On the formation and persistence of superfog in woodland smoke

Dense fogs, comparable to historical fogs in England, have been implicated ill numerous roadway accidents in the southern United States. Many of the fogs have occurred in association with prescribed burning. Direct measurements of superfog (fog reducing visibility to less than 3 m) were taken during burning of forest litter on 22 March 2003. Visibility was measured at 0.1 m implying all extinction coefficient of 39120 km(-1). The number of condensation nuclei required for fog to produce the observed visibility was about 1% of the number of particles released in wood smoke as reported in the literature. A recursive non-gradient mixing model shows (1) maximum excess liquid water (LWC) released was approximately 7.0 g kg(-1), 23 times the LWC in natural fog, (2) superfog can form at the site of combustion then continue to form as it drifts downwind, and (3) superfog can modify air mass stability near the ground and persist for hours until dispersed by changing wind conditions or by solar heating after sunrise. Published in 2008 by John Wiley & Sons Ltd.