In S-390, when is the rate of spread likely to be higher?

Prepare for the NWCG Introduction to Wildland Fire Behavior Calculations (S-390) Test. Study with interactive questions and explanations to ensure you're ready for the challenge.

Multiple Choice

In S-390, when is the rate of spread likely to be higher?

Explanation:
Winds that blow exactly along the slope drive the fire upslope the fastest because they push heat and flames directly toward fuels that lie above the flame front. This alignment boosts convective and radiant heat transfer to fuels upslope, so they ignite more quickly and the fire front moves uphill faster. When the wind is cross-slope, heat is spread more sideways, so the upslope preheating and ignition are less efficient, slowing the rate of spread. No wind eliminates much of the convective heat transfer required to preheat upslope fuels, also slowing the spread. So, the combination of wind flowing upslope is the scenario that yields the highest rate of spread.

Winds that blow exactly along the slope drive the fire upslope the fastest because they push heat and flames directly toward fuels that lie above the flame front. This alignment boosts convective and radiant heat transfer to fuels upslope, so they ignite more quickly and the fire front moves uphill faster. When the wind is cross-slope, heat is spread more sideways, so the upslope preheating and ignition are less efficient, slowing the rate of spread. No wind eliminates much of the convective heat transfer required to preheat upslope fuels, also slowing the spread. So, the combination of wind flowing upslope is the scenario that yields the highest rate of spread.

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