Exploring Kootenay Lake

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.

You can stick your finger into a glass of water, but not through a block of ice.

That pretty well explains what is going on in this morning’s picture of fallstreaks.

Water molecules are not as tightly bound together in the liquid as they are in the solid. This means that, at the same temperature, fewer water molecules will escape (evaporate) from an ice crystal than from a water drop.

Wry comment Over two centuries after it was shown to be nonsense, there are still teachers who ignorantly tell their students that clouds form because “cold air cannot hold as much water vapour as warm air.” This sad misrepresentation of reality is rejected by all modern physics. The issue is discussed on this site in an article called condensation. Alas, the air lacks such a holding power and air is actually irrelevant to the issue.

Imagine a small volume in which water drops and ice crystals are together surrounded by water vapour (It is humid). Yes, all three phases—vapour, liquid, solid—frequently coexist at temperatures below 0C.  The vapour molecules will condense on drops and crystals at the same rate. But, it is much easier for the loosely bound molecules in the liquid to evaporate than it is for the tightly bound molecules in the solid. The result is that often there are more molecules arriving than leaving the ice, while there are more molecules leaving than arriving on the liquid: the crystals are growing; the droplets are shrinking.

In the picture, below, the tufts of whiter cloud in the upper portion of the picture are composed of water drops; the greyer streaks that hang below them are of ice crystals. The original water cloud was larger, but when some of the drops froze, they (now being ice) grew rapidly while the remaining water drops shrunk. The increasingly large crystals had a larger terminal velocity and fell below the original water cloud to produce these fallstreaks.

And all this happened for the same reason that it is harder to stick your finger into a block of ice than a bucket of water.