Foehn Warming and a Mountain Crossing
A north–south ridge rises to 3,200 m. Air approaching from the north at sea level has a temperature of 8 °C, a dewpoint of 3 °C and is stable and saturated only above its condensation level. The saturated adiabatic lapse rate is taken as 0.6 °C per 100 m and the dry rate as 1 °C per 100 m. The forecast wind at ridge level is 040/35 kt, roughly perpendicular to the ridge. A turboprop plans to cross the ridge at 3,800 m from north to south, and the destination airfield south of the ridge is at 400 m elevation. The crew notes widespread lenticular cloud over the ridge and reports of severe turbulence and a strong downslope wind on the southern side.
1. Determine the condensation level, the temperature at the summit on the windward side, and the temperature of the air after descending to the southern airfield.
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Dry ascent until saturation: temperature–dewpoint spread is 5 °C, so condensation occurs after 5 × 100 = 500 m. Condensation level ≈ 500 m, temperature there 8 − 5 = 3 °C.
Saturated ascent from 500 m to 3,200 m = 2,700 m at 0.6 °C/100 m → cooling 16.2 °C. Summit temperature ≈ 3 − 16.2 = −13.2 °C.
Descent on the lee side: assume the air remains saturated only briefly, then descends dry. If all 2,800 m descent to 400 m is dry: warming 28 °C, giving ≈ 14.8 °C at the airfield. If the descent is treated as saturated for the first part, the lee temperature is lower, but the key result is that the lee air is markedly warmer than the 8 °C source air - classic foehn warming caused by the difference between saturated ascent and dry descent.
Note the air is also drier on the lee side, since moisture was precipitated on the windward slope.
2. Assess the hazards of the planned crossing at 3,800 m and state how you would adjust the flight.
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The summit air is about −13 °C, so at 3,800 m it is colder still. Lenticular cloud indicates strong mountain-wave activity, and the reported severe turbulence and downslope wind confirm rotor and lee-wave hazards well above the ridge and far downwind. The 35 kt wind at ridge level perpendicular to the ridge supports significant wave amplitude.
Hazards: severe turbulence, possible mountain-wave-induced icing in supercooled cloud, loss of altitude in downdraughts, and altimeter errors in strong pressure gradients. Crossing exactly at 3,800 m gives little margin above the rotor zone.
Adjustments: increase terrain clearance substantially (for example 4,500 m or more, subject to aircraft ceiling and oxygen), cross at an angle rather than perpendicular if possible, and choose the upwind side of the ridge for the approach so that escape is toward lower terrain. If the aircraft cannot climb well above the wave zone, delay until the wind eases or route around the ridge. Brief passengers and secure the cabin.