FAA Aircraft Dispatcher (ADX)MeteorologyMedium
A dispatcher reviewing surface observations on the leeward (east) side of a mountain range notes a sudden temperature rise, sharply lower relative humidity, and gusty winds following the passage of air over the ridge. This condition is best described as a:
- AKatabatic wind caused by nocturnal radiational cooling of the slope
- BUpslope condition caused by moist, stable air forced against the terrain
- CFoehn (Chinook) wind resulting from adiabatic warming of air descending the leeward slope
- DSea breeze circulation caused by differential land-water heating
Show answer & explanationAnswer & explanation
Correct answer: C. Foehn (Chinook) wind resulting from adiabatic warming of air descending the leeward slope
As air is forced over a mountain range and descends the leeward side, it warms at the dry adiabatic lapse rate (having lost most of its moisture on the windward side), producing the warm, dry, gusty wind known as a foehn or Chinook wind. This differs from katabatic drainage winds, which are driven by nighttime radiational cooling rather than mechanical descent and adiabatic warming.
Why the other options are wrong
- A. Katabatic winds are cold drainage winds driven by radiational cooling, not warming after descent.
- B. Upslope conditions occur on the windward side and typically produce cooling and cloud/fog, not warming.
- D. Sea breezes are coastal, thermally driven, and unrelated to mountain descent.
Foehn (Chinook) Wind
A warm, dry, often gusty wind that occurs on the leeward side of a mountain range as air descends and warms adiabatically after losing moisture on the windward slope.
- Descending air warms at the dry adiabatic lapse rate (~3°C/1,000 ft)
- Produces rapid temperature rise and low humidity leeward of mountains
- Known as Chinook winds in the Rocky Mountains, foehn in the Alps
Memory trick: Foehn = Falls-and-warms wind on the far side