Mountain Meteorology and Aviation Hazards

Mountains alter airflow by forcing air over ridges, funnelling it through valleys and creating temperature differences between slopes and surrounding air. These processes generate local winds, strong vertical air movements and turbulence, which can rapidly change an aircraft's airspeed, attitude or height above terrain.

Heating and cooling produce slope and valley winds

Mountain winds can develop even without a strong large-scale weather system because slopes heat and cool differently from the air around them.

• During the day, sunshine warms mountain slopes, heating the adjacent air and encouraging upslope winds; valley circulations commonly carry air up-valley.
• At night, slopes lose heat by radiation. Air near the ground becomes colder and denser and drains downslope as katabatic winds; down-valley flow may also develop.
• These daily reversals change local wind direction and the distribution of rising and sinking air, affecting approaches, departures and low-level flight.

Ridges and valleys redirect and accelerate airflow

Terrain acts as both an obstacle and a channel. The resulting wind may differ substantially from the prevailing wind measured away from the mountains.

• Air forced up a windward slope produces orographic uplift, while descending air on the leeward side can create hazardous downdraughts.
• Valleys, passes and gaps can funnel airflow and locally increase its speed, producing strong channelled winds and unexpected crosswinds.
• Airflow can separate from steep terrain and form eddies, particularly in the lee of ridges, creating irregular turbulence.
• Adjacent sheltered and exposed areas may have sharply different winds, producing wind shear, a change in wind speed or direction over a short distance.

Stable cross-ridge flow generates mountain waves and rotors

When sufficiently strong wind crosses a ridge in a stably stratified atmosphere, displaced air can oscillate downstream, forming mountain waves. Unlike ordinary surface eddies, these waves can extend well above ridge height.

• Alternating ascending and descending branches produce strong vertical currents; a downdraught can exceed an aircraft's available climb capability.
• Rotors, turbulent horizontal circulations beneath mountain waves on the lee side, can cause severe turbulence and rapid changes in aircraft attitude.
• Lens-shaped lenticular clouds may mark wave activity when sufficient moisture is present, but dangerous waves and turbulence can also occur without visible clouds.
• Preparedness requires mountain-weather assessment, attention to wind direction relative to ridges, adequate terrain clearance and routes that preserve escape options; visual conditions alone do not establish safety.