Knowing about rime ice is an essential and potentially life-saving bit of mountaineering knowledge. As part of our avalanche awareness and planning, it’s crucial that we know where the wind has been blowing from, in order that we can identify the dangerous lee slopes. (If you are not sure about what I’ve said already, then book a course now!).
There are many resources available to help us do this including the SAIS Avalanche Forecast and weather forecasts. But the mountains have their own influence on wind direction, especially on complex cliffs where identifying the local lee slope can be more tricky. One bit of knowledge that can help us is understanding rime ice.
Rime ice (or rime, or ‘riming’), forms when supercooled water droplets (in the air) make contact with a freezing cold surface (in the UK mountains, this usually means rocks). As the wind blows, each successive set of water droplets freezes onto the surface thereby creating increasingly large ice structures. Because of their nature, these grow into the prevailing wind and therefore indicate quite clearly the recent prevailing wind direction at that exact spot.
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.