Types of Clouds

What is the Water Cycle?

Water on Earth is constantly moving, changing state (from liquid, to gas, to solid) and being recycled.  The water cycle describes this journey. There are 4 main stages in the water cycle: evaporation, condensation, precipitation and collection.

Evaporation:
When the sun heats the surface of seas, lakes, rivers and streams, some of the water changes state and becomes water vapour, mixing with the air. Warm air rises so the water vapour rises too.

Condensation:
When the air cools down, the water vapour condenses back into water droplets. These water droplets collect together and form clouds.

Precipitation:
The water droplets in clouds attract other water droplets to them and they grow bigger. When they get too big and heavy they fall to ground as rain. If the air is cold enough the droplets remain frozen and fall as snow or hail.

Collection:
When the water falls to Earth it collects as streams, rivers or lakes. When it falls on land in can filter in to the Earth and become groundwater or it can flow over the land as run off to meet existing bodies of water.

Some of the water may be taken up by plants and animals. Plants take up water from the ground through their roots. They then ‘breathe’ the moisture out of their leaves into the air.Evaporation accounts for about 90% of the water in the air with transpiration accounting for most of the other 10%.

Clouds: Clouds form when water vapour condenses in the atmosphere. When air rises, it cools. A rising current of air is called an updraught (also spelled updraft). As the air cools, its ability to hold moisture is reduced. The water vapour in the rising air condenses, forming clouds. When discussing clouds, we are concerned with cloud cover and cloud type.

Cloud cover: The amount of the sky that is obscured by clouds is known as cloud cover. Cloud cover is measured in oktas (eighths). One okta of cloud cover means that one eighth of the sky is obscured by clouds. Four oktas of cloud cover means that half the sky is obscured by clouds. Eight oktas of cloud cover means the entire visible sky is obscured by clouds. The human eye is used to judge cloud cover.

Cloud type: There are many types of clouds. Clouds are named according to their appearance. The three basic cloud types are cirrus, cumulus and stratus.

• Cirrus clouds: These clouds form high in the atmosphere where they are blown into thin, feathery formations by high level winds.
• Cumulus clouds: The word cumulus means “heap” or “pile”. Cumulus clouds are puffy looking clouds formed when water vapour condenses as a result of strong updraughts. They look like floating heaps of cotton.
• Stratus clouds: These clouds are formed in layers. A stratus cloud looks like a sheet of cloud.

Many other types of cloud display features of more than one of the basic cloud types. For instance a cirrostratus cloud displays features of cirrus as well as stratus clouds. They are high level,thin, feathery clouds formed in layers. It is important to note that the prefix “alto” is used to describe mid level clouds and the terms “nimbo” or “nimbus” are used to describe clouds that produce rain. Therefore, altocumulus clouds are mid-level cumulus clouds and nimbostratus clouds are stratus clouds that produce rain.

A distinct type of cloud is the cumulonimbus cloud. This type of cloud forms when very strong updraughts rise high into the atmosphere forming very tall clouds. The top of this type of cloud may spread out, giving the cloud an anvil shape. Cumulonimbus clouds produce heavy rain and thunderstorms. The diagram below shows various cloud types as well as the altitudes at which they form