In which type of fuel will a fire spread the most rapidly?

A fire will spread most rapidly in fine fuels due to their small size, high surface area-to-volume ratio, and moisture content. Fine fuels, which include materials such as grass, leaves, and small twigs, ignite quickly and burn easily, allowing flames to propagate rapidly. Their ability to catch fire easily and sustain quick combustion facilitates the spread of flames, especially in favorable conditions such as low humidity and wind.

In contrast, compacted fuels are more densely packed and, while they can burn, their rate of spread is generally lower due to reduced airflow. Brush, which consists of larger vegetation, can fuel fires but has a slower rate of spread compared to fine fuels. Similarly, slash, typically referring to cut or fallen timber, has a greater bulk and moisture retention, which can hinder rapid fire propagation. Thus, the nature of fine fuels makes them the most conducive to quick fire spread.