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raindrop—A drop of water of diameter greater than 0.5 mm falling through the atmosphere. In careful usage, falling drops with diameters between 0.2 and 0.5 mm are called drizzle drops rather than raindrops, but this distinction is frequently overlooked and all drops with diameters in excess of 0.2 mm are called raindrops. The limiting diameter of 0.2 mm is rather arbitrary, but has been employed because drops of this size fall rapidly enough (about 0.7 m s−1) to survive evaporative dissipation for a distance of the order of several hundred meters, the exact survival distance being a function of the relative humidity. Drops much smaller than this limiting size fall so slowly from most clouds that they evaporate before reaching the ground. Virga is almost always composed of drops with diameters just below the limiting size assigned to drizzle drops. Raindrops are very much larger than cloud drops. A typical raindrop might have a diameter of 1–2 mm, while a typical cloud drop diameter is of the order of 0.01–0.02 mm. Raindrops fall between 2 and 12 meters per second (depending on altitude); those larger than about 1 mm are increasingly deformed by airflow (with flatter bases), the largest raindrops having a height to width ratio of 1:2. Raindrops may form by coalescence of cloud drops or from melting ice precipitation. Any given rainfall is characterized by a certain drop-size distribution of its raindrops, and even within a given storm this distribution may change its characteristics. The largest drops observed in heavy thunderstorms may have equivalent spherical diameters of 5–8 mm. Raindrops of such large size are rare, but occasionally form in the warm rain process by accretion of cloud water or can result from melting hail.
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