Surface Hoar

The days in Alpe d'Huez have been very warm recently, yet the nights have still been very cold. Cold calm nights create the perfect conditions for surface hoar to grow. Similar to how the windscreen of your car can be covered in frost by the morning, surface hoar is effectively the same thing, but forms as ice crystals growing on the snow. During a clear nights sky a tremendous amount of heat radiates away from the surface of he snow, making the snow surface much cooler than the surrounding air. The moisture in the air above then condenses on the snow creating these tall thin ice crystals. They have a very pretty appearance, glistening in the sun, and as you ski through them you can even hear a light jingling sound as you knock them over.



Despite their pretty appearance, if buried within the snowpack, surface hoar can present a real danger. Stacked up a bit like dominoes, the crystals can take a lot of weight from above, for instance they can hold up metres of subsequent snowfall. But, also like dominoes, when a force acts upon them from the side they will all fall down. In our case this could be a snowboarder making a turn in the snow above, and thus triggering an avalanche.



Furthermore, once buried, surface hoar is very difficult to detect. Also, it tends to form in a hard to predict pattern across the mountain. Sometimes, it may only occur further up the mountain, where higher peaks rise above the clouds. Other times it may only occur in sheltered valleys, where the wind can't blow it over. This can lead to buried 'pockets' across the mountain, where surface hoar is present.

buried surface hoar.jpg

Thankfully, the high temperatures due today will likely melt most of the surface hoar, meaning that when the snow comes again in a few days, the ice crystals will be gone. However, it is worth remembering that on some north facing slopes, the suns impact won't be felt, and the surface hoar could remain a significant danger.