The smaller raindrops are kept nearly spherical by surface-tension forces, but, as the diameter surpasses about 2 mm (0.08 inch), they become increasingly flattened by aerodynamic forces. The terminal velocities of raindrops at ground level range from 2 metres per second (7 feet per second) for the smallest to about 10 metres per second (30 feet per second) for the largest. Raindrops rarely exceed 6 mm (0.2 inch) in diameter because they become unstable when larger than this and break up during their fall. Liquid waterdrops with diameters greater than those of drizzle constitute rain. Slight drizzle produces negligible runoff from the roofs of buildings, and thick drizzle accumulates at a rate in excess of 1 mm per hour (0.04 inch per hour). High relative humidity below the cloud base is required to prevent the drops from evaporating before reaching the ground drizzle is classified as slight, moderate, or thick. It forms by the coalescence of even smaller droplets in low-layer clouds containing weak updrafts of only a few centimetres per second. Liquid precipitation in the form of very small drops, with diameters between 0.2 and 0.5 mm (0.008 and 0.02 inch) and terminal velocities between 70 and 200 cm per second (28 and 79 inches per second), is defined as drizzle. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.
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