2.2.4 Mountainous terrain, 2

Below the graphics are detailed descriptions of each feature.

Area A, fastest snowmelt:

photograph of a mountain with a bare south-facing slope and a snowy north-facing slope

While the sun has a generally uniform effect over snowpack surfaces on flat terrain, it's different in complex terrain. The interaction of aspect (the direction that a slope faces), time of day, temperature, and cloud cover determine how snowpack depth evolves on mountain slopes. In general, in the Northern Hemisphere, south-facing slopes get greater solar exposure, which causes the most frequent and rapid snowmelt, and thus the lowest snow depths.

Area B, the most snowfall from an upslope event

Diagram of an upslope precipitation event

The prevailing wind direction during a precipitation event is critical for the resulting snow accumulation, with the upwind side of a mountain (the upslope side) typically receiving the most snow. Downslope winds tend to dry out the atmosphere and quickly reduce snowfall.

 

 

 

 

Area C, lots of drifted snow

Electron microscopy photo of a single stellar snow crystal with sectorlike extensions with an arm broken off during descent through the atmosphere

Wind has three primary effects on snowfall and snowpack.

  1. It fractures snowflakes in the air, breaking them into small pieces of ice that pack together and form dense layers of wind-deposited snow where they land.
  2. Wind picks up the top portion of the snowpack, fractures the crystals, and deposits them downwind, increasing snow depth and density where they fall.
  3. Wind enhances sublimation, the evaporation of snowpack. When relatively dry air is present, sublimation is always occurring (the top of the pack is always evaporating). Wind enhances the process by quickly removing the water molecules near the ice surface. Note that snow in tree canopies can sublimate particularly quickly due to the enhanced exposure to dry air and wind. In some areas, nearly half of the annual snowfall can sublimate in this way.

Spencer Logan of CAIC discusses wind redistribution of snow.

Here's Spencer Logan of CAIC dicsussing wind and its impact on snowpack: "We're standing in the Front Range of Colorado, where it's very windy. Behind me, you can see the effects of it. Above tree line, we see areas that have been blown bare during the winter, with the snow transported somewhere. Some gets picked up and moved higher into the atmosphere, but a lot of it gets deposited downwind. The big round roll is a very thick deposit of snow, which will probably last through most of the summer."

Remember to select the last option, D.

Area D, little snow accumulation due to slope steepness

The window of terrain steepness for avalanche occurrence.

In general, snow does not accumulate much on slopes steeper than 45 degrees. It slides off in avalanches during or immediately after snowstorms. (An avalanche is a large mass of snow or other material that moves swiftly down a mountainside or over a precipice.)

Note that the steepness angle can be as high as 50- or even 60-degrees in maritime climates, such as the coastal mountain ranges of North America.

Most avalanches tend to occur on slopes between 30 and 45 degrees. These avalanches are often large and can remove much of the snow from the slope. Avalanches rarely occur on slopes less steep than 30 degrees.