"Southern Florida is about to be hit by a major dust cloud, blown across the Atlantic Ocean from storms scouring sand from the Sahara Desert. Madeleine Brand speaks with dust cloud expert Dr. Joseph Prospero, a professor of marine and atmospheric science at the University of Miami."
"MADELEINE BRAND, host:
I'm Madeleine Brand, and this is DAY TO DAY.
The Caribbean islands and south Florida are bracing for their next major weather event. It's not a hurricane, though; it's a dust storm, an enormous cloud of red sand. And get this: It's kicked up in the Sahara Desert and rides the trade winds all the way across the Atlantic. One person who's spent his career watching this sort of event is Dr. Joseph Prospero. He's a professor of marine and atmospheric science at the University of Miami in Florida. And welcome to the program.
Dr. JOSEPH PROSPERO (University of Miami): Thank you.
BRAND: So a dust storm all the way from the Sahara. How does the dust get to Miami?
Dr. PROSPERO: Well, during the summer months, Miami and--well, much of the tropical Atlantic and the Caribbean lies in the trade wind belt. So in Africa, you generate dust and it gets lifted up in the atmosphere and it gets caught up in the trade wind circulation, and the dust goes where the trade wind goes. So we get dust here in Miami.
BRAND: So even though it's riding the winds, it doesn't necessarily get put out, if you will, or moistened by the clouds?
Dr. PROSPERO: Well, when the dust-laden air interacts with a tropical storm or a random cloud in the trade winds, it will be removed by precipitation processes. It will rain out and be deposited on the ocean surface or on islands. And, in fact, much of the soils in the Caribbean islands is derived from African dust, so it does contribute something to the islands. So, yes, there is the removal of dust during the transport process, but a lot of it remains in the atmosphere because a large part of the Atlantic doesn't experience much rain at this time of year, particularly the eastern Atlantic. So it travels quite far.
BRAND: So what does it look like? Does it look simply like a hazy day?
Dr. PROSPERO: Yes, it looks like a very hazy day, and you get very much reduced visibility. Our school is located on Virginia Key; we're about, oh, three or four miles from the shoreline. And when I look to downtown Miami, the buildings will be quite hazy and, you know, relatively obscure. Instead of seeing deep, blue skies, you see a sort of, you know, a milky sort of sky. And around the sun, there'll be a very large aureole. That is, light is scattered by the dust and it forms this large, lit, brilliant area around the sun, and it extends out quite a distance. So it's quite apparent.
BRAND: And I suppose down on the ground things get dusty.
Dr. PROSPERO: Yes, yes. But actually, most of the dust is carried at higher altitudes. The dust typically in the Miami area will extend, oh, to 10, 12,000 feet. And most of it is below that level; some will get higher. But you may very well have the highest concentrations aloft within this what we call the Saharan air layer, which has very characteristic properties. It's a dry, hot layer of air and it has a lot of dust in it.
BRAND: And is this only a eastern United States-Caribbean phenomenon, or do dust storms happen all over?
Dr. PROSPERO: Well, there are a lot of dusty places on our Earth. After all, about 30 percent of the Earth's surface is arid, desert. Africa is probably the single largest source in terms of the amount of material generated and transported over great distances, but there--China puts out a lot of dust every year and, of course, the Middle East. We often see that in the news clips on television in the evening when they show scenes from Iraq. And you'll often notice that it is quite hazy. It's because there's a lot of dust generated in that region.
BRAND: Joseph Prospero is a professor at the University of Miami's Rosenstiel School of Marine and Atmospheric Science in Florida.
Thanks for joining us.
Dr. PROSPERO: You're quite welcome.
BRAND: DAY TO DAY returns in a moment. I'm Madeleine Brand."
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