Snow

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Snow

• 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.

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Snowfall

• 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 atmospheres 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.

4
Source Gray and Male (1981)
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• 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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Source Cotton (1990)
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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.

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• There are 3 main stages in the life cycle of a
  low pressure system (mid-latitude cyclone)
• formation of disturbance on a front (formation
  stage or cyclogenesis)
• 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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Source Gray and Male (1981)
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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 lt 0oC) and
  ice nuclei (particles that serve as nuclei for
  condensation or deposition and ice crystal
  formation) must also be present.

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• 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.)

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Source Stull (2000)
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• There are two types of ice nucleation
• homogeneous nucleation
• heterogeneous nucleation
• Homogeneous nucleation is the formation of pure
  ice crystals without a foreign agent it occurs
  mainly at T lt
• -40oC.
• Heterogeneous nucleation involves freezing of
  water onto a host particle (ice nucleus).

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• 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

15
Source Rogers and Yau (1987)
16
Source Gray and Male (1981)
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• 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.
• At temperatures conducive for the formation of
  snow, a cloud may be just slightly
  super-saturated with respect to water but 10 to
  20 supersaturated with respect to ice.

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Source Stull (2000)
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• 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.

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• The basic habit or shape of an ice crystal is
  determined by the temperature at which it grows
  whereas its growth rate and secondary crystal
  features are determined by super-saturation.
• Ice crystal habits occur in many different
  shapes
• 1) plates
• 2) needles
• 3) sheaths
• 4) columns
• 5) dendrites

21
Source Stull (2000)
22
Source Rogers and Yau (1987)
23
Source Gray and Male (1981)
24
Source Libbrecht 2007
25
Source Libbrecht 2007
26
Source Libbrecht 2007
27
Source Libbrecht 2007
28
Source Libbrecht 2007
29
Source Libbrecht 2007
30
Source Libbrecht 2007
31
Source Libbrecht 2007
32
Source Bentley and Humphreys (1931)
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Source Bentley and Humphreys (1931)
34
Source Bentley and Humphreys (1931)
35
Source Bentley and Humphreys (1931)
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• The rate of mass (m) growth of an ice crystal by
  water vapour diffusion over time (t) is
• dm/dt 4pCDFAC(?8 - ?0), where
• C shape factor
• D water vapour diffusivity in air
• F ventilation factor
• AC function of crystal size
• ?8 water vapour density (wvd) at large distance
  from the ice crystal
• ?0 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.

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• 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 crystals cross sectional area and fall
  distance.
• Aggregation is the adhesion of two or more snow
  crystals after their collision.

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• This process is most effective at T 0oC when
  snow becomes sticky. Aggregation also depends
  on distance covered during snowfall.
• At T gt 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.

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References

• Cotton, W.R., 1990 Storms, Aster Press.
• Bentley, W. A. and W. J. Humphreys, 1931 Snow
  Crystals, Dover 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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 (lt 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.