A fire spreads because the burning zone delivers heat to fuel that has not caught yet, drying it, pyrolysing it and finally igniting it, so the front advances into ground it has already prepared. Radiation from the flame and from glowing char reaches forward through the air, dominating in deep fuel beds and at the surface; convection carries hot gas into the fuel ahead, and becomes far more important once wind or slope tilts the flame over the unburned fuel; conduction matters only within individual fuel particles, since air conducts heat poorly. Direct flame contact is really convection and radiation at very short range. The rule to carry is that spread stops when the delivered heat flux at the fuel ahead falls below its critical value, which is why a firebreak or a change in fuel can halt a front. The confusion it resolves is picturing spread as flame reaching out to grab fuel; the flame arrives at fuel that was already cooked and ready. After this Concept you can explain which heat transfer mode is driving a given fire front and what would break the chain.
Combustion in wildfire behavior is an oxidation reaction that reverses photosynthesis: hydrocarbon fuel plus oxygen combusts to yield water, carbon dioxide, and heat energy, and it requires the three…