Should contain everything covered in lecture

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10 Nov 2010
School
Course
Lecture Nine
x Snow
o Formation of snow begins with the gradual growth of ice crystals in cold
(<< 0 degrees Celsius), high-altitude cloud.
o Water in the liquid state blow 0 degrees Celsius is referred to as super
cooled (freezes if it impacts an object such as airplanes)
o Super Saturation ± when air is saturated (100% relative humidity) with
respect to water it is supersaturated (>100% rh) with respect to ice
o The shapes of snow flakes are controlled by super saturation of water
vapour
x Value of Snow
o Snow is an important water source for groundwater recharge
o Insulates soil reducing frost penetration
o ,PSRUWDQWFRPSRQHQWRIWKH(DUWKVUDGLDWLRQEXGJHWGXHWRLWVKLJK
albedo.
x Blizzards - Extreme winter storms defined as sustained wind speeds greater
than 40km/h, visibility less than 1 km, and wind chills equivalent to -25
degrees Celsius
x Wind Chill ± Relationship between air temperature, wind speed and wind chill.
Freezing to human skin occurs almost instantly at -50 degrees Celsius. In Canada,
80 lives per year are lost due to hypothermia and frostbite (more than to all
geological hazards combined).
x Freezing Rain ± Develops when precipitation falls through layers of the
atmosphere with different temperatures. If the particle falls through a warm air
near but not at the ground, partial melting and super cooling can occur. Such
particles then freeze upon impact with surfaces at the ground.
x Ice Storms ± When the conditions for freezing rain persist for over a period of
hours to days, the events are classified as ice storms.
x Snow Avalanches ± Masses of snow (more than a few m3), that separates from the
intact snowpack and slide downslope.
o Risks
Kill approx. 200 people per year
Most avalanches in Canada are associated with recreational
accidents.
o Ecological Benefits
Shapes vegetation in high mountain valleys
o Initiation
Point ± Releases avalanche (small amount of snow and grow as
they move downslope ± LQYHUWHG³VKDSH
Slab avalanches ± Layers of cohesive snow, fracturing off the
snowpack along a weak layer
o Weak Layers
Less dense and more transparent than adjacent layers (put a
snowpack towards the Sun)
Weak layers in the snowpack can form in three main ways:
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