Cloud Types and their Formation
	Air rises to form cloud in the following ways. Convection When air at the Earth's surface is heated it rises as thermals. Warm air rising over cold Warm air rises and is undercut by cooler air. The boundary between the warm and cold air is called a front.

Upward Motion

Turbulent air rises because of the frictional effect of the surface of the Earth.

Landscape

Air is forced to rise over high ground such as mountains or hills. See relief rainfall.

Ten main cloud types exist. These have sub-types and those named with prefixes indicate their structure more precisely. There are three cloud levels:-

High

This refers to the altitude of between six and twelve kilometres and features the following cloud types:

Cirrus:- having a fibrous apprearance. Their ice crystals make the clouds white and feathery at the edges. They have a dense head with a tail.

Cirrocumulus:- a massive, high swelling cloud, that mostly forms ahead of warm fronts. It has small white patches in rafts that look like small altocumulus.

Cirrostratus:- This cloud slowly approaches and covers the sky and indicates bad weather.

Medium

These have an altitude of between two and six kilometres and features:

Altocumulus:- cumulus at the intermediate level, a sign of changeable weather.

Altostratus:- flat cloud at an intermediate height, appears grey or blue grey. It is associated with coming rain as it precedes nimbostratus cloud which produces precipitation.

Low

At an altitude up to two kilometres, features:

Stratocumulus:- appears in a layer, does not significantly change the weather.

Stratus:- generally appears as mist or fog.

Low with vertical development

Nimbostratus:- associated with prolonged rainfall. This is the cloud of fronts and depression centres.

Cumulus:- when they are not deep, fair weather results. They appear to have the shape of a cauliflower because thermals rise and then curl.

Cumulonimbus:- causes showers and thunderstorms. They are overgrown cumulus with projecting lumps.

Cumulus Clouds

These are formed by convection and can exist in all of the cloud levels. They are lumpy, heaped up, swelling clouds.

Cirrus Clouds

Are also referred to as feather or veil cloud and feature ice prisms in their top storey. High cloud is always cirrus. They are the fastest moving of all cloud types, although their great altitude makes them appear still.

Stratus Clouds

These are mainly unstructured, fog-like, layer clouds that are quite shallow but cover a large area, up to 1000 kilometres wide. The word "Strato" indicates a flattened shape.

Key to cloud types

Cirrus	Ci	Cirrocumulus	Cc
Cirrostratus	Cs	Altocumulus	Ac
Altostratus	As	Stratocumulus	Sc
Stratus	St	Nimbostratus	Ns
Cumulus	Cu	Cumulonimbus	Cb