Fog can occur any time of day throughout the year. In southern Wisconsin, fog is most frequent in the morning, between about 6 and 8 a.m.
Fog formation is the result of two primary processes. First, the air is cooled to the dew point, and the relative humidity reaches 100%. When the air temperature is the same as the dew point temperature, condensation of the atmospheric water vapor occurs forming fog droplets. Another method of fog formation requires water to evaporate from the surface into the air, increasing the relative humidity until condensation occurs.
During this time of year, as the nights are getting longer, we can have clear skies and calm winds at night. These conditions help the ground, and the air near the ground, to rapidly cool. As the temperature of the air drops, the relative humidity increases as the dew point approaches the temperature, leading to fog formation. Light winds can help with fog formation as they gently mix moist air near the ground upward. Strong winds mix the air near the ground with the drier, warmer air above, keeping the air near the surface from saturating.
If you take a long, hot shower, you may “fog up” the bathroom. Some of the warm water droplets from the shower evaporate into the cooler bathroom air, moistening it to saturation and forming a fog. Evaporation fogs also form over lakes when colder air moves over warmer water. Evaporation of the warm water into the air causes the relative humidity to increase and a fog may form.
Fogs often dissipate with the morning daylight. This is sometimes referred to as the fog ‘burning off’, but that analogy is not correct. When the sun rises, the air and ground warm up, which increases the air’s capacity to hold water vapor. Since fog is a cloud of liquid water droplets the warming encourages the fog droplets to evaporate.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
