Steam devils

When it is cold out, the lake often delivers steam fog; and, very occasionally, a steam devil.

Steam fog is not formed by the same mechanism as, say, cumulous clouds or even radiation fog. The latter form when the water vapour in the air is cooled below its saturation point. But, steam fog, and aircraft contrails, form by a non-linear mixing process when warm vapour is mixed with cold vapour. In this case, the warm vapour is in the bottom centimetres above the non-frozen water surface and the cold water vapour is in the overlying much colder air flowing over it. They mix and form a fog, although neither had a cloud in it to begin with.

The main time this happens around here is at night when cold vapour flows down from the mountain side over the warmer lake. One sees the fog over the water with the first light of daytime. And it becomes spectacularly visible when the sun gets up. But, the sun quickly turns off the process by warming the draining vapour from the mountain side and so stops it from flowing out over the lake. Thus, there is perhaps a quarter or a half hour of good observing before it all stops.

Now, steam fog is nice, but sometimes a strong wind shear turns a convective plume of steam fog into a steam devil. The word, devil, comes from a Greek word, diabolos, meaning to throw, which probably is the origin of devil in names such as dust devil and steam devil for they throw things. And a steam devil is a marvellous vertical structure: a whirlwind with a hollow core. 

Today’s brief view did provide a couple of steam devils, but none were quite as grand as one I saw sixteen years ago (which is shown last).

The sun gets up and temporarily shows steam fog on the lake, before it warms the mountain side and turns off the drainage.

A steam devil extends up from the lake. There is a hint of a hollow centre.

And another steam devil.

The steam devil from 2009, showing a well developed one.