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Water drops falling through the atmosphere; the chief form of precipitation. Raindrops range in size up to 4 mm diameter; if they are smaller than 0.5 mm the rain is called drizzle. The quantity of rainfall (independent of the drop size) is measured by a rain gauge, an open-top vessel which collects the rain, calibrated in millimeters or inches and so giving a reading independent of the area on which the rain falls. Light rain is less than 25 mm/h, moderate rain 25 to 75 mm/h, and heavy rain more than 75 mm/h. Rain may result from the melting of snow or hail as it falls, but is commonly formed by direct condensation.

When a parcel of warm air rises, it expands approximately adiabatically, cooling about 1K per 100 m. Thus its relative humidity rises until when it reaches saturation the the water vapor begins to condense as droplets, forming clouds. These droplets may coalesce into raindrops, chiefly through turbulence and nucleation by ice particles or by cloud seeding. Most air may be lift by convection, producing convective rainfall; by forced ascent of air as it crosses a mountain range, producing orographic rainfall; and by the forces within cyclones, producing cyclonic rainfall.

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