Page 334 - PPL-engelsk 2025

Meteorology

Mountain waves are dangerous because they cause severe turbulence and

unpredictable updrafts

and downdrafts on the

leeward side, which can

cause an aircraft to lose
control.

For mountain waves to form, the following conditions are required:

- Stable air.

- Wind direction nearly perpendicular to the mountain range—up to +/- 30
degrees.

- Wind speed over the mountain top >25 knots.

- Wind speed must increase with height above the mountain range.

Mountain waves most commonly occur during the colder months of the year.

The waves are stationary, and the air is lifted in front of the wave. If the air is

moist enough, stationary clouds will form here due to adiabatic cooling.

On the backside of the wave, the clouds will dissipate in the descending air.

The clouds formed in connection with mountain waves are high Lenticularis

clouds (lens-shaped clouds) and lower-laying rotor clouds.

Rotor clouds form as a chain of CU-like clouds parallel to the mountain range

on the leeward side beneath the wave crests. These clouds can grow into CB

clouds with thunderstorms.

On the windward side of the clouds, there are very strong updrafts, while there

are downdrafts up to 5000 feet per minute on the leeward side.

In and below the rotor cloud, there are areas with extremely strong turbulence.

Updrafts and downdrafts of 1000 to 1500 feet per minute are not unusual.

Along with the updrafts and downdrafts, there are also severe wind gusts.

A cap cloud forms on the windward side of the mountain, covering the

mountaintop and extending down the leeward side like a waterfall.
Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 334