Obstruction by raised terrain

Mountain ranges obstruct wind flow in a similar manner to barriers such as trees and buildings but the effects are generally magnified. Ranges stop, deflect or disturb airflow depending on the speed of the airflow, the angle of impingement, the stability of the atmosphere and the structure of the range itself.

Disturbances are particularly noticeable when strong winds flow over, around or through rugged and steep terrain of significant elevation. Sometimes, even in moderate winds of 15 to 20 knots, airflow is significantly disturbed to great heights and turbulent eddies form in the lee of elevated terrain dangerous to aviation operations.

Blocking of wind flow

Wind flow at low levels (below ridge top), within a cool stable layer, can be blocked by the terrain of any height. Blocked (stalled) air can stagnate in front of terrain or flow at a low level around it.

Some low-level air may escape through valleys or over lower ridge tops.

Within the blocked flow, winds tend to stagnate or flow parallel to the terrain contours.

Blocking is depicted in The Figure above It is most often observed to occur in the early hours of the day, before surface heating erodes the low-level inversion, thus decreasing stability at low levels.

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