New Zealand Mountan Safety Council

Online Course - Terrain - Aspect

 

Aspect – Exposure to Wind and Sun

The aspect or direction a slope is facing plays an important role in the creation of avalanche conditions.

Measure the aspect of a slope using a compass.


Lee Slopes

When the wind blows, snow is taken from upwind areas and gets redeposited in downwind, sheltered areas. We call the sheltered areas lee slopes (from leeward). You can see this a lot around terrain which changes abruptly like ridge lines. Signs of lee slope snow loading include pillow shaped snowdrifts and cornices.

Cornices often form on the lee side of ridges and are a great clue to show you which way the wind has been blowing. In the two photos below the wind has been blowing from left to right. Note how the wind has stripped off the snow on the windward side (left hand side) of the ridge in the first photo.

 


 

 

Lee slopes are not always at the top of a slope. In the picture below you can see where the wind has swept across the mountain face (left to right), overloading the side of the vertical gullies. We call this ‘cross loading’.


Mountains act like large sails sticking up into the sky. They bend and deflect the speed and direction of the wind as it flows over the terrain. We call this ‘local wind effect’, and it is always present. Example: Even if you know it’s been blowing Northwest all night, and expect to see wind drifted snow piled up on Southeast (lee) aspects, there will be variation due to ‘local wind effect’.

Shaded Slopes

Slopes facing south (southerly aspects) will remain colder as they receive little or no sun. New snow will take longer to stabilise, and weaknesses in the snowpack can persist.


Sunny Slopes

Slopes facing North (northerly aspects) receive the most heat from the sun. When this heating is intense or rapid it can quickly weaken the snow on these aspects. It is common in springtime to see naturally triggered loose snow avalanches in the middle of the day.

 

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Avalanche Forecast Regions:
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Mountainf Safety Council websiteAdventure Smart websiteNew Zealand Avalanche Center