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Physical Characteristics of Snow Crystals Atmospheric ice particles are classified based on the different shapes and growth processes of the crystal types: plate, stellar crystal, column, needle, spatial dendrite, capped column, irregular crystal, graupel, ice pellet and hail. Within each category the precipitation type may be distinguished according to: broken crystals, rimed particles, clusters, wet or melted, and the maximum particle dimension. Individual snow crystals observed at the earth's surface range in maximum dimension from about 50 μm to 5 mm. Particle densities range from approximately 100 to 700 kg m-3. 1 Physical Characteristics of Snowflakes Snowflakes may consist of two to several hundred snow crystals joined together. Generally, for snowflakes to form, a myriad of crystals should be moving at different velocities at air temperatures slightly lower than 0oC. Because they have abundant radiating arms, dendritic crystals tend to aggregate more readily than other types and are often found as the constituent crystals in snowflakes. Areal Distribution of Snowfall The following atmospheric conditions are important in determining the occurrence of significant amounts of snowfall: 1) Sufficient moisture and active nuclei at a temperature suitable for the formation and growth of ice crystals, 2 2) Sufficient depth of cloud to permit growth of snow crystals by aggregation or accretion, 3) Temperatures below 0oC in most of the layer through which the snow falls, and 4) Sufficient moisture and nuclei to replace losses caused by precipitation. The two areas of relatively heavy snowfall in North America are on the east and west coasts of the continent. Certain areas of western British Columbia the Yukon and Alaska adjacent to the mountain ranges which parallel the Pacific Coast, receive seasonal values exceeding 400 cm. However, amounts are highly variable; e.g., along the southern coast of B.C., near sea level, the air temperatures are normally above freezing so that most of the winter precipitation is rain, and the seasonal average snowfall is less than 60 cm. 3 Snowfalls are also relatively light in areas to the lee of the mountains. Widespread heavy snowfall also occurs in eastern Canada throughout central Ontario, southern Quebec, much of the Atlantic provinces, Labrador, and the east coast of Baffin Island where the seasonal amounts range from 250 to 400 cm. These parts of Canada lie on or near several principal tracks of transient low pressure systems which are frequently vigorous and well developed. Varying amounts of moisture are supplied to these areas from the Pacific and Atlantic Oceans and the Gulf of Mexico. In addition, the Great Lakes serve as an important moisture source for local precipitation, e.g., average seasonal snowfalls greater than 250 cm occur southeast of Lake Huron. 4 Snowfall amounts decrease rapidly in the southward direction from the eastern Ontarionorthern New England area to the southeastern United States. This is mostly a result of increasing temperatures, as opposed to decreasing precipitation. Over the prairie provinces of Alberta, Saskatchewan, and Manitoba the seasonal snowfall is considerably lower than in the eastern or western regions of Canada, averaging between 75 and 140 cm. The small amounts of snowfall over these regions can be attributed, in part, to the infrequent occurrence of vigorous weather systems. Also, the relatively flat terrain is not conducive to snowfall formation since the Pacific air moving inland subsides because of the downward slope in topography from the Rocky Mountains. 5 The western half of the Arctic Islands receives less snow (< 80 cm) than most other parts of Canada. Although this area experiences long winters; it is remote from major moisture sources; the extremely low temperatures over the region reduce the moisture holding capacity of the air to extremely low values thereby reducing snowfall amounts. 6