The Mountain Wave and the Icing Layer
You are crossing a north-south ridge at 11,500 ft AMSL. The ridge crest is 9,000 ft. Winds at ridge-top level are 35 kt from 270°, perpendicular to the ridge, and the air is stable with a moist layer between 8,000 and 14,000 ft. The temperature at 11,500 ft is −2°C and the dew point is −4°C. Your aircraft has a service ceiling of 14,000 ft, no de-icing, and you are 20 NM west of the ridge. You have already experienced a 1,500 ft/min updraught and a 1,000 ft/min downdraught in the last 5 minutes.
1. Identify the phenomenon and estimate the vertical extent of the hazardous zone relative to the ridge.
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The perpendicular wind of 35 kt across a stable, moist layer over a 9,000 ft ridge is a classic mountain wave (lee wave) setup. The 1,500 ft/min updraught followed by a 1,000 ft/min downdraught confirms rotor and wave activity.
Vertical extent: The wave extends from the ridge top up to the tropopause, but the hazardous rotor zone is typically within 1–2 times the ridge height above the crest - here up to about 9,000 + 9,000 = 18,000 ft, well above your service ceiling. The moist layer at 8,000–14,000 ft means lenticular cloud and rotor cloud are likely, and the wave amplitude is greatest just above the crest.
Horizontal extent: The downdraught on the lee side typically extends 5–15 NM downwind of the ridge, with the strongest sink within a few NM of the crest. At 20 NM west (upwind), you are in the approach region where the updraught builds, so the worst is ahead.
2. Assess icing risk and decide whether to continue at 11,500 ft or change level.
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Icing: At 11,500 ft the temperature is −2°C and the dew point is −4°C, a spread of only 2°C. In the wave, the air is being lifted and cooled, so the spread will close and cloud will form. Supercooled water at −2°C in a moist, stable layer is ideal for moderate to severe clear icing. With no de-icing, this is a hard limit.
Options:
- Climb: Service ceiling 14,000 ft. Climbing to 14,000 ft puts you at about −7°C, still in the moist layer and still in icing conditions, and still in the wave - no benefit, more risk.
- Descend: Below the ridge top is not possible for terrain clearance unless you route around. Descending to 10,000 ft keeps you at about +1°C, above freezing, which eliminates icing - but it is still in the rotor zone and close to the ridge.
- Turn back / divert around the ridge: The safest option. Route around the end of the ridge or return to the upwind side and wait for the wind to ease.
Recommendation: Do not continue at 11,500 ft. The combination of wave turbulence and icing in a non-de-iced aircraft is unacceptable. Turn back or divert around the ridge; if you must cross, do it in daylight with a de-iced aircraft and a forecast of lighter winds.