Mix of Factors Weakened Hurricanes This Season The Atlantic hurricane season has come to an end, and it was a quiet year in the United States. But in Mexico and Nicaragua, two Category 5 storms did a lot of damage. Scientists argue about whether global warming is affecting hurricanes, and this year won't help settle the question.
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Mix of Factors Weakened Hurricanes This Season

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Mix of Factors Weakened Hurricanes This Season

Mix of Factors Weakened Hurricanes This Season

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MELISSA BLOCK, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

ROBERT SIEGEL, host:

And I'm Robert Siegel.

It's the last day of November and the end of the Atlantic hurricane season. This year has not helped to settle scientific argument over whether global warming is affecting hurricanes. Fourteen named storms appeared in the Atlantic and the Caribbean. Not all of those were hurricanes. In fact, most of them were relatively weak. And in the past, some might not have been named at all.

NPR's John Hamilton has more.

JOHN HAMILTON: Only one hurricane made landfall in the U.S. this season. A Category 1 storm named Umberto struck the northeast coast of Texas in September. It cost about $50 million of damage.

Greg Jenkins, an atmospheric scientist at Howard University says it would be easy for Americans to draw the wrong conclusion about the hurricane season.

Dr. GREG JENKINS (Atmospheric Scientist, Howard University): We tend to say, well, if we don't get hit by a big one, then things weren't that bad. But then if you go into Central America or you go into Mexico, well, they have the Katrinas of 2005.

HAMILTON: Except they were named Dean and Felix. More on those storms in a minute. But first, some reasons why the U.S. got off so easy. Jenkins says early on, conditions favored lots of storms coming off the west coast of Africa.

Dr. JENKINS: In May and June, we were thinking, wow, we had three storms already. This is going to be a busy season.

HAMILTON: Then, Jenkins says, winds began to appear that took air from the Sahara Desert and pushed it out over the Atlantic Ocean.

Dr. JENKINS: And those are bad because what they do is they make the environment very stable. The air is very dry.

HAMILTON: They're bad if you're a budding hurricane. Storms need warm water and wet, unstable air to grow. Jenkins says the storms that did make it across the Atlantic stayed south of the U.S. because of a high-pressure system that acted like a barrier to hurricanes.

Dr. JENKINS: And it pretty much kept those storms on the west to west-northwest track. So they never really turned north towards the U.S.

HAMILTON: The way Katrina did. Scientists at NASA Goddard Space Flight Center say they noticed something else. Scott Brown, a research meteorologist there says satellite images showed a lot of relatively cool water near Africa.

Mr. SCOTT BROWN (Research Meteorologist, NASA Goddard Space Flight Center): Temperatures in the Eastern Atlantic Ocean were cooler in 2007 than they were in 2005 and 2006. So that means that disturbances in the Eastern Atlantic would have had less energy available to them due to the cooler ocean temperatures.

HAMILTON: As a result, several potentially large storm systems petered out over the ocean. But Brown says two of the ones that made it became the monsters named Dean and Felix.

Mr. BROWN: They were able to reach the very warm waters of the Western Atlantic and Caribbean.

HAMILTON: Dean plowed across Mexico's Yucatan Peninsula in August, bringing winds of 165 miles an hour. Two weeks later, Felix pummeled the border between Nicaragua and Honduras with similar intensity. The 2007 hurricane season raises a troublesome question for meteorologists: How to classify marginal storms like the first one of the season named Andrea?

Mr. BROWN: In years prior to satellite data, we certainly wouldn't have figured that there was something out there worth investigating by aircraft. And so it probably would not have been called a subtropical storm.

HAMILTON: That makes it hard to compare the number of storms in recent years to those many decades ago. And that's a big concern to people who are trying to decide whether the total number of storms has increased because of global warming.

John Hamilton, NPR News.

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