Every summer Miami is witness to a mass aerial migration — the journey of huge quantities of dust carried by winds from Africa to the Caribbean and South Florida. This journey begins over North Africa, 4,000 miles to 6,000 miles away. It takes a week for dust to reach our shores. TV weather news will forecast the dust arrival, often showing a satellite image of a huge, hazy plume extending from the coast of Africa to our shores.
You can see for yourself when the dust arrives. Just look toward the sun — but be sure to block the disk with your thumb or stand behind a utility pole. You will see a brilliant aureole enveloping the sky around the sun. The dome of the sky will appear opalescent, silver-grey instead of the usual crystalline blue. These effects are produced by the scattering of solar radiation by dust particles suspended in the air.
Research at the University of Miami Rosenstiel School shows that dust extends to altitudes of several miles in what is known as the Saharan Air Layer (SAL). The SAL is a hot, dry layer whose characteristic temperature-humidity profile is clearly seen in meteorological soundings. Because of dust, the entire sky becomes a source of light that illuminates the landscape with a diffuse soft warm light.
Distant objects are heavily obscured by the dense dust haze. A striking example occurred in early May when there was a particularly strong dust incursion. Since then we have had an unusually active dust season with several African “dust storms” passing over our city. Last week, we experienced another major several-day event.
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African dust affects our environment in many ways. The dense column of dust reduces the solar radiation falling on the Earth’s surface. This leads to a reduction in land and sea-surface temperatures and a redistribution of heating in the atmosphere above, affecting its thermal stability. In this way, African dust and the SAL could serve to modulate the occurrence and intensity of tropical cyclones and hurricanes.
African dust also plays an important role in our geology by forming soils. In Florida, we live on a limestone platform. Accordingly, we would expect our soils to be dominated by limestone particles. Yet red soils are widespread in Florida, the Bahamas and islands in the Caribbean, largely made up of African dust deposited over many millennia.
You can see the effects of dust deposition today. Collect rain in a white container and on occasion you will see a red-brown deposit on the bottom. After brief showers you might see reddish-brown spots on your car or as a fine line along the edge of your windshield wiper blades — it is African dust. Rub it with your finger and you will find that it has the texture of fine cosmetic rouge.
Because the particles are so fine, they penetrate deeply into our lungs. The Environmental Protection Agency and the World Health Organization have established air-quality guidelines for respirable particulate matter (PM). These guidelines are based on epidemiological studies that relate ambient PM concentrations to various health metrics — for example, premature mortality and increased hospital admissions and emergency visits.
Our research shows that dust events in Florida and the Caribbean can far exceed these air quality guidelines. In the Caribbean, where the impact is greatest, PM air quality is compromised to a degree comparable to that in major urban areas despite the fact that local pollution sources are generally minimal. However African dust comprises “clean” soil particles, and there is no clear evidence that it would have the same health affects as particulate matter in polluted urban areas.
Nonetheless, while the health effects of such dust are uncertain and largely unexplored, they are of concern.
There is a clear need for community health studies to assess this issue. How will changing climate affect dust transport to our region? And how will these changes affect our health? Our weather, including tropical cyclones and hurricanes? As atmospheric climate models become ever more sophisticated and observations more precise, we look forward to improving our understanding of our dusty future.
The future climate in North Africa is a critical issue for us. At this time, climate models are evenly divided on the issue of rainfall (and, therefore, dust activity) in the African source regions. About half of the models predict drier conditions — meaning more dust transport — and half predict wetter, and less dust. In this regard, our dust future in Florida is hazy.
Joseph Prospero is professor emeritus of the Department of Atmospheric Sciences & CIMAS at the Rosenstiel School of Marine and Atmospheric Science.