Which of the following factors primarily affects the rate and direction of the spread of exterior fires?

The rate and direction of the spread of exterior fires are significantly influenced by wind direction. Wind acts as a catalyst for fire, pushing flames and heat ahead of the fire front, which can increase both the speed and spread of the fire. Wind also has the ability to transport embers and sparks to new locations, potentially igniting additional fires.

While factors like temperature, humidity, and type of fuel are also important in fire behavior, they do not have the same direct and dynamic impact on the immediate spread of fire as wind does. Temperature can influence how readily materials ignite, humidity can affect the moisture content of fuels and combustion efficiency, and the type of fuel determines how flammable and how quickly materials burn. However, wind is the primary driver for how quickly and in what direction an exterior fire will spread in real-time conditions, making it a crucial element for firefighters to monitor during operations.