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While prescribed fires are common tools in wildland management, a combination of smoke and fog, known as superfog, has in some cases crossed over major roadways, leading to multicar pileups and fatalities in visibility of less than 3 meters.
New research led by the University of California, Riverside, and sponsored by the USDI/USDA Joint Fire Sciences Program, has for the first time produced superfog in a laboratory. With a better understanding of how superfog forms, foresters may be able to add additional criteria in planning future prescribed burns.
Team - Smoke - Particle - Size - Distribution
The team also identified the smoke particle size distribution and concentration, ambient liquid water content, ambient temperature, ambient relative humidity, fuel moisture content and wind speed that leads to superfog formation. The authors caution, however, that the science of predicting when some of these conditions will be met is still in its infancy.
Fog forms when water molecules condense around microscopic solid particles suspended in the air, somewhat like dew forming around a blade of grass. Particles come from many sources, including dust, vehicle emissions, and smoke. In order for water to condense, the ambient air temperature must be cool enough to become saturated with water vapor introduced by processes like wind, evaporation, or plant respiration. When nearly saturated air blends with smoke and moisture released by smoldering organic materials, a dense, low-lying superfog can form.
Superfog - Researchers - Laboratory - Setup - Conditions
Because superfog is uncommon and hard to study naturally, the researchers designed a laboratory setup to explore conditions that create it. They burned wildland fuels, such as pine needles, in an air conditioned, custom-made fire wind tunnel under varying environmental conditions and fuel moisture content.
The team found that when water content is low, particle size needs to be small enough to create droplets no larger than one micron, small enough to fit 50 droplets in the diameter of human hair,...
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