This tip is all about Sastrugi which, along with rime ice, are very useful in determining what the local wind direction has been recently. This knowledge plays a crucial part in our avalanche decision-making process by indicating where the dangerous lee slopes may be.
Sastrugi are slightly reminiscent of sand dunes in the snow. Like rime ice, the direction of the wind that they indicate is slightly counter-intuitive. The wind erodes and carves sastrugi out of areas of hard-packed snow. The wind gradually wears away at the windward side of some hard-packed snow. Then it deposits the snow crystals on the sheltered side of the increasingly growing feature. The steep, scarp side is the side that has been carved and shows the direction of that the wind has come from.
Sastrugi can be just a centimetre or so high, but in time they can also become several feet high. When they have grown to sizes like this, they can be physically difficult to move across. This is particularly so if you are on skis or towing a sledge.
Knowing where the wind has come from then makes it clear where the leeward (sheltered) side of any slopes are. This is where the wind will deposit snow, potentially (or most likely) creating areas that are dangerously avalanche-prone.