Clouds    

Clouds are formed when water vapour in the air is cooled and condenses as part of the water cycle. Clouds consist of billions of tiny water droplets (and even ice crystals) floating in the sky and appear in a variety of shapes and sizes depending on how and where they formed. However, there are three main types of cloud:

Cirrus

– high-level, wispy clouds. The name originates from the Latin word meaning "curl of hair". These feathery clouds form very high up in the sky (at altitudes between 5 km and 14 km) where it is very cold. They are therefore made up of tiny ice crystals rather than water droplets. Cirrus clouds occur in warm air which is being slowly lifted over a large area by an approaching cold front, and they are therefore often the signal of bad weather.

Mare’s tails – cirrus clouds are often called mare’s tails. This is because strong winds high in the air blow them into wispy curls like the tail of a horse.

Cumulus

– fluffy, cumulus clouds are named after the word "heap". These are the most familiar clouds and look like heaps of cotton wool or large cauliflower’s. Cumulus clouds are found at a height of about 500 metres and are composed of tiny water droplets. They form when sunshine warms pockets of moist air and causes them to rise quickly. As they get higher, the pocket of air billows out and forms the familiar fluffy shape as the moist air cools and condenses into water droplets. These clouds are usually seen in fine weather, when the sky is blue.

Stratus

– low-level blankets of cloud. The name "stratus" means "layers" in Latin, although you in fact rarely see the layers in stratus clouds. Instead they appear as a grey, shapeless sheet of cloud extending in all directions across the sky. They are usually only about 1 km thick, but can be as much as 1000 km wide. Stratus clouds build up when a layer of warm, moist air rises slowly over a mass of colder air. These clouds are often dark and gloomy, and are associated with rain and drizzle. Stratus clouds can sometimes rest on the ground or sea instead of up in the air, and they are then called ‘fog’.

Although there are only three basic types of cloud – cirrus, cumulus and stratus – these can combine to produce other types, such as cumulonimbus, cirrostratus and stratocumulus. In all there are about ten different varieties, which we will explore in more detail in the next section.

We know that clouds form when rising air cools and the moisture in it condenses to form water droplets. But do you know what makes the air rise in the first place? Let me tell you.

Air rises for three main reasons:

• Sunshine – heat from the sun or warm ground warms the air and makes it lighter. It therefore rises into the sky.

• The terrain – air may rise as it is forced upwards due to changes in the terrain (landscape). This often occurs when wind blows air either over mountains, or over cliffs onto land from the sea.

• A front – air can also rise at a weather front. At cold fronts, cold air is pushed under warm air, forcing it upwards and at a warm front, warm moist air is forced up and over the cold air.

In all three of these cases, the warm moist air will cool as it rises, and so the moisture it contains will condense into water droplets – forming clouds.

Let's now move on to the Cloud Watching section and have a look at clouds in more detail.