Download Physical Characteristics of Snow Crystals

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.
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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,
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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.
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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.
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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.
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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.
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