3. Snow refers to forms of ice crystals that precipitate
from the atmosphere and undergo changes on the
Earth's surface. It pertains to frozen crystalline water
throughout its life cycle, starting when, under suitable
conditions, the ice crystals form in the atmosphere,
increase to millimeter size, precipitate and
accumulate on surfaces, then metamorphose in place,
and ultimately melt, slide or sublimate away.
WHAT IS SNOW ?
4.
5. Orographic or relief snowfall is caused when masses of air
pushed by wind are forced up the side of elevated land
formations, such as large mountains. The lifting of air up
the side of a mountain or range results in adiabatic cooling,
and ultimately condensation and precipitation. Moisture is
removed by orographic lift, leaving drier, warmer air on the
descending, leeward side.The resulting enhanced
productivity of snow fall and the decrease in temperature
with elevation means that snow depth and seasonal
persistence of snowpack increases with elevation in snow-
prone area
MOUNTAIN EFFECTS
6.
7. An avalanche is a rapid flow of snow down a sloping
surface. Avalanches are typically triggered in a starting
zone from a mechanical failure in the snowpack when the
forces on the snow exceed its strength but sometimes
only with gradually widening After initiation, avalanches
usually accelerate rapidly and grow in mass and volume as
they entrain more snow. If the avalanche moves fast
enough some of the snow may mix with the air forming a
powder snow avalanche, which is a type of gravity current.
They occur in three major mechanisms.
AVALANCHE
8.
9. Most avalanches occur spontaneously during storms under
increased load due to snowfall. The second largest cause
of natural avalanches is metamorphic changes in the
snowpack such as melting due to solar radiation. Other
natural causes include rain, earthquakes, rockfall and
icefall. Artificial triggers of avalanches include skiers,
snowmobiles, and controlled explosive work. Contrary to
popular belief, avalanches are not triggered by loud sound;
the pressure from sound is orders of magnitude too small
to trigger an avalanche.
WHEN OCCUR ???
10.
11. Avalanche initiation can start at a point with only a
small amount of snow moving initially; this is typical
of wet snow avalanches or avalanches in dry
unconsolidated snow. However, if the snow has
sintered into a stiff slab overlying a weak layer then
fractures can propagate very rapidly, so that a large
volume of snow, that may be thousands of cubic
meters, can start moving almost simultaneously.
HOW IT IS FORMED ?
12.
13. Slab avalanches occur in snow that has been deposited, or redeposited by wind.
They have the characteristic appearance of a block (slab) of snow cut out from
its surroundings by fractures. These account for most back-country fatalities.
Powder snow avalanches result from a deposition of fresh dry powder and
generate a powder cloud, which overlies a dense avalanche. They can exceed
speeds of 300 kilometres per hour (190 mph), and masses of 10,000,000 tonnes
(9,800,000 long tons; 11,000,000 short tons); their flows can travel long
distances along flat valley bottoms and even uphill for short distances.
Wet snow avalanches are a low-velocity suspension of snow and water, with the
flow confined to the surface of the pathway.[55] The low speed of travel is due
to the friction between the sliding surface of the pathway and the water
saturated flow. Despite the low speed of travel (~10 to 40 kilometres per hour (6
to 25 mph)), wet snow avalanches are capable of generating powerful
destructive forces, due to the large mass, and density.
TYPES OF AVALANCHE
14.
15. If caught in an avalanche, try to get off the slab. In most instances, this is not
easy. Skiers and snowboarders can head straight downhill to gather speed,
then veer left or right out of the slide path. Snowmobilers can punch the
throttle to power out of harm's way. No escape? Reach for a tree. No tree?
Swim hard. The human body is three times denser than avalanche debris
and will sink quickly. As the slide slows, clear air space to breathe. Then
punch a hand skyward.
Once the avalanche stops, it settles like concrete. Bodily movement is nearly
impossible. Waitand hope for a rescue. Statistics show that 93 percent of
avalanche victims survive if dug out within 15 minutes. Then the survival
rates drop fast. After 45 minutes, only 20 to 30 percent of victims are alive.
After two hours, very few people survive.
WHAT TO DO IN AVALANCHE ?
