Spotting is fire spread that skips ground. Burning fragments, called firebrands or embers, are lifted from bark, branches or structures into the convection column, carried downwind while still glowing, and land in unburned fuel, where they act as pilot ignition sources. Three separate things must all succeed for a spot fire to start: the ember must be produced and lofted, which depends on the fire's intensity and on bark and fuel type; it must survive its flight without burning out, which favours dense chunky brands over light flakes; and it must land in a receptive fuel bed, dry and fine enough to be ignited by a small glowing source. Spot distances range from a few metres to many kilometres in extreme fires. The rule this gives is that spotting breaks the assumption a continuous front rests on, so a firebreak, a river or a road stops a spreading front and does not stop spotting. After this Concept you can explain why a fire appears far ahead of its front and what conditions make that likely.
Wildfire spread is governed by three physical mechanisms: convective flame impingement, radiative heat transfer, and firebrand showers, in which combusting material loses structural integrity, is lof…