Mountain wave formation & cloud signs
Mountain (standing) wave: airflow wave downwind of a ridge.
Needed (all three):
| Factor | Requirement | Cite |
|---|---|---|
| Wind speed | Over about 50 kt | TC AIM AIR 1.5.6 |
| Wind direction | Within 30 degrees of perpendicular to ridge main axis | TC AIM AIR 1.5.6 |
| Air | Crosses a stable layer | AWH 16.2 |
Stability decides outcome (AWH 16.2):
| Air and wind | Result |
|---|---|
| Weak wind or dense air | Mountain dams the flow |
| Unstable, moist air | Thunderstorms or deep convective cloud |
| Only strong wind through stable air | Wave |
Reach:
- Can extend 100 NM downwind of the mountains, at altitudes many times higher than the ridge (AIR 1.5.6).
- Mountain wave activity (MWA) not limited to the immediate vicinity of a range (RAC 11.7.12).
- Most likely near high ranges (Rockies); also over lower ranges (Appalachians) (AIR 1.5.6).
Cloud signs:
| Cloud | Marks | Where and what beneath |
|---|---|---|
| Lenticular | Wave crest position only, not intensity (AIR 2.10) | PIREPs beneath frequently report moderate-to-severe turbulence; wind below variable, gusty, surface to cloud base (AWH 16.2.4) |
| Rotor | Severe, tumbling turbulence beneath the wave | Downwind (lee) side of ridge, near or below ridge level, under wave crest (AWH 16.2.5) |
- Strong wave in dry air can show no cloud at all (AIR 2.10).
- Rotor rolling moments can exceed aircraft roll authority; worst low, on takeoff and landing: slow, high-drag configuration (AWH 16.2.5).
Trap: cross-ridge wind under about 50 kt, or dense air, builds no wave; air must also be stable. Lenticular marks position only, so cloud does not show wave strength.
References
- TC AIM AIR 1.5.6
- TC AIM AIR 2.10
- TC AIM RAC 11.7.12
- FAA Aviation Weather Handbook (FAA-H-8083-28) 16.2
- FAA Aviation Weather Handbook (FAA-H-8083-28) 16.2.4
- FAA Aviation Weather Handbook (FAA-H-8083-28) 16.2.5
- FAA Aviation Weather Handbook (FAA-H-8083-28) 16.2.7