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Neither rain nor snow, nor freezing rain, for that matter. That is how one must often characterize winter precipitation. Rain and snow are just the most common members of the winter precipitation congregation, but not the only ones. Others include: ice pellets, snow grains, graupel, ice crystals and ice needles and also known by local names in different regions such as sleet, hail, soft hail, snow pellets and diamond dust.
Almost all forms of these hard rains begin their lives as ice/snow crystals in the clouds. But as they descend, they traverse a layer of warmer air whose temperature is above-freezing. There the ice/snow melt or partially melt depending on the depth of the layer and its departure from freezing. If they continue to fall through non-freezing temperature air, they fall as rain or perhaps a mixture or rain and snow. For these special hard rains, another cold air layer lies beneath the warm, melting layer of some depth above the planet's surface as the accompanying diagram shows. If the surface cold layer is very shallow, the precipitation will fall as cold rain which may freeze when it contacts the cold surface. If the falling rain passes through a layer of subfreezing air, the drops may supercool but not freeze, which leads to the formation of freezing rain and glaze or rime ice formation when they alight. However, if the layer is of proper depth, the rain drops or partially melted snow may refreeze or partially refreeze forming one of the hard rains. And if the falling drops are caught in a series of updrafts and downdrafts and make multiple passes through the warm and cold layers, they may collect several layers of ice much like a hail stone. These mixed precipitation events are typically found in the pre-warm frontal region of a wave cyclone as show in the accompanying diagram. That is not to say all are formed there. Winter precipitation around large water bodies and high terrain often has mixes of hard rains with the softer varieties where the over-ridding air mass may have many different temperature layers. Hard rains may fall either as showers or continuous precipitation, but some types fall in a preferred manner. Like snow and rain, ice pellets may fall continuously or in showers. Snow grains and ice crystals usually fall as steady precipitation whereas showers are the dominant method of fall for graupel (snow pellets and soft hail).
The copyright of the article Neither Rain Nor Snow in Meteorology is owned by . Permission to republish Neither Rain Nor Snow in print or online must be granted by the author in writing.
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