Observations of Fire Whirls during a Prescribed Fire

We observed a prescribed fire on a grass hill to determine the conditions leading to the occurrence of fire whirls and their properties. Infrared and visible images showed that multiple fire whirls occurred in a large whirl. The large whirl occurred over two fire lines on a slope near the summit. Thirteen fire whirls occurred on and near the two fire lines. Nine fire whirls for which the direction of rotation could be determined rotated cyclonically. Their direction of rotation was the same as that of the large whirl. Although the large whirl travelled, all the fire whirls appeared only where the large whirl was present. The circulations of three fire whirls that could be determined were approximately the same. These results indicate that all of the fire whirls may have been caused by the large whirl. We determined the size, tangential velocity, location, travel speed, and duration of the fire whirls and the conditions of the fire and meteorology.