In favourable atmospheric conditions, large hot fires can produce pyrocumulonimbus (pyroCb) cloud in the form of deep convective columns resembling conventional thunderstorms, which may be accompanied by strong inflow, dangerous downbursts and lightning strikes.  These in turn may enhance fire spread rates and fire intensity, cause sudden changes in fire spread direction, and the lightning may ignite additional fires.  Dangerous pyroCb conditions are not well understood and are very difficult to forecast. 

Building on a conceptual study of the thermodynamics of fire plumes that examined the influence of a range of environment and fire property factors on plume condensation, a method was proposed (in previous work) for identifying on an atmospheric sounding the minimum plume height (z_fc) and plume buoyancy (b_fc)at which free moist convection can develop.  Applying z_fc and b_fc to solutions to the Briggs analytical plume model the net buoyancy flux and total heat flux (often expressed as “firepower”) required for pyroCb formation can be estimated from a single atmospheric sounding.  This quantity is the PyroCb Firepower Threshold (PFT).  Spatial maps of PFT will be presented that provide valuable insight into how and why pyroCb developed in a number of fires.