For decades researchers have postulated that fog typically forms in the early morning hours following a calm, clear night. But a new study shows that a higher percentage of fog incidences–at least in the Midwest–occur under overcast skies when rain has fallen overnight.

Dense fog forming in conditions that are not well understood increases the difficulty of forecasting fog accurately, according to Nancy Westcott, climatologist at the Illinois State Water Survey, University of Illinois at Urbana-Champaign.  Fog is a safety hazard for ground and air travelers and can be particularly costly when air traffic is delayed.

Using hourly surface airways data of dense fog occurring at Peoria, IL, in 1970–1994, Westcott noted that 57 percent of fog events occurred under low cloud cover. Dense fog is defined as having at least one hour in which the visibility is one-quarter mile or less.

These findings differ from studies showing that fog most often forms under clear skies with a radiation-caused cooling effect at the ground, which then decreases the air temperature just above the surface and causes the air to become saturated with moisture.

In Westcott’s studies of the 1970 to 1994 period, 302 dense fog events occurred in Peoria between October and March. The most notable feature of the events was that rain or snow often occurred within one hour at the onset of dense fog events with low pressure systems and approaching storm fronts.

In the classic clear skies scenario, cooler temperatures can be detected at the ground surface. With cloud cover, there often is no evidence of cooling at the surface. Westcott speculated that radiation cooling at the top of the cloud layer can penetrate into the cloud deck, overcoming weak warm air advection and cooling the layer to supersaturation and fog.

In an in-depth case study of November 6–7, 2006, in Champaign, precipitation moistened the lower atmosphere and lowered the cloud base. Dense clouds exited Illinois by the early morning hours as the precipitation ended.

"As the low pressure system moved across the area, light rain falling helped moisten and precondition the region for fog," Westcott said.

Fog formed several hours later behind the departing clouds, suggesting that radiational cooling at the top of the low-cloud layer was critical in promoting early morning dense fog.

"This study provided some insight into the physical processes that can lead to the development of dense fog when low clouds are present," she said.  "It is important for forecasters to know that fog can occur with low clouds as well as during clear conditions."

This study was co-authored with David Kristovich, head of the ISWS Center for Atmospheric Science, and published in the Journal of Applied Meteorology and Climatology.

The Illinois State Water Survey, a division of the Institute of Natural Resource Sustainability at the University of Illinois at Urbana-Champaign, is the primary agency in Illinois concerned with water and atmospheric resources.