Meteorological aspects of the Tumut fire experiment

A mass fire experiment was held on 1 April 1968 in Bondo state forest near Tumut, New South Wales, in which a 32.6 acre compartment of felled, dried eucalypt timber was ignited at many points simultaneously. The weather was mild at the time of ignition, the ambient temperature being 66°F, the relative humidity 45 percent and the surface wind velocity WSW at 4 m sec-1. The surface wind flow was turbulent up to and at the time of ignition. Atmospheric soundings in the region indicated conditional instability to 14,000 ft or higher, so that the release of heat energy and water vapour by the are led to the formation of a cloud-topped convection column, a process which was assisted by the light wind structure aloft; wind speeds to 8000 ft ranged from 1 to 4 m sec-1. Six anemometers at 2 m and three at 10 m above ground level were located up to 90 ft from the fire around its perimeter. After allowing for the ambient wind, the fire induced an inflow with speeds as high as 15 m sec-1 normal to the fire perimeter. Fluctuations in the total inflow coincided with variations in fire intensity. There was some evidence of a light wind component tangential to the fire perimeter and directed in a cyclonic sense, but the effect was largely masked by an upslope component of the fire-induced wind. Vertical wind speed was measured at 10 m above ground near the fire centre. The record showed very turbulent wind flow during and following the active phase of the fire, with sustained but light downdrafts for periods of about one minute. The mean upward speed reached a maximum value of 2.9 m sec-1. A significant positive correlation was found between the range of vertical wind gusts at 10 m and the inflow component of the wind at the 2 in level.