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Boulder's downslope winds

Lines of lenticular (lens-shaped) clouds like these often accompany severe downslope windstorms

The city of Boulder is located at the eastern foothills of the Colorado Rocky Mountains, where the mountains meet the plains. Strong winds often sweep down over Boulder from the eastern slope of the Rockies. With gusts of 160 kilometers per hour or more, these downslope winds can turn violent, ripping roofs from buildings, snapping power lines, shattering windows, and sandblasting paint from cars.

The city of Boulder is located at the base of the eastern foothills of the Colorado Rockies, where the mountains meet the plains. Strong winds often sweep down over Boulder from the eastern slope of the Rockies. Sometimes these downslope winds turn into violent gusts of 160 kilometers (100 miles) per hour or more, ripping roofs from buildings, snapping power lines, shattering windows, and sandblasting paint from cars. This photo shows damage to property in south Boulder that took place on 8 January 1969, the day after one of Boulder's worst windstorms. Photo by Edward Zipser.

Although downslope windstorms strike Boulder most frequently in January, they have been recorded in every month except July. Damage from Boulder's winds averages about a million dollars per year, with one exceptionally strong storm on 17 January 1982 resulting in more than $10 million worth of damage. As new construction spreads toward the foothills, other areas along the eastern slope, such as Fort Collins and the southwest Denver suburbs, are reporting increasing wind damage to homes and other property.

Boulder's downslope winds differ from the winds common on the plains to the east. The Rockies, which are the largest north-south mountain barrier in the world, stand in the path of the prevailing west-to-east atmospheric circulation over North America. When large-scale weather patterns produce a strong, deep flow of air across the Rockies, the peaks along the Continental Divide act like rocks in a streambed. Just as a rock in flowing water produces a series of ripples in the surface of the stream, the mountains cause ripples in the atmosphere. Forced up by the peaks, the air seeks to return to its original level but oscillates through several up-and-down cycles before settling back into a horizontal flow over the plains.

Because the crests and troughs remain stationary even though the air that forms them moves rapidly, this flow pattern is known as a standing wave. A standing wave produced in the atmosphere by mountains is often called a mountain wave or lee wave, because it extends out from the lee (downwind) side of the mountains. Where the wave dips down, as it does along the foot of the eastern slope of the Rockies, it creates violent winds at the earth's surface. Aircraft flying through the upper levels of the wave often encounter strong, choppy gusts known as clear-air turbulence. The wave's crests are sometimes marked by lines of lenticular (lens-shaped) clouds parallel to the mountains.

Downslope winds, typically warm and dry, occur in many parts of the world where mountains stand in the path of strong air currents. In the European Alps, where they were first identified, they are known as the foehn. The foehn has other names in other places: zonda in Argentina, halny wiatr in Poland, koembang in Java, and Santa Ana in California. In the Rocky Mountains, where warm, dry downslope winds can melt a foot of snow in less than a hour, they are called the chinook--after Native Americans of the Pacific northwest, where the winds originate.

Since the mid-1960s, NCAR scientists have collaborated with researchers at the National Oceanic and Atmospheric Administration, the Institute of Arctic and Alpine Research of the University of Colorado, and other organizations to study Rocky Mountain downslope winds. Instrumented aircraft have flown into the winds at various levels to probe their structure. Mathematical models have been developed to test prediction techniques. Scientists have worked with local government officials to develop realistic building codes, tie-down regulations for mobile homes, and other policies designed to reduce wind damage. More recently, NCAR scientists have studied the possible role of strong downslope winds as a hazard to small aircraft.

Research can lead to improved prediction of high winds and provide warning time for protecting lives and property. Damage can be reduced by such measures as constructing more substantial buildings, covering windows with heavy shutters, and planting trees as wind breaks. There may not be a way to reduce the force of a wind, but researchers are working to better understand the interplay of natural elements that creates winds.

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Edited by Rene Munoz, munoz@ucar.edu

Prepared for the Web by Jacque Marshall
Last revised: Mon Apr 10 14:14:36 MDT 2000