Fuel size

Size of the material also affects ignition and burning. Fine or light fuels like grass, stubble, brush and leaves can ignite quickly, spread flames rapidly and burn out quickly. They have much greater surface area than larger fuels, which means more of the available fuel is exposed to the air necessary for combustion. Larger materials have a smaller surface to volume ratio and only the outside layer can burn, as with a log that must burn from the outside in. This is part of the reason for splitting firewood; smaller logs burn more intensely and efficiently.

Light fuels are also more easily affected by relative humidity and reach equilibrium with the air quickly. This means they can dry and become flammable relatively fast and can vary greatly throughout the day as conditions change. Fuels that were nonflammable in the morning due to an overnight shower or morning dew can become highly flammable later in the day if sunny conditions warm and dry the fuels, air temperature rises, humidity drops, and winds increase. Light fuels are also more easily dried by the heat of the fire itself. These characteristics tend to make fires in light fuels flashier and more intense.

While fires in heavy fuels take more heat to burn, and spread more slowly, these larger fuels also hold heat and burn for a longer time. When heavy fuels are involved, the fire may require a long period of mop up and monitoring to make sure it stays out and does not cross the firebreak. Mop up of a burn in warm season grasses can be completed in a short time; a burn in timber and slash fuels may need to be mopped up and monitored for days.