Watch for steam devils in late November/early December

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Watch for steam devils in late November/early December

One of the many advantages of living near large lakes in temperate climates is the steam fog that shrouds them in fall and early winter. Fog is essentially a ground-hugging cloud, composed of tiny liquid water droplets.

Steam fog will develop at this time of year if the air above the lake has very low relative humidity, and a lot of lake water evaporates into invisible water vapor. The more vapor in the air, the closer the air comes to being saturated. When the air finally becomes 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 and the water at the lake surface, there will also be considerable turbulence in the air over the lake. When a strong cold front passes through, strong winds will carry cold air toward us, and 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 physically taxing due to the cold, if you ever have a chance to watch steam devils parade across a cold, watery lake, do not pass it up!

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.