FAA Private Pilot Helicopter (PRH)WeatherMedium
A pilot plans an early morning departure from a helicopter pad located in a narrow mountain valley. Winds have been calm and skies clear overnight. What wind condition should the pilot expect near the valley floor at departure time?
- ANo local wind effects since surface winds have been calm
- BAn upslope (anabatic) wind flowing up the mountainsides
- CStrong, gusty winds from an approaching cold front
- DA downslope (katabatic) wind draining cold, dense air into the valley
Show answer & explanationAnswer & explanation
Correct answer: D. A downslope (katabatic) wind draining cold, dense air into the valley
On clear, calm nights, mountain slopes cool rapidly through radiation, chilling the adjacent air, which becomes denser and drains downslope into the valley. This nighttime downslope flow is called a katabatic (or mountain) wind and is typically strongest just before sunrise.
Why the other options are wrong
- A. Calm winds aloft do not preclude local katabatic drainage winds near the surface.
- B. Anabatic (upslope) winds develop during daytime heating, not before sunrise.
- C. No frontal passage was mentioned; this is a local thermal/gravity wind effect.
Katabatic (Mountain/Valley) Wind
A local wind caused by cold, dense air draining downslope into valleys at night due to radiational cooling of mountain surfaces.
- Occurs mainly on clear, calm nights
- Opposite of daytime anabatic (upslope) wind
- Strongest near sunrise before daytime heating begins
- Can create low-level turbulence and wind shear near valley floors
Memory trick: 'Katabatic comes at night, cold air takes flight downhill'