What factor is critical in determining fire spread?

Wind speed and direction play a critical role in determining fire spread because they directly influence how flames propagate and can significantly alter the behavior of a fire. When wind blows, it can provide additional oxygen to the flames, resulting in faster combustion and enabling the fire to jump to new fuel sources. The direction of the wind can also carry embers and ignited materials to nearby areas, potentially igniting new fires at considerable distances from the original source. In contrast, factors such as humidity levels can affect the moisture content of fuels, while the type of extinguishing agent used is more pertinent to fire control rather than its spread. Time of day may have indirect effects on temperatures and relative humidity but does not have the immediate, dynamic impact on fire behavior that wind does. Consequently, wind speed and direction are key elements in understanding and predicting how a fire will spread in a given environment.