THE CONDITIONS ASSOCIATED WITH DUST STORM GENERATION IN NORTH AFRICA

Dust storms are a persistent phenomenon in some locations, but highly responsive to drought conditions in others. North Africa contains a wide diversity of dust sources, including the most productive dust sources in the world. Whether or not a given location might act as a dust source depends on its landform and climatology. The landform, as determined from satellite imagery, represents the combination of availability of wind-erodible sediments and the sheltering role of crusts, larger clasts, and topography. Records from 232 meteorological stations across North Africa are examined for the period 1931-2003 and monthly values of wind speed, wind direction, and visibility calculated. Wind speed is the driver for dust mobilization while visibility acts as a proxy for the dust content of the atmosphere. For 32 of these stations, modeled monthly precipitation values is also extracted. Antecedent precipitation is used as a proxy for the influence of wet and dry periods on vegetation cover which, in turn, strongly influences the ability of winds to mobilize dust. Rank correlations between wind speed, precipitation, and visibility indicate locations where atmospheric dust (visibility) is related to wind power, soil moisture (current precipitation), vegetation (antecedent precipitation), or all of these. At most sites where dust storms occur, wind speed and visibility are negatively correlated. Antecedent rainfall is correlated with visibility only in the Sahel, where the influence of vegetation cover on dust sources is thought to be strong. It is in this region of North Africa, south of the Sahara, where population pressures are greatest, and the influence of drought is expected to have the largest impact on dust sources.