The technique of prescribed fires is a common tool in land management across the United States. Some of the major concerns associated with prescribed fires are operating within prescription parameters, air quality and visibility impacts. In rare cases, visibility impacts by the smoke resulting from prescribed burns have been intensified by surrounding micrometeorology and have traveled over roadways causing major vehicular accidents. Such events have occurred in 2000 on the I-10 and I-95, 2001 on the I-4, 2006 on the I-95, and 2008 on the I-4 causing numerous fatalities, injuries, and damage to property. The combination of smoke and fog in these situations reduced visibility over the roadways to less than three meters, forming a condition defined as superfog. Our research focuses on the formation of the rare phenomena of superfog with the goal of enabling land managers to effectively plan prescribed burns and prevent such events.

A physical model in the laboratory has been developed to characterize the properties of smoke resulting from smoldering pine needle litters at the PSW Forest Service in Riverside, CA. Temporal measurements of temperature, relative humidity, sensible heat flux, radiation heat flux, convective heat flux, particulate matter concentrations and visibilities have been carried out.

The range of temperature was between 30-70 degrees centigrade and relative humidity between 10-20% measured by Campbell Sci. HMP45C temperature and relative humidity sensors. Sensible heat flux measurements of up to 3 kW m^-2 were obtained using Campbell Sci. CSAT3 sonic anemometer. Fine particulate concentrations of over 160 mg m^-3 were measured with TSI DustTrak. Radiation and convective heat flux measurements, obtained by Hukseflux Thermal Sensors RC01, were less than 1 kW m^-2 showing convective forces dominated in the smoldering phase. A custom visibility measurement system that utilizes a laser and an optical power meter was designed. For specific cases, a meter above the fuel bed, visibilities of less than 1 meter were measured. A detailed description of methods, experimental parameters, and measurement results will be presented with a special focus on the parameter ranges that lead to visibilities of less than 1 meter.