Orographic lift
Orographic lift: air forced up rising terrain. Forms whenever moist air crosses a mountain range, regardless of wind direction (ASL 3/2025).
| Slope | Air | Humidity | Cloud and precipitation |
|---|---|---|---|
| Windward | Rises, expands, cools | Relative humidity climbs | Form at and above condensation level |
| Leeward | Descends, warms | Relative humidity drops | Dissipate |
- Result: two climates on one range. Wet windward, dry leeward.
- Rain shadow: dry conditions can extend hundreds of miles to the lee of a prominent range (FAA AWH 12.4.1).
- Fair weather on one side often means poor weather on the other, and vice versa (ASL 3/2025).
Mountain obscuration: mountains or ridges hidden by cloud, precipitation or smoke. A planning hazard.
- Elevation varies widely around mountains: valley station can report a VFR ceiling while the ridge sits in fog (FAA AWH 18.2.2).
- Valley station report does not clear the route through the peaks (ASL 3/2025: few reporting stations in mountains, weather varies along any route).
Trap: orographic cloud forms on WHICHEVER slope is windward that day, so the wet side flips with wind direction.
References
- FAA AWH 12.4.1 Orographic Effects
- FAA AWH 18.2.2 Mountain Obscuration
- ASL 3/2025 Mountain Flying