Q: What is an ice fog?
A: Each year about 700 fatalities occur in the United States as a result of traffic accidents during fog. A combination of high speed and low visibility is often to blame.
Fog is a cloud in contact with the ground. When the relative humidity approaches 100 percent, water vapor condenses on tiny particles suspended in the air to form a suspension of small water drops. The air in contact with the ground can reach high humidity if it cools or when water from the surface evaporates into it. Either of these processes increases the relative humidity of the air.
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Ice fog is a type of fog consisting of fine ice crystals suspended in the air. It occurs only in cold areas of the world as water droplets suspended in the air can remain liquid down to minus 40 degrees.
More common than an ice fog is a freezing fog. A freezing fog occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime. If there is a very light wind on a day with freezing fog, the wind can blow the droplets in one particular direction. As a result, small spikes of ice can grow into the wind on objects like trees and fences.
The freezing fog can make roadways very slippery and dangerous. The danger is compounded by the poor visibility that accompanies the fog.
"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences.