Like a thin tornado in reverse, steam devils form spiraling columns of microscopic droplets of liquid water, reaching up towards the skies above. While tornadoes stretch down from the clouds and become waterspouts when over bodies of water, steam devils originate primarily on small bodies of water with an ambient temperature significantly higher than the air above them, then extend upwards in a spiraling vortex.
Steam devils are ephemeral, meaning short-lived, atmospheric phenomena, lasting from just a few seconds to several minutes. They are also very rare, because they require specific conditions to form, and even when those conditions are met, there is still a large element of chance involved.
They can occur when a frigid or arctic air mass displaces a warmer air mass over a lake or pond, normally in a narrow region of the planet encompassing the border between the temperate and sub-arctic zones. This results in the ambient temperature of the water being considerably higher than that of the air above it. Water vapor evaporates from the surface of the lake, but because the air above is so much colder, the saturation vapor pressure for water in air at that temperature is quickly met, forcing some of the water vapor to condense back into liquid water in the form of micro-droplets.
Under normal circumstances, micro-droplets of water are not visible beyond perhaps a slight haze. But in such situations as this, they are sufficiently close together to be visible. This is the same basis for the visibility of mists, which are also composed of micro-droplets of liquid water. In fact, if steam devils do not form, mist is what you are likely to see instead, or as well, even if they do form.
Air with a high water vapor content is “lighter” than air with a low water vapor content, because the molecular mass of water vapor is 18.02 while the molecular mass of dry air, all the components of air besides water vapor, is 28.98. Air pressure is directly related to its mass, so the air just above the lake’s surface has a lower pressure than the air above and to the sides, causing it and the micro-droplets of water it contains to rise. Again, this is exactly how mists rise from the surface of lakes.
This is where the high winds and chance come in. If the winds strike the rising micro-droplets just right, they can impart an angular velocity that causes their rise skywards to be in a spiraling motion. Once started, the spiral or vortex can self-perpetuate for varying lengths of time, up to a few minutes. During this time, the steam devil may climb as much as 1500 feet, giving the appearance of a thin white tornado.
