Exploring the Wind Limit: A Collaborative Project with the Institute for Building and Home Safety
Wind is a dominant environmental variable in wildland fire spread and intensity. The fire model used to support wildland fire management in the U.S. assumes that above some wind speed the fire will actually decrease in rate of spread. A widely referenced study on this phenomenon suggests that this limit is about 25 mph for fires in grasses. However, no other data exist exploring this phenomenon based on controlled experiments. A research team from the Missoula Fire Sciences Lab, including FFS Mechanical Engineer Bret Butler, Physical Scientists Cyle Wold and Paul Sopko, and Meteorologist Larry Bradshaw recently traveled to South Carolina to study how wind affects fire spread rates. The Institute for Building and Home Safety (IBHS) exists to study how wind, rain and hail affect buildings and homes. This facility has the capability to generate winds up to 130 mph and to replicate actual fluctuations in wind speed and direction. Bret and IBHS scientists designed a series of experiments to explore the response of fire in beds of pine needles to high-speed winds. The research team completed 17 separate experiments over the course of one week. Each experiment consisted of two parallel pine needle beds measuring 3 m wide by 24 m long by 7 cm deep. They then recorded the time for each bed to burn using temperature sensors, video cameras and infrared cameras. The team tested wind speeds from 10 mph to 45 mph at the surface of the fuel bed, which corresponds to winds at 20 feet more than 50% greater. The data suggest that even at the highest wind speeds fire rate of spread continues to increase, contradicting the earlier reports based on fires in grass.