you boil a kettle of water. Close to the spout, you can see a clear zone of water vapour; further away, the water vapour cools and condenses to form clouds of water droplets that we know as steam.
In an occluded front, the region of water air loses heat to the cool air below, and vapour condenses as clouds. On average, about half of the Earth is covered by clouds at any time – and it is not only when fronts mix that they form. Whenever air carrying water vapour cools below a certain temperature, clouds of water droplets or ice crystals form.
Clouds sometimes form when a wind carrying water vapour hits a mountain range. The wind is suddenly forced upwards, so it cools and water vapour condenses out to form clouds. This explains why mountain ranges are often very wet on the windward side and less so on the leeward (sheltered) side.
Another way for clouds to form is when a small bubble of air over a particularly hot piece of land is heated so that it rises through colder surrounding air. Eventually it gets high enough to cool down to the point where water vapour condenses out to form a cloud. A typical example of this type of cloud is the air-weather cumulus.
There are different types of clouds, each name is based on its height and appearance, and each has its own altitude. Stratus, Nimbostratus, Stratocumulus and Cumulus are under 2 km above sea level. Altostratus, Altocumulus and Cumulonimbus are between 6 km and 2 km. Cirrostratus, Cirrocumulus and Cirrus are between 12km and 6 km.
Fog and mist
The sailor’s curse and the motorist’s nightmare, fog, like cloud, is formed when moist air cools and cannot hold as much water vapour. Typically, sea fog occurs when a warm, moist tropical air mass moves over a cold sea. Close to the water, the air cools and the water vapour that it contains condenses to form tiny droplets. Over land, fog occurs when air cools close to the Earth’s surface and, again, water droplets condense. This often produces a morning mist when the land has cooled rapidly overnight under a clear sky.