As the sun rose yesterday, a neat choreography of a katabatic wind created ephemeral sprites of steam fog, and abruptly ruffled calm waters farther offshore. In fifteen minutes, it was all over.
The night had been clear, so the infrared radiation lost by the ground was not offset by compensating radiation from the clouds. The mountainside cooled, which in turn caused the adjacent air to cool and begin to flow downslope. This drainage wind is called a katabatic wind, but it is usually so gentle as to be almost undetectable. Upon reaching the valley bottom, the air moved out over the Lake.
The katabatic air flowing out over the Lake was cooler than the centimetres-thick air already over the water. Certainly, the Lake had also lost energy by infrared emission, but convection in the water had brought warmer water from below to the surface and kept the adjacent air warm. It also kept this thin layer of air moist.
There are two distinct mechanisms that can produce condensation. The best known is vapour cooling. This is responsible for the formation of cumulus clouds and wave clouds. (It is usually explained by saying that cold air cannot hold as much water vapour as warm air, but this glib assertion has been known to be nonsense for over two centuries.) The mechanism for the formation of the steam fog seen here was that of vapour mixing. There was no net temperature change as warm and cool vapour from two sources mixed. (This mechanism also gives rise to the contrails trailing the engines of jet planes.) As the katabatic wind, with its cool vapour, flows over the surface air of the Lake, with its warm vapour, the condensation forms little convective towers of steam fog that drift with the wind.
As if the katabatic wind and the formation of steam fog were not interesting enough, something else odd took place. Lines of the steam-fog sprites trace the air flow offshore. Initially the wind does not ruffle the water’s surface — then abruptly, a little way out, it does. This pattern is strikingly consistent across the Lake: smooth water abruptly transforms into ruffled water.
We are used to seeing the wind drag on a water surface and raise waves. However, waves on water have a minimum speed of 23 cm/s. If the wind is gentler than this (or if a swimming bug moves slower than this), no waves are created. The katabatic wind flowing down the mountainside has been slowed by trees. But, as the air moves out over the water, it accelerates and at the threshold number of 23 cm/s, it begins to abruptly ruffle the water’s surface as it can now create waves.
This waltz between wind, water, and waves originated with a cooling of the mountainside by the net loss of infrared radiation. It ended quickly when the rising sun began to warm the mountainside. This killed the katabatic wind, and over a period of only a few minutes, the air stopped flowing out over the water, the steam fog vanished, and the Lake became calm.
Thank you for the dance.
A katabatic wind flows over warmer water making steam fog and ruffling the water offshore.