Onset of flow-induced fingering in bushfires

On a relatively large length-scale, the fire-fronts of wind-driven bushfires are sometimes seen to develop into curved shapes, suggesting that a linear fire-front becomes unstable. A mechanism for this instability can be identified if the hot plume of the fire is considered to partially block the air-flow from below, while stratification of the atmosphere restricts upward displacement. Downwind of the fire this causes a speeding up of the component of the average horizontal flow in the direction normal to any part of the fire-front. The perturbation in the horizontal wind that results from a perturbed shape of the fire-front shows an increase in the flow of air into the fire at more advanced parts of the front, normally resulting in an increased burning rate which would therefore increase the size of the perturbation.