A foehn wind is air forced across a mountain barrier and down the far side. Descending air is compressed and warms at roughly \( 9.8\,^{\circ}\mathrm{C} \) per kilometre while its water content stays fixed, so relative humidity collapses even as temperature climbs; terrain channelling then accelerates the flow, and the result is hot, very dry, very strong, often nocturnal wind. Regional names describe the same mechanism: the Santa Ana and Sundowner winds of southern California, the Diablo winds of the San Francisco Bay area, the Chinook of the Rocky Mountain front, the Zonda, the Bora and the original Alpine Föhn. These events dominate the record of catastrophic fires because they deliver extreme wind, single-digit humidity and the failure of overnight humidity recovery at the same time, and because they persist for days. The confusion this resolves is treating them as merely strong winds rather than as a combined wind-and-drying regime. After this Concept you can explain why a named downslope wind event, and not a hot day, precedes most catastrophic fire runs.
Foehn winds form through a mechanism in which air forced to rise over a mountain barrier cools via adiabatic expansion (decompression due to reduced atmospheric pressure at altitude), losing moisture…