Mauritania went from two dust storms a year to about eighty
Sand grains hopping across a desert build up static charge, and that charge shakes loose even more grains. The result can be a wall of dust over a kilometre high. Saharan storms have multiplied roughly tenfold since the 1950s, stripping African topsoil while feeding rainforests an ocean away.
A storm starts when strong wind meets dry, loose ground. Sand grains first tremble, then bounce along the surface in a motion called saltation; each impact chips off finer dust that rises and travels suspended in the air, while heavier grains creep along the ground. A 2008 study found that bouncing sand becomes negatively charged relative to the ground, and the resulting electric field frees yet more grains, doubling the particle count older theories predicted.
The winds come from thunderstorm outflows of rain-cooled air, from dry cold fronts like those behind the American Dust Bowl, or from broad pressure gradients. A temperature inversion can pin the dust into a shallow layer; otherwise fine dust, though not sand, may climb to 6,000 metres. Drought, wildfire, heavy tillage and overgrazing all expose soil to the wind. Local names mark the phenomenon: the simoom off the Sahara and the haboob around Khartoum, most frequent in summer.
The Sahara is a prime source, especially the Bodélé Depression and the region where Mauritania, Mali and Algeria meet, and its dust sometimes reaches central Europe and Britain. Oxford geographer Andrew Goudie reports that Mauritania saw only a couple of storms annually in the early 1960s but roughly 80 each year since 2007. In June 2007, dust leaving Africa was five times the level of a year earlier, perhaps cooling the Atlantic enough to trim that autumn's hurricane activity.
Costs are heavy. Storms carry off the lightest, most nutrient-rich soil, sandblast seedlings, ground aircraft and snarl roads. Inhaled dust worsens asthma, can cause dust pneumonia and, with prolonged exposure, silicosis, and it lofts bacteria and fungal spores. Yet deposits help too: Saharan minerals fertilise Central and South American rainforests, dust brings iron to iron-poor seas, and ancient loess in northern China and the American Midwest makes rich farmland. Mars has storms that can wrap the whole planet, though its thin air, about 1 percent of Earth's pressure, keeps them from hurricane force.
Source: Dust storm
