Two fuels of identical chemistry behave completely differently depending on particle size, and the property that captures this is the surface-area-to-volume ratio: exposed surface divided by the particle's volume, in units of one over metres. A grass blade or pine needle has an enormous ratio, so nearly all of its mass sits within a millimetre of a heated surface and it reaches ignition temperature almost immediately. A log has a small ratio, so heat must travel inward by conduction and most of its mass stays cool while its outside chars. This is why fire scientists sort dead fuels into size classes, and why those classes also set how quickly a fuel exchanges moisture with the air, the timelag idea taught with fuel moisture elsewhere in this path. The rule is simple: high ratio means fast to dry, fast to ignite, fast to be consumed; low ratio means slow in all three. The confusion it clears is that a big fuel is not a dangerous fuel for ignition purposes; fine fuel carries the fire and coarse fuel sustains the heat afterwards. After this Concept you can predict which fuels in a mixed fuel bed will carry the flame front.
Wildland fuel characteristics govern fire behavior through a set of structural and moisture-related parameters: surface area-to-volume ratio (how much fuel surface is exposed to the environment, cont…