While skiing in the Jura Mountain Range, in Switzerland, several skiers were treated to this incredible sight, a wave of fog seemingly rolling over and down a mountain.
A combination of geographic and weather conditions is required for fog avalanches to form.
A long, flat ridge is required. The windward side of the ridge needs to have a long steep slope – ideally a cliff – fronted by several kilometres of level country. On the other side of the ridge, which is sheltered from the wind, the slope needs to be less steep so that the fog can slowly drift downwards.
At the same time, to get conditions like those seen in the Jura, you need an upper fog line at the same altitude as – or just above – the ridge line. A high-pressure area is also needed to ensure stable atmospheric conditions. In addition, the wind must also be blowing down on the ridge line as close as possible to vertical so that fog can be pushed along. Otherwise, it dissipates immediately.