Ice Fog

Nature Watch
January 28, 2009

By Susan Benson,CNHM Director of Education

“Wind chill advisories” and “below zero temperatures;” these are phrases we have become quite familiar with as regular parts of our vocabulary this winter. Throughout all of these cold nights, I’ve been interested by the fog we’ve woken up to on and off in the past few weeks, and I had to look further for more information.

I first looked to recent weather forecasts to find some mention of fog, and was surprised to discover ice fog, a phenomena I was unfamiliar with. Ice fog, perhaps something that happens less often in northern Wisconsin, has been in forecasts in the past few weeks. In mid-December ice fog shrouded Big Bay State Park as arctic air met warmer Lake Superior waters, and ski trails are still impassable yet today as a result of this event. An event that arctic regions are more familiar with, ice fog can occur during long, cold nights, as the snow-covered ground radiates heat to space, and air temperatures at the surface become increasingly colder. Ice fog occurs when water droplets have frozen into tiny crystals of ice in midair. Usually, this occurs when temperatures are at or below -30 degrees Fahrenheit.

During clear night skies, incoming solar radiation is greatly reduced, and the snow surface loses more energy than it gains. Because of this energy loss, the snow surface gets colder, cooling the air in contact with it, and making a colder layer of air that develops over the snowpack. This condition is called a temperature inversion because it is opposite of normal conditions when the air close to the ground is warmer than the air above it. When calm air is present at night, this inversion can develop and become cooler, sometimes going even several hundred meters above the ground surface, and creating conditions for ice fog.

Ice fog can also occur in urban areas due to water vapor present in automobile exhaust fumes or smokestack emissions. Normally these pollutants would rise into the atmosphere because they are warmer and lighter than the air into which they are emitted, but when conditions are right, these strong temperature inversions cause the polluted air to become trapped at ground level, resulting in the formation of ice fog.

Some of us might be feeling cabin fever as these cold temperatures and wind chills keep us feeling stuck more often indoors, but there are still interesting events that can be learned about and experienced here in the north woods. If you find these winter blues keep you indoors, perhaps you will find your way to your local library to read more about the life here in the cold, enjoy winter views through the windows of your warm home, or just dress for success in many warm layers for your outdoor excursions.


Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 13470 County Highway M or on the web at http://www.cablemuseum.org/ to learn more about exhibits and programs.