BACKGROUND: Researchers at the University of Wisconsin, Madison, have proposed an intriguing theory as to what might be causing stronger and more frequent hurricanes. They discovered a surprising link between hurricane frequency in the Atlantic Ocean and thick clouds of dust that periodically rise from the Sahara Desert and blow off Africa's western coast. The findings are significant because they show that long-term changes in hurricanes may relate to different factors. If scientists conclusively prove that dust storms help to squelch hurricanes, weather forecasters could one day begin to track atmospheric dust, factoring it into their predictions for the first time.
A WEATHER CONUNDRUM: The question of what, if anything, might be causing stronger and more frequent storms remains open. Some scientists point to rising ocean temperatures, brought on by global warming, while others think the upswing is simply part of a natural cycle in which hurricanes become worse for a decade or two before dying down again. The UWM scientists' findings add another piece to the puzzle. They pored over 25 years of satellite data from 1981 to 2006 and noticed a correlation between periods of intense hurricane activity and scarce amounts of dust in the atmosphere. In years when stronger dust storms rose up, on the other hand, fewer hurricanes swept through the Atlantic.
DUST TO DUST: Dust storms form primarily during the summer and winter months in the Sahara, but some years they barely form at all, and scientists are unsure why. Attention has turned to the environmental impact of dust since it became clear that in some years, millions of tons of sand rise up from the Sahara Desert and float across the Atlantic Ocean, sometimes in as few as five days. The sand rises when hot desert air collides with the cooler, dryer air of the Sahel region -- just south of the Sahara -- and forms wind. As particles swirl upwards, strong trade winds blow them west into the northern Atlantic. It's also possible that these dust storms might suppress the development of hurricanes. Sahara Desert dust storms impact the atmosphere in three ways: (1) dust storms are extremely dry and cover a large area; (2) dust storms have strong winds; and (3) dust absorbs heat and prevents cloud formation. Dry, dust-ridden layers of air may help to dampen brewing hurricanes, which need heat and moisture to fuel them. That effect could also mean that dust storms have the potential to shift a hurricane's path further to the west, giving it a higher chance of hitting US soil.
ABOUT HURRICANES: A hurricane is a type of tropical cyclone, a low-pressure system that usually forms in the tropics and has winds that circulate counterclockwise near the earth's surface. Storms are considered hurricanes when their wind speeds surpass 74 MPH. Every hurricane arises from the combination of warm water and moist warm air. Tropical thunderstorms drift out over warm ocean waters and encounter winds coming in from near the equator. Warm, moist air from the ocean surface rises rapidly, encounters cooler air, and condensed into water vapor to form storm clouds, releasing heat in the process. This heat causes the condensation process to continue, so that more and more warm moist air is drawn into the developing storm, creating a wind pattern that spirals around the relatively calm center, or eye, of the storm, much like water swirling down a drain. The winds keep circling and accelerating to form a classic cyclone pattern.