Mid-flame wind speed is approximately what percent of 20 ft winds?

Mid-flame wind speed is approximately what percent of 20 ft winds?

• A. 30%
• B. 50%
• C. 70%
• D. 90%

Answer: (A)

Wind in the flame zone is much lower than the free-stream wind measured at standard heights because surface roughness and fuel drag slow air movement as it moves toward the ground. The rule of thumb used in S-390 is that the wind speed at mid-flame height is about one-third of the 20‑ft wind. In other words, mid-flame wind is roughly 30% of the 20‑ft wind. This matters for fire behavior because the air flow inside and just above the flames drives preheating and flame tilt, so using about 30% of the 20‑ft wind gives a practical estimate for how the wind influences the flame zone. For example, a 20 mph 20‑ft wind translates to ~6 mph at mid-flame height. Real conditions can vary with terrain and fuel, but 30% is the standard approximate value.

Wind in the flame zone is much lower than the free-stream wind measured at standard heights because surface roughness and fuel drag slow air movement as it moves toward the ground. The rule of thumb used in S-390 is that the wind speed at mid-flame height is about one-third of the 20‑ft wind. In other words, mid-flame wind is roughly 30% of the 20‑ft wind. This matters for fire behavior because the air flow inside and just above the flames drives preheating and flame tilt, so using about 30% of the 20‑ft wind gives a practical estimate for how the wind influences the flame zone. For example, a 20 mph 20‑ft wind translates to ~6 mph at mid-flame height. Real conditions can vary with terrain and fuel, but 30% is the standard approximate value.