| Section D | D index | 471-479 of 573 terms |
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driven snowSnow that has been moved by wind and collected into snowdrifts. A wind speed of about 13 mph will move light surface snow. In weather reports, snow lifted less than 6 ft above the surface is drifting snow, more than 6 ft is blowing snow.
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driving-rain indexQuantity that gives a measure of the amount of wind-driven rainfall on a vertical surface. It is the product of the average rainfall and the average wind over a specified period.
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drizzle dropA drop of water of diameter 0.2–0.5 mm (0.008–0.02 in.) falling usually (but not always) from low stratus or stratocumulus cloud. Although this is the correct term for this size range, all water drops of diameter greater than 0.2 mm are frequently termed raindrops, as opposed to cloud drops. Should such drops reach the ground, they can be felt on the upturned face.
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drizzle(Sometimes popularly called mist.) Very small, numerous, and uniformly distributed water drops that may appear to float while following air currents. Unlike fog droplets, drizzle falls to the ground. It usually falls from low stratus clouds and is frequently accompanied by low visibility and fog. In weather observations, drizzle is classified as 1) light, the rate of fall being from a trace to 0.3 mm (0.01 in.) per hour; 2) moderate, the rate of fall being from 0.3 to 0.5 mm (0.01–0.02 in.) per hour, and 3) heavy, the rate of fall being greater than 0.5 mm (0.02 in.) per hour. When precipitation equals or exceeds 1 mm (0.04 in.) per hour, all or part of the precipitation is usually rain. However, true drizzle falling as heavily as 1.3 mm (0.05 in.) per hour has been observed. By convention, drizzle drops are taken to be less than 0.5 mm (0.02 in.) in diameter. Larger drops are considered raindrops. Compare mist.
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drogueA sea anchor or other parachute-shaped device for use in water. Drogues suspended at desired depths from buoys are used to determine the set and drift of currents at those depths, by tracking the motions that they give to the buoys at the surface.
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drop breakupThe disruption of raindrops caused primarily by collisions with other raindrops. The distribution of drop fragments resulting from collisions is a complicated function of drop sizes, but generally the number of fragments increases with drop size. Thus, larger raindrops are more susceptible to breakup. Drop breakup is a limiting factor to the growth of raindrops by the collision–coalescence process.
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drop-size distributionThe frequency distribution of drop sizes (diameters, volumes) that is characteristic of a given cloud or of a given fall of rain. Most natural clouds have unimodal (single maximum) distributions, but occasionally bimodal distributions are observed. In convective clouds, the drop-size distribution is found to change with time and to vary systematically with height, the modal size increasing and the number decreasing with height. For many purposes a useful single parameter representing a given distribution is the volume median diameter, that is, that diameter for which the total volume of all drops having greater diameters is just equal to the total volume of all drops having smaller diameters. The drop- size distribution is one of the primary factors involved in determining the radar reflectivity of any fall of precipitation, or of a cloud mass.
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