Transport of Firebrands by Line Thermals †

Abstract

The motion of a strong line thermal in an unstratified atmosphere is modeled to estimate a bound for its capability to lift firebrand particles. It is found that the maximum height of a viable firebrand is roughly proportional to the square root of thermal strength. The horizontal distance traveled from the point of origin to the point where free descent begins is calculated for two wind-speed profiles with height, assuming the transporting thermal to be embedded in the windfield. This downwind drift distance is shown to be both significant and sensitive to the windspeed profile.