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9.16 Clouds and Precipitation


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Types of Precipitation

When water vapour changes state by condensing onto dust, carbon dioxide, pollen or any nuclei, water droplets or ice crystals begins to form. These droplets of water and ice crystals are suspended in the atmosphere by currents of air dragging them around until they become too heavy. When they are too heavy their drag cannot overcome their weight so they start to fall as precipitation to the earth at a speed relative to their drag.

Drizzle is fairly steady, light precipitation. The drop size of drizzle is smaller than that of rain. Though drizzle may be produced by low cumuliform clouds, it is more commonly associated with stratiform clouds.

Rain is defined as precipitation of liquid droplets, either in the form of drops or of smaller widely scattered drops. Rain drops are normally larger than drops of drizzle. Nevertheless drops falling on the edge of a rain zone may be as small as drizzle drops, owing to partial evaporation.

Showers are defined as precipitation that begins and ends abruptly and lasts for short periods ranging from less than a minute up to a half hour or more. Usually there are rapid fluctuations in the intensity of the precipitation, and there is a noticeable brightening of the sky after or between showers. Snow pellets and hail always occur in the form of showers, but rain, snow, and ice pellets can occur with either showery or non-showery characteristics.

Virga is precipitation that falls from a cloud but evaporates before reaching the ground. At high altitudes the precipitation falls mainly as ice crystals, before melting and finally evaporating.