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Clouds are sometimes the most fascinating as well as the most informative of all weather phenomena. They vary from the silky wisps of high altitude cirrus to the towering and menacing  dark storm bearing cumulonimbus. Clouds are easily as variable as the weather itself.

Different types of clouds provide clues and indications of their own as to the weather conditions. Observers here in Samoa at two of our key climate stations (Apia and Faleolo International Airport) report cloud types every hour with information as to the cloud types present, heights and the amount of sky covered overall.

CLOUD FORMATION

Clouds all over the world have their origins in the water that cover up to 70 per cent of the earth's surface. Millions of tons of water vapour are evaporated into the air daily from the oceans, lakes and rivers, and by transpiration from trees, crops and other plant life. Samoa being surrounded by Pacific Ocean sees various cloud types coming and going daily (see cloud spotting, under Activities for an active method for learning about different cloud types).

Rising moist air encounters lower pressures and expands a result becoming cooler in the process. As this air cools, less water vapour can be held and so eventually becomes saturated. It is at this point that some of the water vapour will condense into tiny water droplets to form cloud. About a million cloud droplets are contained in one rain drop. Thus, whenever clouds appear they provide visual evidence of the presence of water in the atmosphere.

 There are 4 ways in which moist air can be lifted to form clouds. they are:

Orographic Lifting: 
 When air is forced upwards by a barrier of mountains or hills

Convective Lifting: 
 When air is heated at the earth's surface and rises in the form of thermal currents or bubbles

Widespread Ascent: 
 the result of interaction of air masses or the movement of a cold air mass forcing warm air to rise

Mechanical (or frictional) turbulence: 
 occurs when the air flow is deformed into a series of eddies as it moves over the earth's surface.

Classification of Clouds
 Clouds are classified using the Latin system developed by Luke Howard, an English naturalist, who described clouds as they would appear to a ground observer.  He classified four basic types of clouds:

Stratus (Latin for "layer") - a sheet-like cloud

Cumulus ("heap") - a puffy cloud

Cirrus ("curl of hair") - a wispy cloud

Nimbus ("violent rain") - a rain cloud

 Howard's work was later expanded by Abercromby and Hildebrandsson in 1887.  These scientists identified the clouds by the height of the cloud's base above the Earth's surface.  The four major cloud groups and their most common types along with the abbreviations for the clouds are shown below.
 
Group 1.  High Clouds
        Cirrus (Ci)
        Cirrostratus (Cs)
        Cirrocumulus (Cc)		 Group 2.  Middle Clouds
        Altostratus (As)
        Altocumulus (Ac)
Group 3.  Low Clouds
        Stratus (St)
        Stratocumulus (Sc)
        Nimbostratus (Ns)		 Group 4.  Clouds with Vertical Development 
        Cumulus (Cu)
        Cumulonimbus (Cb)

High clouds range in height from 16,000 to 60,000 ft (5000 to 18,000 m) depending on the latitude region.  Middle clouds range in height from 6500 to 26,000 ft (2000 to 8000 m).  Low clouds range in height from the surface to 6500 ft (0 to 2000 m).

Samoa Meteorology Division weather observers note the above grouping of clouds and the World Meteorological Organization's (WMO) classification of the 9 individual types contained in each of the Groups 1 - 3, including those in Group 4 as they occur.

Do the Cloud Spotting Activity to learn more about clouds!!!