How are backing fire, head fire, and flank fire distinguished in terms of spread direction?

• A. Head fire moves with the wind and uphill and spreads fastest; flank fire spreads sideways; backing fire moves slowly against the wind and uphill
• B. Head fire moves against the wind
• C. Backfiring is the same as backing fire
• D. Flank fire spreads fastest when aligned with wind direction

Answer: (D)

Spread direction relative to wind defines how each edge of a fire behaves. The leading edge, moving with the wind (and often uphill), is the fastest part of the fire under typical conditions. The trailing edge fights the wind, so it advances slowly. The sides advance more on the order of sideways spread and aren’t as directly driven by the wind as the head.

When the wind lines up with a flank, that edge receives strong reinforcement along its length, pushing the flame front along the flank more rapidly. In that orientation, the flank can spread faster than when the wind is at an angle to it, or than the other edges in many conditions. That’s why the statement about flank fire spreading fastest when aligned with the wind direction best captures how wind alignment can boost the speed of spread on a flank edge.

Other options misstate how wind affects the edges or mix up terms (for example, backing fire generally moves against the wind and is slower, and backfiring is a different tactic from backing fire).