Clouds

This page provides a general introduction to clouds. More detailed discussion of specific types of clouds can be found on the following pages.

You may also want to have a look at:

• the gallery of cloudscapes, and attempt to identify the clouds that you can see.
• the clouds at twilight page, for some beautiful sunsets

How Clouds Form

Even in clear air water is present, but it is in the form of water vapour, a gas, which is invisible. The warmer the air more water vapour it can contain. This is easily understood if we think about demisting a car wind screen or using a hair drier: the extra heat helps to evaporate the moisture. Conversely, if air is cooled, when it reaches dew point, condensation occurs, i.e. the water vapour is converted into minute water droplets.

If this happens on a grassy surface, or on a car, it forms dew (or hoar frost if the temperature is below freezing). In this case, only the air very close to the ground surface has suffered enough cooling to cause condensation. If a greater depth of air near the surface is cooled, fog is formed. Fog is simply ground level cloud.

Clouds at higher levels are formed by similar cooling processes. Their size and shape are controlled by the movement of air within and around them.

What type of cloud is that?

We have all noticed that there are different types of cloud, and most of us are able to interpret what clouds indicate about the weather that is likely to occur in the next hour or so. However, that’s where it normally ends. Like so many phenomena in nature, there are no really sharp boundaries between one cloud type and the next, just a gradation. To keep things relatively simple, however, we can divide clouds up into three main groups shown below.

Cirrus

These are the highest of the clouds.  They are made of ice crystals and frequently take the form of thin strands and wisps.  They are normally not thick enough to obscure the sun or the moon.

The wispy form of cirrus clouds is caused by falling grains of snow, which are blown by the wind.  The air is so dry at these high altitudes that the snow grains 'sublimate' (evaporate) before they have descended very far.

The longer and more horizontal the virga the stronger the wind at that altitude.

Stratus

These are essentially 'layer clouds'.  They are usually sub-horizontal and can occur at low medium and high levels. At the lower and medium levels they can form thick sheets which blot out the sun, and cause grey, dull weather. They can produce rain, drizzle or snow.

The layered form of stratus clouds indicates that the air is moving more or less horizontally, with very litte vertical uplift.

Cumulus

These are the cauliflower shaped clouds that are usually associated with showery weather. They can produce rain, snow or hail, which might be accompanied by thunder and lightning. The cauliflower form of cumulus indicates rapidly rising pockets of air.