Sprites and a devil

Steam fog There are two distinct processes that may lead to the formation of cloud or fog. One is vapour cooling; the other is vapour mixing. Steam fog results from the second: two volumes of vapour with different temperatures and (vapour) pressures are mixed. Unexpectedly, even though the average temperature and pressure are not changed by such mixing, the humidity can now be above 100% resulting in condensation. See:
• a discussion of condensation
• other shots of steam fog and steam devils

Each clear morning for a month, the Lake has been a stage upon which there appeared dancing sprites. These animated elves are the translucent and ephemeral manifestations of steam fog.

Steam fog is a harbinger of autumn. As the season shifts, the lake water does not cool as rapidly as does the atmosphere. On a clear night gentle drainage breezes (katabatic winds) flow down the mountain slopes and out over the Lake. This places a shallow (~1m) layer of cool dry air over the top of a tiny surface (~1cm) layer of warm moist air immediately adjacent to the water. Steam fog forms as the vapour in the two layers mix in tiny convective towers. The fog gives visual form to the convection to produce the sprites that dance in the breeze over the Lake’s surface.

Steam fog is much harder to see before sunrise. It is the first touch of sunlight that usually reveals the world of dancing sprites. Alas, the sunlight will also often end the show when it warms the mountain side and stops the drainage winds.

Frequently there is a delightful scene of water birds swimming through the steam fog. These are Common Mergansers.

Solar warmth can put a quick end to the fog by warming the valley sides that had locally supplied the cool drainage wind. However in steep valleys, some slopes may remain in shadow and so continue to send cold air out over the water. Alternatively, the cold air might have blown in from more distant locations. In these cases, one might enjoy steam fog throughout the day. In this midday shot, the constantly shifting sprite-like convective columns stand out.

If there is steam fog and a somewhat brisker wind, I always watch for steam devils. These whirl winds are similar to dust devils, but it is fog, rather than dust, that enables them to be seen. My best pictures of steam devils were taken a few years ago (steam fog and steam devils), but last week I saw a distant one (and its reflection). Incidentally, this use of the word, devil, has no religious overtones. The root meaning of devil is to throw, and such whirl winds rightly gained the name because they throw things around.