Conditions associated with cold season dense fog in the midwestern United States.

摘要

Dense fog events in the Midwestern United States can severely impact air and ground transportation. According to Illinois Department of Transportation data, in Illinois outside of the Chicago area, some 4000 collisions occur annually under foggy conditions, resulting in some 30 deaths per year and millions of dollars in damage. A physically-based operational forecast model, however, remains elusive for this region.; Environmental conditions associated with cold season dense fog were examined through the development of a climatology based on 49 years of hourly surface data. The study focused on Peoria, Illinois, a site centrally located in the Midwest and minimally impacted by topographic and urban effects. Daily precipitation and snow depth data, 4-inch soil temperature measurements, and twice daily upper air data also were examined. Events from 1970--1994 were classified by synoptic type employing 3-hour surface weather data. Fog events were grouped by dense fog longevity, with long duration events lasting more than five hours and short events less than three hours.; The most common synoptic regimes for dense fog events were: high pressure (26%), low pressure (23%), pre-frontal (18%) and frontal (15%). The frequent association of fog with fronts and low pressure verified results of earlier studies (Stone, 1936; George, 1940; and Byers, 1959). Further, light precipitation often was observed upwind and at dense fog onset for both long and short duration events associated with fronts and/or low pressure, suggesting precipitation may be an important source of moisture for these fog events.; The physical dimensions and onset time of dense fog appeared to be the best predictors of fog duration for all types of fog. Longer lasting events tended to form earlier in the night, to be more opaque and widespread, and to dissipate several hours later than short-lived events. The longer durations may result from the absence of the destructive influence of solar radiation, and more opaque and widespread events may require more time for dissipation.