This mineral-rich African dust is both helpful and harmful to ecosystems.

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. 

Typically occurring from late spring to early fall, these dust clouds, or Saharan dust plumes, are born when tropical waves (areas of low pressure that are elongated rather than circular) move along the southern edge of the Sahara Desert. As these ripples 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.  

A visible satellite image of the historic June 2020 Sahara Dust plume, taken on June 18.

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. It was not only long-lasting, but exceptionally large, too. 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.

The Sahara Desert in Morocco.

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.

Particulate Matter

Particulate matter (PM) refers to tiny solids or liquid droplets that remain suspended in the air, such as dust, pollen, and mist. They're so infinitesimally small, that they're measured in microns (one millionth of a meter; a human hair is about 70-90 microns in diameter). If inhaled, PM can cause serious health effects. It's also considered a form of air pollution.

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 fertilizes plants on land and at sea (such as phytoplankton) which need these micronutrients for proper growth.

An algal bloom colors the Gulf of Mexico waters a rusty brown.

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.  

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, it 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.

Human Impacts

If you suffer from asthma, allergies, or have other health sensitivities, you may want to enjoy those SAL sunsets from indoors. The high concentration of particle pollution associated with Saharan dust events can be extremely hazardous if inhaled. For example, a study in the journal Environmental Health found that on Saharan dust days, emergency room visits for asthma, chronic obstructive pulmonary disease (COPD), and respiratory infections increased by 38%, 57%, and 60%, respectively. Findings from a working paper released by the National Bureau of Economic Research explain that Saharan dust is also linked to a 22% increase in infant mortality. 

Saharan Dust: Key Takeaways

• 180+ million tons of Saharan dust exits North Africa's Saharan Air Layer every year, from late spring to early fall.
• Saharan dust promotes plant biodiversity and reduces tropical cyclone activity.
• Fine and ultrafine grains (size PM2.5 and smaller) can trigger code orange air quality days, leading to respiratory and cardiovascular illness.