Most incidents happen on smaller fires or isolated portions of larger fires.
Fires respond quickly to shifts in wind direction or wind speed. Flare-ups generally occur in deceptively light fuels. Fires run uphill surprisingly fast in chimneys, gullies, and on steep slopes. Some suppression tools, most notably helicopters and air tankers, can adversely affect fire behavior. The blasts of air from low flying helicopters and air tankers have been known to cause flare-ups and cause weak trees to fall.