“It’s been an unusually active year for dust coming out of Africa,” says Joseph M. Prospero, a University of Miami atmospheric and marine chemist who has studied the phenomenon for more than three decades. “We’ve had spectacular dust outbreaks starting in late February. I haven’t seen anything like this in many years.”

Dust pollution occurring near the source of an African outbreak has been known to contaminate food and drinking water, cause highway accidents, close airports, disrupt radio and satellite communications, and even suffocate cattle.

Borne aloft by trade winds, the heavier particles quickly drop away. Those that survive the journey across the ocean are a hundred times smaller than the diameter of the finest human hair. The dust normally arrives in the Caribbean by mid-June, as reliable as an unwelcome house guest, and doesn’t leave until fall. Islanders see the results most vividly at sunset: the opalescent glow that ordinarily lights the sky in these regions becomes a muddy shade of burnt orange.

“BLOOD SNOWS”

At various times dust from Africa can be found as far west as South America and as far north as central Europe, where it is blamed for “blood snows” in the Alps and Pyrenees.

“It originates wherever there is a desert,” says Lester Machta, of the National Oceanic and Atmospheric Administration (NOAA). “When strong winds occur, they just pick up the dust and carry it along. Most of it falls out over the water, but enough comes across to be detectable.”

Africa isn’t the only source. Springtime windstorms in western and central China pick up large quantities of yellow dust and deposit them across Japan, especially in the south, where they’re called “kosa.” Particles that bypass Japan scatter across the Pacific Ocean to destinations as far north as the Bering Strait between Alaska and Siberia, and as far south as the Antarctic Circle.

Saharan dust periodically prompts the National Weather Service Office in San Juan, Puerto Rico, to issue an air pollution alert. Thomas Cahill of the University of California at Davis recently has found traces of Saharan dust in the Southeast and Eastern United States.

“When the dust comes up here to Miami, which is not as often as farther south, it is pretty dramatic,” says the University of Miami’s Prospero. “The visibility may be reduced to five or six kilometers, and if it rains, we get mud spots on our cars. It is not a subtle change.”

Prospero, who monitors dust through a network of air-sampling stations in both the Atlantic and Pacific, says that by the time the particles reach the United States, for the most part they have lost their harmful properties.

GULF WAR SYNDROME CULPRIT?

Dust outbreaks are likely to be most harmful close to the source. Airborne dust is one of the suspected culprits in the range of maladies labeled “Gulf War Syndrome” resulting from the 1991 U.S.-led military campaign in Kuwait. Roland Draxler of NOAA’s Air Resources Laboratory estimates that roughly 30 percent of the surface of Kuwait was disturbed by the war, resulting in higher than usual levels of dust.

However, some scientists theorize that aeolian dust may have beneficial effects. Since the particles are chemically alkaline, they may dilute sulfuric acid—a major component of acid rain. They might even be helping to offset the current trend toward global warming. (See sidebar.)

Dust can make noticeable differences in weather. In a process similar to the formation of acid rain clouds from industrial air pollutants, airborne moisture may form around the tiny particles, which become cloud droplets. The chemistry of the resulting rainfall depends on the composition of the particles that made the cloud.

An obvious question is whether human activities such as deforestation and other disturbances of the land have increased the levels of aeolian dust on a global basis. Although some scientific models in specific areas have shown as much as a 50 percent increase in dust resulting from human impacts, Prospero isn’t prepared to make any global generalizations.

“The surface of the earth is very mobile,” he says. “The continents move around, materials are moving around. Dust is another manifestation of the dynamic nature of the earth.”

Eye in the Sky is a weekly series that brings you the story behind the headlines using satellite imagery, remote sensing, aerial photography, and maps. This feature is developed by National Geographic News with the sponsorship of the National Imagery and Mapping Agency (NIMA) and Earth-Info. Check out maps and imagery at http://www.earth-info.org.