Calima, Saharan Air Layer, Sahara, Canary Islands, Africa

In the case of Africa, winds blow twenty percent of dust from a Saharan storm out over the Atlantic Ocean, and twenty percent of that, or four percent of a single storm's dust, reaches all the way to the western Atlantic. The remainder settles out into the ocean or washes out of the air with rainfall. Scientists think that the July 2000 measurements made in Puerto Rico, nearly 8 million tonnes, equaled about one-fifth of the total year's dust deposits.
This phenomenon can happen at any time of year but is usually associated with the hot air found over the islands during the summer months, ranging in time scale from a few hours up to a week. Calima as it is called, is caused by a duststorm that is stirred up by high winds in the Sahara and is then driven over the Canary Islands by south easterly winds. The fine sand particles cause the air to become thick and visibility becomes rather like that experienced during a thick fog, depending on the severity. During the calima, every surface will be covered in fine reddish brown dust.
These clouds of dust are visible in satellite photos as a milky white to gray shade, similar to haze.
Findings to date indicate that the iron-rich dust particles which often occur within the SAL reflect solar radiation, thus cooling the atmosphere. The particles also reduce the amount of sunlight reaching the ocean, thus reducing the amount of heating of the ocean. They also tend to increase condensation as they drift into the marine layer below, but not precipitation as the drops formed are too small to fall and tend not to readily coelesce. These tiny drops are subsequently more easily evaporated as they move into drier air laterally or dry air mixes down from the SAL aloft. Research on aerosols also shows that the presence of small particles in air tends to suppress winds. The SAL has also been observed to suppress the development and intensifying of tropical cyclones, which may be related directly to these factors.