What is a cloud? Although they may look fairly solid and may be beloved of poets, they are in fact merely a collection of water droplets, ice crystals or a complex mixture of both. Clouds are formed by climatic forces acting on moisture in the air. This moisture is constantly renewed by water molecules escaping from land surfaces and water into the air and evaporating as a gas or vapour. The amount of the water vapour that can be held in the air depends on the temperature. The cooler the air, the less water it can hold (which is why areas near the equator of often cloudy.

Once the air has become cold enough some of the water vapour condenses to form a visible mass of tiny droplets. This condensation depends on the presence in the atmosphere of minute particles. Often these particles (known as condensation nuclei) can be salt from ocean spray or particles from fires or volcanic eruptions. When such condensation from vapour to droplets occurs on the ground, it is called dew. Near the ground it is called fog or mist. Up in the sky it is called clouds.

A classification of clouds was first introduced by Luke Howard (1772-1864) who used Latin words to describe their characteristics.

• Cirrus - a tuft or filament (e.g. of hair)
• Cumulus - a heap or pile
• Stratus - a layer
• Nimbus - rain bearing

There are now ten basic cloud types with names based on combinations of these words (the word 'alto' meaning high but it is also used nowadays to denote medium-level cloud).

Clouds form when moist air is cooled to such an extent that it becomes saturated. The main mechanism for cooling air is to force it to rise. As air rises it expands, because the pressure decreases through the atmosphere, and therefore cools. Eventually it may become saturated and the water vapour then condenses into the tiny water droplets mentioned above. If the temperature reaches below about -20C many of the cloud droplets will have frozen so the cloud mainly composes of ice crystals.

There are four main ways in which air rises to form cloud:

• Rapid local ascent when heated air at the earth's surface rises in the form of thermal currents (convection).
  
  
• Slow widespread mass ascent where warm moist air is undercut by cold air (the barrier between the warm and cold air is called a 'front').
  
  
• Upward motion associated with turbulent eddies resulting from the frictional effect of the earth's surface.
  
  
• Air forced to rise over a barrier of mountains or hills.

The first of these tends to produce cumulus-type clouds, whereas the next two usually produce layered clouds. The last can produce with cumulus-type cloud or layered cloud depending upon the state of the atmosphere. The range of ways in which clouds can be formed and the variable nature of the atmosphere give rise to the enormous variety of shapes, sizes and textures of clouds.

Other features in the Weather Basics series:

- Clouds
- Day and Night
- Fog and Mist
- Fronts
- Frost
- High Pressure
- Humidity
- Jet Streams
- Lightning
- Low Pressure
- Mirages
- Northern Lights
- Rain or Showers?
- Thunder
- Watercycle