Rain

Rain, water drops falling through the atmosphere, the liquid form of precipitation. Raindrops range in size up to 0.16 in (4 mm) in diameter; if they are smaller than 0.02 in (0.5 mm), the rain is called drizzle. The quantity of rainfall is measured by a rain gauge, an open-top vessel that collects the rain, calibrated in inches or millimeters and so giving a reading independent of the area on which the rain falls. Light rain is less than 0.1 in (2.5 mm) hr, moderate rain up to 0.3 in (7.6 mm) hr, and heavy rain more than 0.3 in/hr. Rain may result from the melting of falling snow or hail, but it is commonly formed by direct condensation. When a parcel of warm air rises, it expands, almost without loss of heat; thus its relative humidity rises until reaching saturation, water vapor begins to condense as droplets, forming clouds. These droplets coalesce into raindrops chiefly through turbulence and nucleation by ice particles (and also cloud seeding). Moist air may be lifted by convection, producing convective rainfall; by forced ascent of air as it crosses a mountain range, producing orographic rainfall; and by the force within cyclones, producing cyclonic rainfall.