| Section S | S index | 561-569 of 1376 terms |
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snow-generating level—A layer in the middle or upper troposphere in widespread precipitation in which ice crystals form in small convective cells and fall to lower altitudes. Such layers are thought to be characterized by convective instability. Small convective cells called generating cells that develop within the layer may produce ice crystals that then settle into lower altitudes. The base of the convectively unstable layer is called the snow-generating level. On time–height displays from vertically pointing radars, generating cells are typically observed to extend about 1–2 km above the snow-generating level. Snow trails, or streamers, are terms used to describe the trails of precipitation that subsequently emerge from the base of the generating cells.
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snow grains—(Also called granular snow.) Precipitation in the form of very small, white opaque particles of ice; the solid equivalent of drizzle. They resemble snow pellets in external appearance, but are more flattened and elongated, and generally have diameters of less than 1 mm; they neither shatter nor bounce when they hit a hard surface. Descriptions of the physical structure of snow grains vary widely and include very fine, simple ice crystals; tiny, complex snow crystals; small, compact bundles of rime; and particles with a rime core and a fine glaze coating. It is agreed that snow grains usually fall in very small quantities, mostly from stratus clouds or from fog, and never in the form of a shower.
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snow line—1. In general, the outer boundary of a snow-covered area. It has at least two specific applications: 1) the actual lower limit of the snow cap on high terrain at any given time; 2) the ever-changing equatorward limit of snow cover, particularly in the Northern Hemisphere winter. 2. Same as firn line. See climatic snow line.
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snow mat—A special device used to mark the surface between old and new snow. It consists of a piece of white duck 28 inches square, having in each corner triangular pockets in which are inserted slats placed diagonally to keep the mat taut and flat. See snow board, snow gauge.
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snow pellets—(Also called soft hail, graupel, tapioca snow.) Precipitation consisting of white, opaque, approximately round (sometimes conical) ice particles having a snowlike structure, and about 2–5 mm in diameter. Snow pellets are crisp and easily crushed, differing in this respect from snow grains. They rebound when they fall on a hard surface and often break up. In most cases, snow pellets fall in shower form, often before or together with snow, and chiefly on occasions when the surface temperature is at or slightly below 0°C (32°F). It is formed as a result of accretion of supercooled droplets collected on what is initially a falling ice crystal (probably of the spatial aggregate type).
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