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Clouds are vital to sailors for predicting the arrival of changes in the weather and altering course accordingly. Formed of water droplets and/or ice crystals, clouds are characterized by their shape, height, and vertical or horizontal spread.

Cloud observation is the surest way of predicting the weather in the immediate future. Meteorologists describe 10 types of cloud, divided into 4 categories according to their height:

(c) DR
© DR

High-level clouds: 4,000-12,000 meters
Cirrus, Cirrostratus and Cirrocumulus Made up of ice crystals, generally announcing a disturbance coming in the next 12-24 hours

Mid-level clouds: 1,500-6,000 meters
Altostratus and Altocumulus Which are thicker, grey and precursors of the arrival of rain.

Low-level clouds: 0-2,000 meters
Stratus and Strato-cumulus. Dense and grey, give drizzle or fine snow.

Vertically-developed clouds: 400-12,000 meters
Small Cumulus are fair-weather clouds, large Cumulus, Cumulonimbus and Nimbostratus announce ice crystals, rain, hail or snow.