African Dust Linked To Hurricane Strength

MICHELE NORRIS, Host:

This hurricane season in the Atlantic is shaping up to be a busy one after two relatively quiet years. Scientists at the cutting edge of hurricane research say one reason we're seeing more storms this year may have to do with African dust.

NPR's Jon Hamilton reports.

JON HAMILTON: Big hurricanes like Gustav usually start out as small storms coming off the coast of Africa. These so-called African easterly waves tend to get stronger when they encounter favorable winds and warm water. They tend to dissipate if they hit cool water, disruptive air currents or dust.

GREG JENKINS: Last year, we saw African easterly waves just get taken apart by dust storms.

HAMILTON: Greg Jenkins is a hurricane expert at Howard University. He says dust also seem to weaken a lot of storms in 2006. But Jenkins says this year, the satellite images are telling a different story.

JENKINS: We don't see any dust - any significant dust. But we do see Josephine, Ike and we see Hanna.

HAMILTON: It's not clear why the amount of dust vary so much from year to year. But Jenkins is one of a growing number of scientists who believe that dust helps explain why some storms become hurricanes and some don't. He says the dust get swept up into the atmosphere as winds whip across the deserts of northern Africa.

JENKINS: So in the summertime, we see a layer, basically about three miles up to probably six, seven miles high where we see a dust layer. We call it the Saharan air layer.

HAMILTON: And it carries a lot of dust, millions of tons each season. For the past few years, scientists have been trying to figure out just what that dust does to baby storms.

In 2006, NASA spent a month flying planes near developing storm systems along the African coast. During one of those flights, NASA scientist Robbie Hood described what happened to one of the storm system she was tracking.

ROBBIE HOOD: The first day we flew it, it was pretty much clear in the center of it, just normal moisture pattern. The next day we flew it, we got in the center of it and it was filled with dust.

HAMILTON: The storm petered out. That sort of research has added to the evidence that dust tends to tamp down developing storms. Bill Lau, who works at NASA Goddard Space Flight Center in Maryland, has published several studies on dust and storms. He says they all reach the same conclusion.

BILL LAU: More dust, less hurricane and more hurricane, less dust.

HAMILTON: But it's not entirely clear why. Lau says one likely reason is that dust particles absorb some of the sun's heat. They also reduce the amount of sunlight that reaches the ocean.

LAU: So as a result, the ocean absorbs less heat from the sun, and therefore, it has an effect of cooling the ocean.

HAMILTON: Meaning, less potential energy to fuel a hurricane. And Lau says there's another factor.

LAU: Accompanying the dust are always dry air from the Sahara. And hurricane does not like dry air. So both of these effect, the dry air itself and also the blocking(ph) of solar radiation can suppress a hurricane.

HAMILTON: Dust researchers say they are not trying to downplay other factors affecting the hurricane season - things like sea surface temperature and the presence of El Niño wind patterns. But they say it may be time to start considering dust when making seasonal forecasts. Greg Jenkins says the lack of dust this year tells him that Josephine won't be the season's last storm.

JENKINS: I'm a little nervous because unless dust really starts to develop or we get some other factors to weaken them, you know, it may be a rough ride in September and in October.

HAMILTON: After all, we're still only halfway through the 2008 hurricane season.

Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.