Wednesday, November 19, 2008

Forecast: Grit Showers

The Arabic words for wind and phenomenon emerge from the term haboob, which scientists use to describe the enormous sand and dust storms that are a regular feature of life in desert climes. Although haboobs form primarily in a broad swath of arid land stretching from N. Africa to Iraq, they are known to occur in other places including the SW USA.

I think of haboobs as dry monsoons that pelt the ground with particulate matter rather than rain. They generally form when the low-pressure conditions that creat thunderstorms collapse. As cold air from high altitudes rushes into the former area of low pressure, it first thrusts towards the ground, then is deflected outward. The resulting wind may pick up as much as several hundred tons of loose desert debris. The unforgettable result (I have seen it) is a giant brown or black wall of violently churning grime.

In this pic, a haboob bearing sand and dust form the Sahara Desert rolls into Khartoum, the capital of Sudan. The people of the Middle East are so familiar with such storms that they give them specific names. The season's first storm, which usually arrives at the end of May, is called al-Haffar, or the Driller, because it scrapes huge holes in desert sand dunes. The next, in early June, is called Barih Thorayya, because it arrives with the dawn star, Thorayya. The last storm is called al-Dabaran, the Follower; it is infamous for carrying a particularly penetrating layer of microcopic dust that finds its way into seemingly every crevice and corner in its path.

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