What’s that cloud?

Of all weather phenomena, clouds are among the most fascinating. From the silky, high-altitude cirrus to the towering, threatening mass of storm-bearing cumulonimbus, clouds are as varied as the weather itself.

Cloud formation

Clouds have their origins in the water that covers 70 per cent of the Earth’s surface. Millions of tons of water evaporates into the air daily from oceans, lakes and rivers, and by transpiration from trees, crops and other plant life.

As this moist air rises, it encounters lower pressures, causing it to expand and cool. As air cools it can hold less water vapour and eventually will become saturated. At that point, some of the water vapour condenses into tiny water droplets (a million times smaller than raindrops), forming clouds. Thus, whenever clouds appear, they provide visual evidence of the presence of water in the atmosphere.

Cloud classification

There are ten main cloud types, which are further divided into 27 sub-types according to their height shape, colour and associated weather. Clouds are generally categorised by where they sit in the atmosphere: low—from the earth’s surface to 2.5km; middle—2.5 to 6km; or high—above 6km.

Clouds are given Latin names which describe their characteristics—such as cirrus (a hair), cumulus (a heap), stratus (a layer) and nimbus (rain-bearing). All clouds are actually white in colour, but when viewed from the ground some appear grey or dark grey according to their depth and shading from higher cloud.

You’ll generally notice two different appearances of clouds—layers of cloud that look like sheets, and clumpy clouds that look like smoke plumes.

Ten cloud types. In the high atmosphere: Cirrostratus, Cirrus and Cirrocumulus. In the middle layer: Altostratus and Altocumulus. In the lower layer: Stratus, Stratocumulus, Cumulus. Extending from the lower layer into the middle layer: Nimbostratus. Extending across all three layers: Cumulonimbus.

 

Read more at: http://media.bom.gov.au/social/blog/895/whats-that-cloud/?cid=002fb02

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