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Effects of Moisture Released during Forest Burning on Fog Formation and Implications for Visibility

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  • 1 Forestry Sciences Laboratory, Southern Research Station, U.S. Department of Agriculture Forest Service, Athens, Georgia
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Abstract

Smoke from wildland burning in association with fog has been implicated as a visibility hazard over roadways in the United States. Visibilities at accident sites have been estimated in the range from 1 to 3 m (extinction coefficients between 1000 and 4000). Temperature and relative humidity measurements were taken from 29 “smokes” during 2002 and 2003. These data were converted to a measure of the mass of water vapor present to the mass of dry air containing the vapor (smoke mixing ratio). Smoke temperatures were processed through a simple radiation model before smokes were mixed with ambient air with temperature and moisture observed during the early morning on the days following the burns. Calculations show supersaturations implying liquid water contents (LWC) up to 17 times as large as LWC found in natural fog. Simple models combining fog droplet number density, droplet size, and LWC show that the supersaturation LWC of smokes is capable of reducing visibility to the ranges observed.

Corresponding author address: G. Achtemeier, Forestry Sciences Laboratory, Southern Research Station, USDA Forest Service, Athens, GA 30602. Email: gachtemeier@fs.fed.us

Abstract

Smoke from wildland burning in association with fog has been implicated as a visibility hazard over roadways in the United States. Visibilities at accident sites have been estimated in the range from 1 to 3 m (extinction coefficients between 1000 and 4000). Temperature and relative humidity measurements were taken from 29 “smokes” during 2002 and 2003. These data were converted to a measure of the mass of water vapor present to the mass of dry air containing the vapor (smoke mixing ratio). Smoke temperatures were processed through a simple radiation model before smokes were mixed with ambient air with temperature and moisture observed during the early morning on the days following the burns. Calculations show supersaturations implying liquid water contents (LWC) up to 17 times as large as LWC found in natural fog. Simple models combining fog droplet number density, droplet size, and LWC show that the supersaturation LWC of smokes is capable of reducing visibility to the ranges observed.

Corresponding author address: G. Achtemeier, Forestry Sciences Laboratory, Southern Research Station, USDA Forest Service, Athens, GA 30602. Email: gachtemeier@fs.fed.us

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