Q: What are steam devils?

A: One of the many characteristics of large lakes is that they are often shrouded in steam fog in the fall and early winter.

Fog is essentially a ground-hugging cloud, composed of tiny liquid water droplets. The steam fog that occurs at this time of the year will develop if the air above the lake has very low relative humidity. In such a case, there is a lot of evaporation of the lake water into invisible water vapor. The more vapor that resides in the air, the closer the air comes to being saturated. When the air actually does become saturated, some of the vapor begins to condense into liquid water and the steam fog begins to form.

When there is a large difference in temperature between the air above the lake and the water at the surface of the lake, there will also be considerable turbulence in the air over the lake.

If, as when a strong cold front makes its way through Madison, the winds that carry to cold air towards us are strong, the combination of steam fog production, turbulence over the lake, and strong background winds can create one of nature’s most awesome spectacles – steam devils.

Steam devils are swirling columns of steam fog (analogous to dust devils) that can look like tornadoes of steam fog on a windy, cold winter day. Though it can be a physically taxing experience due to the cold, if you ever have the chance to watch steam devils parade across Lake Mendota or Lake Monona, you should not pass it up.

- Steven A. Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA Radio (970 AM) the last Monday of each month at 11:45 a.m.

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