Steam fog

The coming of steam fog signals the coming of fall. Steam fog forms when the lake water is much warmer than the air over it. This often happens when the surface water is still warm from the summertime, but a clear autumnal night causes katabatic winds to spread somewhat colder air over the Lake.

The formation of the steam fog is, itself, interesting. Many other clouds form when water vapour is cooled. However, when steam fog forms it is the result of the mixing of packets of warm and cold vapour, the average temperature of which is the same after as before the mixing. Oddly, this can produce condensation: the fog.

Steam fog is seen here both in the foreground and in the background, where it is strikingly visible against the shaded mountain. The locally variable nature of katabatic winds produces a patchy and rapidly shifting fog. As the sun warms the mountainsides, the katabatic winds die and the steam fog vanishes.