This paper studies fire whirls formed behind an L-shaped wall in a crossflow. Wind-tunnel experiments at various crossflow velocities were conducted, and it was found that there was a narrow range of cross-flow velocity that led to the formation of an intense and stable fire whirl, i.e., the existence of a critical wind velocity. Scaling analysis and computational fluid dynamics (CFD) calculations of different scales suggested that the Froude number is the governing parameter of the phenomenon; the critical wind velocity is therefore roughly proportional to the square root of the fire size. Particle image velocimetry (PIV) measurements showed that the rotational velocity component was reduced near the bottom floor, which then induced a fast radial inflow toward the axis in the vicinity of the floor. This radial inflow pushes the flame toward the fuel surface, enhancing heat transfer between flame and fuel and thereby leading to the formation of an intense fire whirl. The inflow velocity was much slower when the crossflow velocity was outside the critical range. Finally, it was demonstrated that the formation of an intense fire whirl could be prevented by blocking the near-floor flow using an obstacle. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.