The Cycle Of Snow

Snow crystals form in the atmosphere when water vapor is precipitated at temperatures below freezing. They form around centers of foreign matter, such as microscopic dust particles, and grow as more ice formed from atmospheric water vapor is deposited on them. Tiny water droplets may also contribute to snow crystal growth. The crystals generally are hexagonal, but variations in size and shape are almost limitless and include plates, columns, and needles. The particular shape depends on the air temperature and the amount of water vapor available.

When a snow crystal falls through air masses with different temperature and water-vapor conditions, more complex or combined types may develop. Crystals in air that has a temperature near freezing stick together to become snow flakes, aggregates of individual crystals. When snow crystals fall through air that contains water droplets, the droplets freeze to the crystals, forming the rounded snow particles called graupel (soft hail).

The density of new-fallen snow depends on weather conditions. The lowest-density snow

(lightest, driest) falls under moderately cold and very calm conditions. At extremely low temperatures, the new snow is fine and granular, with somewhat higher densities. The general rule is that the higher the temperature, the more dense (heavier, wetter) the snow, though density varies widely in the range of 20 to 32 degrees Fahrenheit (-7 to 0 degrees Celsius). The very highest densities are associated with graupel or needle crystals falling at temperatures near freezing. The percentage of water in new-fallen snow ranges from 1 to 30 percent, sometimes even higher, with the average for mountain snowfall being 7 to 10 percent. Wind affects snow density, for high winds break up falling crystals into fragments that pack together to form dense, fine-grained snow. The stronger the wind, the denser the snow.

Two types of snow form right at ground level.

Rime is the dull white, dense deposit formed from the freezing of droplets of water on trees, rocks, and other objects exposed to the wind. Rime deposits build up toward the wind. Rime may form large feathery flakes or a solid incrustation, but

Fig. App. 1-1. Snow crystal forms: a, plates; b, stellar crystals; c, columns; d, needles; e, spatial dendrites (combinations of feathery crystals); f, capped columns; g, irregular particles (compounds of microscopic crystals); h, graupel (soft hail); i, sleet (icy shell, inside wet); j, hail (solid ice).

lacks regular crystalline patterns.

Hoarfrost, on the other hand, displays distinct crystalline shapes: blades, cups, and scrolls. Hoarfrost forms on solid objects by the process of sublimation—the direct conversion of atmospheric water vapor to a solid. Deposited on top of snow, it is known as surface hoar and is generally produced during a cold, clear night. The crystals appear fragile and feathery, and sparkle brilliantly in sunlight. A heavy deposit of surface hoar makes for fast, excellent skiing.

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