Snowfall Formation & Distribution
Definition: Snow is defined as particles of
ice formed in a cloud that are large enough
to fall toward the ground. Snow is therefore
solid precipitation formed of white or
translucent ice crystals, chiefly in complex
hexagonal form and often agglomerated into
snowflakes. It does not include other forms
of solid precipitation such as ice pellets or
hail.
Definition: Snowfall is falling snowflakes or
snow crystals or also the accumulation of
snow during a specified period of time.
The formation of snow requires at least 3
conditions: 1) atmospheric moisture and 2)
mechanisms to convert this water vapour
into precipitation (i.e. vertical motion of air),
3) temperatures (T) below 0oC.
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Moisture is supplied by evaporation from
oceans, rivers, soils and plants, as well as
sublimation from snow and ice.
The atmosphere’s capacity to hold water
decreases exponentially with temperature
(Clausius-Clapeyron relation) such that there
is potential for heavier snowfall when T
nears the freezing point.
The Pacific Ocean is the main source of
atmospheric moisture for B.C.
Vertical motion yields adiabatic expansion
and thus cooling of air that may lead to the
condensation of water vapour into clouds.
Four main types of atmospheric processes
lead to vertical motion of air:
1) horizontal convergence
2) orographic lift
3) convective lift
4) frontal lift
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The intensity of the precipitation and hence
snowfall accumulation depends on the rate
of vertical motion.
For B.C. fronts associated with synoptic
weather systems and enhanced lifting along
the Western Cordillera are the two main
mechanisms for vertical motion that yield
snowfall.
There are 3 main stages in the life cycle of a
low pressure system (mid-latitude cyclone):
1) formation of disturbance on a front
(formation stage or cyclogenesis)
2) increase in amplitude (developing
stage)
3) occlusion (mature stage or cyclolysis)
Orographic upglide increases the rates of
condensation and precipitation. Mountains
also retard storm motion and increase
duration of snowfall events.
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Summary: The formation of snow depends
on many variables including ambient air
temperature, moisture and motion.
Supercooled water (liquid water at T < 0oC)
and ice nuclei (particles that serve as nuclei
for condensation or deposition and ice
crystal formation) must also be present.
Different types of snow (rimed crystals,
graupel, snow crystals or snowflakes) arise
depending on environmental conditions in
which they form.
Ice nuclei from 0.01 to 1 μm are abundant in
the atmosphere and consist of aerosols of
natural and anthropogenic sources (dust,
clay-silicate particles, organic matter from
vegetation, pollutants, etc.)
There are two types of ice nucleation:
1) homogeneous nucleation
2) heterogeneous nucleation
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Homogeneous nucleation is the formation of
pure ice crystals without a “foreign” agent;
it occurs mainly at T < -40oC.
Heterogeneous nucleation involves freezing
of water onto a “host particle” (ice nucleus).
There are 4 ice nucleation mechanisms:
1) heterogeneous deposition
2) condensation followed by freezing
3) contact
4) immersion
Once formed, an ice crystal grows by
several mechanisms:
1) deposition
2) riming
3) aggregation
An ice crystal embedded in a cloud of water
droplets will grow at the expense of the
droplets because the vapour pressure at the
ice surface is less than at the water surface.
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At temperatures conducive for the formation
of snow, a cloud may be just slightly supersaturated with respect to water but 10 to
20% supersaturated with respect to ice.
Supersaturation is air with relative humidity
greater than 100%, i.e. having more water
vapour than is needed to produce saturation
with respect to a plane surface of pure water.
This yields a net transfer of water vapour
from the cloud droplets to the ice surface,
leading to preferential growth of ice crystals
over raindrops (the Bergeron process).
Once the snow crystals become large
enough, they accelerate downward through
gravitational acceleration.
The basic habit or shape of an ice crystal is
determined by the temperature at which it
grows whereas its growth rate and secondary
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crystal features are determined by supersaturation.
Ice crystal habits occur in many different
shapes:
1) plates
2) needles
3) sheaths
4) columns
5) dendrites
The rate of mass (m) growth of an ice crystal
by water vapour diffusion over time (t) is:
dm/dt = 4πCDFAC(ρ∞ - ρ0)
C = shape factor
D = water vapour diffusivity in air
F = ventilation factor
AC = function of crystal size
ρ∞ = water vapour density (wvd) at large
distance from the ice crystal
ρ0 = wvd at the crystal surface
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This equation is only valid if crystal
diameters are less than a few 100 microns,
after which growth by collision with cloud
droplets becomes dominant.
Riming is a growth process by accretion of
cloud droplets that collide and adhere to a
snow crystal.
The collision efficiency is defined as the
ratio of the number of droplets that actually
impact the crystal to the number of droplets
that are swept out by it inside a column of
air enclosed by the crystal’s cross sectional
area and fall distance.
Aggregation is the adhesion of two or more
snow crystals after their collision.
This process is most effective at T ~ 0oC
when snow becomes “sticky”. Aggregation
also depends on distance covered during
snowfall.
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At T > 0oC, snow will melt rapidly and fall
as rain if the freezing level isotherm is high
enough above ground.
Through deposition, aggregation and riming,
a 1 mm diameter snowflake can grow to 10
mm in about 20 min. and if melted form a
raindrop of about 1 mm in diameter.
References
Cotton, W. R., 1990: Storms, Aster Press.
Gray, D. M. and D. H. Male, 1981:
Handbook of Snow, The Blackburn Press.
Rogers, R. R. and M. K. Yau, 1987: A Short
Course in Cloud Physics, Pergamon Press.
Stull, R. B., 2000: Meteorology for
Scientists and Engineers, Brooks/Cole
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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.
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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.
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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) 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.
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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.
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.
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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.
Snowfall amounts decrease rapidly in the
southward direction from the eastern
Ontario-northern 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
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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.
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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