Steam Devils take place because of low level instability over a moisture source. What they do is focus transport of latent heat of evaporation from a warm source to a cool source. They typically take place in the autumn and early winters over bodies of water that contain residual summer heat. They can also occur, in fact quite frequently, just post a summer shower where ample sunshine took place during the forenoon hours. Another name for them is ‘steam dogs’.
To understand more fully how they work think of micro-tornadoes. They are rotating columns of air, just as in their cousins Dust Devils and their more distant cousins Tornadoes and even Hurricanes. All of these act as organized convective processes that operate to balance instability, in other words, make conditions stable.
It is a typical June morning. The day dawns with dew on the rose buds that quickly evaporates as the sun climbs the sky and the air begins to heat. All those dew drops evaporating off the land-scape, and even from the soil moisture and water sources, lifts water vapor in huge quantities into the air with the heating of the day. Eventually, rising water vapor-rich air forms cumulus clouds. On days when these convective clouds are particularly vigorous they will produce showers. These showers may move across your location in a brief interval of heavy rain, perhaps even some thunder. Just as soon as the event began, the storm cell moves off and the sun reappears.
The shower cools the airmass down to a more stable temperature – a temperature where the air is no longer compelled to rise. However, the sun shining on all those puddles of water, or the wet street, and roof tops, heats those surfaces very quickly in the near Solstice sun. Steam is created as moisture explosively attempts to evaporate into the cooler surrounding air mass. Usually, the steam just rises in a vagariously wavy fashion, unpredictably lurching from left to right. However, in some instances, the combination of lefts and rights will combine in such a way to create small-scale vorticity; whereby the rising steam plumes find a more efficient means to balance the system.
This question of vorticity is a complex one. In simplest explanation, it is a measure of a fluid medium’s rotation. Once that rotation is established, the air immediately above the steaming surface interface actually rises faster than in the interface its self. This stretches the tube of rotating air, speeding up the rate of rotation do to centripetal acceleration (inward force). An analogy is a figure skater pulling his/her arms inward, speeding up their rate of rotation. The same process takes place speeding up the rotation rate of the Steam Devil. Relative to the surrounding atmosphere, this creates lower pressure in the core of the rotating column of air. This draws the surrounding atmosphere (usually more steam) into the column. This sustains the Steam Devil action for a duration of time. The steam is visible as it rotates into the column air.
They are quite interesting to watch if you are a weather phenomena enthusiast. They typically take on structures similar to a stove pipe and may at times sinuously wobble around a common axis. You can see them dancing across black-top parking lots, along some roof tops in the summer, or perhaps over pond surfaces in the early morning of chilly autumn days. They typically last for just a moment or two and only extend a few feet vertically. In extreme cases, however, arctic air masses passing over the Great Lakes and oceanic areas can cause exaggerated instability of this same kind. These can be considerably larger in diameter (up to a few meters), and can extend to altitudes of several hundred feet! They are the weakest of the rotating air phenomenon, however, and will not produce any damage.
