Abrupt surface drying and fire weather Part 1 : overview and case study of the South Australian fires of 11 January 2005 Graham

It is well known that low relative humidity is an ingredient that leads to higher fire danger due to the relatively rapid response of fine fuels to atmospheric humidity. Rapid fluctuations in relative humidity, though, have not been addressed extensively in fire weather studies or operations, perhaps partly due to the difficulty of observing such fluctuations and the even greater difficulty in forecasting these. In spite of this, fire managers have for many years been well aware of the threat posed by ‘dry air aloft’. In recent years, however, a small number of studies have begun to emerge that discuss the association of some extreme fire behaviour events with short-period reductions in relative humidity near the surface. Mills (2005a) described the association of mid-tropospheric dry air, clearly visible in 6.7 μm ‘water vapour channel’ imagery (WVI) from the GMS-5 geostationary satellite with the observed extreme reduction in relative humidity observed at Canberra Airport on the day of the disastrous fires on 18 January 2003, and questioned whether this dramatic reduction in surface humidity may have affected fire behaviour on that day through its effect on fine fuel moisture content (Luke and McArthur 1978, p.88). Mills (2005a) Abrupt surface drying and fire weather Part 1: overview and case study of the South Australian fires of 11 January 2005