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Shamal Winds
Dust and sand storms are a persistent problem in the region, but they are most prevalent in the spring and summer months due to the strong (northwesterly shamal) winds that characterize the weather during the winter-spring seasonal transition. Specifically, dust and sand storms occur when the strong (mostly dry) storms — that often accompany well-defined cold fronts — stir up these particles. Dust and sand lift both ahead of and (even more so) behind cold fronts (since winds tend to be stronger behind the front than ahead of it). This seasonal trend can best be characterized as a combination of two separate weather systems: the sub-tropical jet stream pushing up from south of the Arabian Peninsula and a polar front jet stream pushing down from the European continent. When these two systems come into close proximity, they create much more dynamic weather than is usually found in this region, especially the strong northwesterly “shamal” winds (shamal winds at several Southwest Asia international airports have been recorded as high as 43 knots or 49 mph).The larger the particles, the stronger the wind required to lift them into the air. But for there to be any long-range transport, there also needs to be considerable vertical motion. The vertical speed determines how much the particulate matter is lifted into the air. Another factor that influences the impact of the shamal is the dampness of the sand. Even a very small amount of precipitation can keep a tremendous amount of sand from entering the air.The unique topography and human intervention within the region also contribute to the frequency and intensity of dust and sand storms in this region. The natural funneling of large air masses by the high mountains in Turkey and Iran, combined with the high plateaus in Saudi Arabia, help to funnel air across the Mediterranean into the Arabian Gulf. Dust and sand storms can persist for days, however, because the air is so dry in this region, there are wide diurnal temperature differences that can influence dust and sand storms (especially during the summer months). In other words, rapid heat loss at night lowers the temperature inversion, helping to settle the dust and sand. Therefore, dust and sand storms generally subside at the source soon after sunset and are strongest in the late morning and afternoons.Sand Versus Dust Storms Technically speaking there are subtle, yet distinct differences between dust and sand storms. Sand Storms: A “sandstorm” is basically a wind storm that carries sand through the air, forming a relatively low cloud near the ground. Typical sandstorms only reach heights of up to 15 meters (49 feet), contain sand particles with average sizes between 0.15 to 0.30 millimeters, have wind speeds exceeding 10 miles per hour and last as long as wind speeds persist. When wind reaches a critical velocity, grains of sand begin to roll forward along the ground surface. For higher wind speeds, sand particles in a sand storm move by “saltation,” a process under which particles are temporarily lifted and then bounced along the surface in a hopping/jumping motion. When one saltating grain collides with another, the impact may lift either particle into the air. Once aloft, these particles are subjected to the forces of gravity (pulling them down) and horizontal wind velocity... and the process starts all over again. Once a dust storm starts, it roughly increases with the cube of the wind speed. Dust Storms: Dust storms are a similar phenomena but have distinctly different characteristics. Dust storms form in semi-arid and arid regions where small dust (and sand) particles are literally blown into the air. Unlike in pure sand storms, dust particles are small enough to be lifted aloft by currents of turbulent air and carried into suspension. Ironically, however, research has shown that wind does not usually pick up dust-sized particles less than 0.05 mm in diameter along many completely “smooth” surfaces bec|
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