How does fog form?

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How does fog form?
fog hovers over 8-lane freeway in California

Photo: NASA

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 relative humidity if it cools or when water from the surface evaporates into it.

Fogs are named for the ways in which they form. Common types of fog in Wisconsin include: radiation fog, advection fog, and evaporation fog. Radiation fogs form on clear, calm and long nights when the ground and the air near it cool by radiation. As the air temperature drops, the relative humidity increases and fog can form. Early morning fogs are often radiation fogs.

When warm air is advected (blown horizontally) over a cold surface, the air near the ground cools as it exchanges energy with the surface. The relative humidity of the air increases with cooling and an advection fog may form.

If you take a long hot shower, you may “fog up” the bathroom. Some of the warm water from the shower evaporates into the cooler bathroom air, moistening it and forming a fog – an evaporation fog. Evaporation fogs also form over lakes when much colder air moves over warmer water.

Each year about 700 fatalities occur in the United States as a result of traffic accidents during fog. High speed and low visibility are a dangerous combination.

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.