Gap Winds

Terrain interacts with the atmosphere in many ways to alter the weather situation, for example, presenting various angled slopes that differ in the surface absorption of sunlight and thus the heating of the air. On all scales, from the global circulations to the microscale, topography's interaction with the wind produces special flow situations. Winds flowing up mountain slopes often form clouds at higher elevations where the air's moisture may drop as rain or snow. When that flow crosses the summits and flows down the descending slope, its compression produces the warm and dry chinook and Santa Ana winds. If winds push through the terrain in spots, rather than going over it, another set of wind phenomena arises, the gap winds.

Gap winds are low-elevation winds associated with gaps or low elevation areas in mountainous terrain, including flow through valleys and canyons and among peaks. These wind fields arise in terrain gaps ranging from hundreds of metres to hundreds of kilometres wide and produce local increases in speed and often large changes in wind direction that more or less parallel the gap axis. Gap winds typically blow over shallow depths that do not exceed a kilometre and are often only a few hundred metres deep. (A special form of gap wind blows in urban areas, the street canyon wind, caused by gaps between buildings rather than terrain.) Gap winds are strongest when a large pressure gradient exists across the gap, although some gap winds do not depend on the large-scale pressure gradient but on the wind entering the gap.

My home on southern Vancouver Island sports one of the most well-known gap winds on the continent when strong, low-level winds blow through the Strait of Juan de Fuca between the Olympic Mountains of western Washington State and the Insular Mountains of Vancouver Island. These winds can be especially hazardous to sailors and boaters on the strait because they can arise swiftly under seemingly benign weather conditions, raising small-craft warnings and at times gale warnings while surrounding waters have no warnings or lesser warnings.

When winds blowing from the wide-open Pacific Ocean meet the topography of the rugged western North American coast, the high terrain places an impediment in the path of the flow. Usually, the winds are deflected upward to cross over the mountain ridges. Where gaps such as the Strait of Juan de Fuca occur, however, the surface winds converge on the opening in order to pass through the sea-level route. Just as a crowd trying to push through a small door or gate exerts pressure on those near the opening, forcing them to

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