3rd March 2006
Rain
Source: Excerpt from The Book " Weather "
Distribution: Worldwide, except polar regions. Cause: Water droplets or ice crystals falling from a cloud under the influence of gravity. Associated Weather: Increased humidity at ground level. Hazard Warning: Prolonged downpours may cause flooding. 
Rain is defined as precipitation that reaches the ground in liquid form. Initially, rain forms in a cloud as either water droplets or ice crystals. These then grow large enough to fall from the cloud under the influence of gravity, the ice crystals melting on their way to the ground.
Sometimes, water droplets or ice crystals fall from a cloud but evaporate in mid-air. This creates an effect that resembles a dark fringe hanging from the cloud base. This phenomenon is known as virga, and it occurs when there is a deep layer of dry air, or a shallow layer of extremely dry air, beneath the cloud. Because virga does not reach the ground, it cannot be classified as precipitation. However, the evaporation that produces virga increases the water vapor content in the layer of dry air, making it increasingly likely that subsequent falls will reach the ground.
Classifying Rain
Rain that does reach the ground can be defined in a number of ways. In the United States, liquid precipitation is classified on the basis of the size of the raindrops and the extent of the associated visibility. Precipitation that consists of raindrops that are less than 1/50 inch (0.065 mm) in diameter and fall close together is defined as drizzle. Drizzle is categorized as light, moderate, or heavy depending on the visibility. Larger raindrops, or smaller drops that are widely separated, are classed as rain, which is defined as light, moderate, or heavy according to the amount that falls and the visibility.
While this system is precise, the amateur weather-watcher is unlikely to make use of it. A simpler and more practical distinction, adopted in some other countries, defines the type of precipitation according to the cloud that produces it. Under this system, liquid precipitation is classed as either rain or showers. Rain refers to falls from stratiform clouds, particularly stratus and altostratus. These clouds normally cover a wide area, so rainfall from stratiform clouds tends to be widespread and relatively long-lasting.
Showers, on the other hand, refers to falls from cumuliform clouds. These falls tend to be localized, and may only last for a minute or so. However, some showers may be heavy, particularly when associated with thunderstorms. Dry spells between showers normally last much longer than the shower itself. However, if there is a lot of cloud about, a number of showers may occur with only short dry spells between them.
Flooding may be caused by both rain from stratiform clouds and showers from cumuliform clouds. Persistent rain may lead to flooding over an extensive area, while heavy showers are more likely to cause flash floods.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
