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Winds Blowing Across Hilly Terrain

On windy occasions such as the approach of a tropical cyclone, air streams of high wind speed may emerge from mountain gaps. Lying between these high-speed air streams are air streams of lower wind speed. Aircraft traversing through alternating high-speed and low-speed air streams may encounter headwind losses and gains at different locations along the approach and departure corridors.

In particular, if an aircraft flies from a low-speed air stream to a high-speed air stream, it may experience a large headwind gain leading to a lift of the aircraft. If the aircraft moves from a high-speed air stream to a low-speed air stream, it may experience a large headwind loss resulting in a sinking motion.

windshear induced by winds blowing across hilly terrain

Figure 2

Figure 2 - Wind pattern captured by the Terminal Doppler
Weather Radar during the passage of
Severe Tropical Storm Hagupit in September 2002