I've seen dust devils during nil wind forming on a frozen lake. (On a = cloudy day). Any explanations for that then?

From : Don Stackhouse

There's a very common misconception out there that says that the air has to be warm to form thermals. This is not true, thermals can form at essentially any temperature that's warmer than the freezing point of air. Trust me, if it's too cold for thermals to form, you have much more important things to worry about!

Thermals are driven by DIFFERENCES in air temperature; it's not that warm air rises, it's that WARMER air rises. A patch of 3 deg.F air will rise in a zero deg.F overall air mass just the way that a patch of 73 deg.F air will rise in 70 deg.F air, or a patch of 103 deg.F air will rise in a 100 deg.F air mass. It's not the temperature that matters, it's the temperature difference.

Anything that causes a significant difference in the temperature of a local chunk of air relative to its surroundings can sire a thermal. Note, in general it takes a massive amount of heat energy to influence a significant amount of air to form a usable thermal (the heat from a backyard barbecue grill, for example, doesn't heat a large enough volume of air to make a big enough thermal for most models to use effectively, and the tiny thermal it does make is too small, concentrated and turbulent for practical flying).

Just to put this in perspective, just the water droplets alone in a small cumulus cloud (which is a very small percentage of the mass of the air in that cloud) typically weigh more than several tons. The heat energy involved in just one average-sized summer thunderstorm is roughly equivalent to thirty of the atomic bombs dropped on Hiroshima and Nagasaki. Nature operates on a truly grand scale.

The traditionally discussed thermal generators involve large surfaces (such as a plowed field) that get warmer than their surroundings in response to sunlight, warming the air above them and causing a thermal. However, the inverse effect works as well. For example, a forest absorbs just as much sunlight per square foot but generally heats up more slowly in the daytime sunlight than the plowed field beside it, so during the day there is usually an updraft over the plowed field and a downdraft over the cooler forest. However, as dusk approaches and the sun gets low in the sky, that plowed field will cool off faster than the forest, and soon the forest becomes the warmer area. In the evening you will often find an updraft over the places that had downdrafts during the day.

Large bodies of water can absorb massive amounts of heat energy with relatively little change in temperature. They can also give off massive amounts of heat while staying very temperature stable. This is the reason for the familiar on-shore and off-shore breezes typically found along coastlines. During the day, the sun warms up the land while having little effect on the water temperature. The air sinks over the cooler water and rises over the warmer land, causing the wind to blow from the water towards the land. At night, the land cools off rapidly, soon causing the water to be the warmer surface. The air now rises over the water and sinks over the now cooler land, causing the wind to blow in the opposite direction.

Justin, that lake spent all summer absorbing energy from the sun, energy that is now stored in the millions of gallons of relatively warm water that's still underneath that relatively thin layer of ice. Meanwhile, the surrounding land and air mass has cooled off till it's much colder than the water temperature. We know that this is the case because the water on the surface has frozen to form that layer of ice, which means that the air must be absorbing heat from the water fast enough to chill the surface layer of water below the freezing point (if it didn't, then the above-freezing water below the surface would melt all of that ice).

Since the air above the lake is obviously absorbing massive amounts of heat energy from the water in the lake, then the air over the lake is getting heated to a warmer temperature than the air over the surrounding colder land. This creates a temperature difference between the two air masses, resulting in a thermal over the lake, powered by the sun's energy that was slowly and patiently accumulated during the entire summer by the water in the lake. Pretty cool, eh?

Don Stackhouse
DJ Aerotech