Saharan Dust: Definition, Properties, and Impact

Hurricanes aren’t the only storms to roll off the west coast of Africa and travel across the Atlantic Ocean. Saharan dust storms — massive clouds of wind-blown sand and silt from the surface of the Sahara Desert — also journey across the Atlantic, sprinkling over 180 million tons of mineral-rich Saharan dust over Europe, the Mediterranean, the Caribbean, and North America each year.

How Saharan Dust Plumes Form

Typically occurring from late spring to early fall, Saharan dust plumes form when tropical waves (elongated areas of low pressure) move along the southern edge of the Sahara Desert.

As these tropical waves move, they kick up clouds of dust and sand into the air. And as this dust accumulates, it forms a very dry, dusty, warm 2- to 2.5-mile-thick air mass, known as the Saharan Air Layer (SAL).

Because the SAL, which sits a mile or so above the desert surface, can extend 5,000 to 20,000 feet into the atmosphere, it’s in the perfect position to be swept offshore by Earth’s east-to-west-blowing trade winds, which exist at similar altitudes.

SAL outbreaks tend to last for a day or two, then settle and stir again, giving rise to a series of dust plumes that travel westward toward the United States every three to five days during the peak SAL months of June and August.

However, in June 2020, a historic dust plume caused continuous dust emissions for 4 days. The long-lasting plume was exceptionally large: It spanned a 5,000-mile distance from the African continent to the Gulf of Mexico, was roughly the size of the contiguous United States, and filled U.S. skies from Texas to North Carolina.   

Properties of Saharan Dust

Saharan dust is composed of various minerals, including silicates such as quartz (SiO₂). Besides silicates, the most abundant components are clay minerals (kaolinite and illite); carbonates, such as calcite (CaCO₃); iron oxides, such as hematite (Fe₂O₃); salts; and phosphates. As you may have guessed, it’s the iron oxides that lend Saharan dust its ochre hue.

Descended from past rocks, these mineral sediments range in size from coarse large grains measuring over 10 microns in diameter (PM10 and larger) to fine grains measuring less than 2.5 microns in diameter (PM2.5 and smaller).

According to an article in the journal Epidemiology, 99.5% of the dust aerosols that reach the western Atlantic are the ultrafine type; the larger particles get “sifted out” by gravity earlier on in the 2,000- to 6,000-mile-long trek.

Environmental Impacts

As mineral-rich dust sprinkles onto the landscapes below, it interacts with the air, land, and ocean in a myriad of ways, both beneficial and detrimental. For example, the iron and phosphorus in Saharan dust fertilize plants on land and at sea (such as phytoplankton) which need these micronutrients for proper growth.

On the other hand, if too much phosphorus or iron overfeeds saltwater and freshwater algae, harmful algal blooms can occur. From 2017 to 2018, a bloom of the red tide organism Karenia brevis off the coast of Southwest Florida turned waters a murky red and poisoned countless fish, sea birds, and marine mammals exposed to its toxins, which can be ingested and inhaled. In humans, such toxins can cause symptoms ranging from respiratory irritation to gastrointestinal and neurological effects.  

Weather Impacts

Saharan dust can affect weather, too. If it mingles with showers or thunderstorms, especially in nearby Europe, it can trigger “blood rain” events — red-tinted rainfall that results when raindrops condense onto grains of the rust-colored dust. 

The dry, windy conditions associated with the SAL also suppress hurricane activity. Not only does SAL air contain half of the moisture tropical cyclones require, but its strong vertical wind shear can literally blow a storm's structure apart. Sea surface temperatures within a dust plume's wake can also be too cool — up to 1.8 degrees F cooler than normal — to power storm strengthening, since the dust acts as a shield, reflecting sunlight away from Earth’s surface.

Not only does Saharan dust reflect more sunlight, but it also scatters more of it, too. This leads to spectacular sunrises and sunsets since the more molecules there are to scatter the violet and blue light waves away from our eyes, the more unadulterated (and therefore, the more vivid) the red and orange light waves we ordinarily see in the morning and evening skies will be.