What Causes Clouds?

Clouds are formed when the invisible water vapour in the air condenses into visible water droplets or ice crystals. For this to happen, the parcel of air must be saturated (i.e. unable to hold all the water it contains in vapour form, so it starts to condense into a liquid or solid form). There are two ways by which saturation is reached.

1. By increasing the water content in the air, e.g. through evaporation, to a point where the air can hold no more.
2. By cooling the air so that it reaches its dew point - this is the temperature at which condensation occurs, and is unable to 'hold' any more water. In general, the warmer the air, the more water vapour it can hold. Therefore, reducing its temperature decreases its ability to hold water vapour so that condensation occurs.

Method (2) is the usual way that clouds are produced, and it is associated with air rising in the lower part of the atmosphere. As the air rises it expands due to lower atmospheric pressure, and the energy used in expansion causes the air to cool. Generally speaking, for each 100 metres which the air rises, it will cool by 1°C, however the rate of cooling will vary depending on the water content, or humidity, of the air. Moist parcels of air may cool more slowly, at a rate down to around 0.5°C per 100 metres.

Therefore, the vertical ascent of air will reduce its ability to hold water vapour, so that condensation occurs. The height at which dew point is reached and clouds form is called the condensation level.

There are five factors which can lead to air rising and cooling.

1. Surface heating: the ground is heated by the sun which heats the air in contact with it causing it to rise. The rising columns are often called thermals.
2. Topography: air forced to rise over a barrier of mountains or hills. This is known as orographic uplift.
3. Frontal: a mass of warm air rising up over a mass of cold, dense air. The boundary is called a 'front'.
4. Convergence: streams of air flowing from different directions are forced to rise where they meet.
5. Turbulence: a sudden change in wind speed with height creating turbulent eddies in the air.

Another important factor to consider is that water vapour needs something to condense onto. There are millions of minute salt, dust and smoke particles in the air known as condensation nuclei which enable condensation to take place when the air is just saturated.