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Hurricanes Need Right Mix to Form


Officially, the Atlantic hurricane season runs from June through November, and halfway through the 2008 season, we've already seen 10 storms. Three of them - Gustav, Hanna and Ike - created havoc and headlines in just the past week. Rose Hoban takes a look at why these storms form.

Ryan Boyles, North Carolina's state climatologist, says there are a few essential ingredients to make a hurricane, starting with warm water. "Very warm water. It's that warm water that fuels everything."

By the end of summer in the Northern hemisphere, oceans are warm enough that water evaporates like crazy. It rises up and makes big puffy clouds. Those thunderheads can make a storm, but alone, they're not enough to stir up a hurricane.

Boyles says the atmosphere has to be just right. For one thing, it can't be too windy. "If you take a dandelion and you blow on it, it blows off," he points out, comparing that to what can happen in the upper atmosphere. "Imagine high winds in the upper part of atmosphere blowing the cloud tops off of a hurricane… the thunderstorms can develop but they can't get organized."

So, how do they get organized? Meteorologist Matt Parker, an associate professor at North Carolina State University, explains that for a hurricane to form, you need spin. He uses a person standing on the earth as an example. "If you could go stand on the North Pole, as the planet turns, you'd be spinning like a top. And if you could go stand on the equator, you wouldn't be spinning like a top. You'd be doing something more like when a diver does a full somersault or some such thing."

Hurricanes can't form right at the equator, because there's no spin. But as you move away from the equator and closer to the poles, you begin to get more spin.

And clouds? Floating above the earth? They would really prefer to move in a straight line. But they end up traveling around a curve instead, because the earth's spin causes them to swirl around. As the clouds spin, they suck more water out of the ocean.
But even with all that, you still need something to get it all started. Disturbed weather. And in the Atlantic, says NC State meteorologist Anantha Aiyyer, most of the disturbed weather systems come off the west coast of Africa.

Aiyyer spends a lot of time looking at weather in the Cape Verde islands; they lie about 500 kilometers west of North Africa. He says a lot of Atlantic storms form there. "These storms actually are very robust, essentially a collection of thunderstorms that form over land, over Africa, and they cluster together. And these clusters, when they go over the ocean, they rapidly organize themselves into a hurricane."

And it turns out Cape Verde Islands sit in just the right spot for hurricane formation: far enough away from the equator to put some spin into the clouds, close enough to the equator so there's lots of warm water.

And our recipe for a hurricane is complete… until that warm water is removed. Climatologist Ryan Boyles points out, "Take it away and the hurricanes fall apart. Which is exactly why as soon as they move inland and they are removed from that warm water source they tend to degrade very quickly."

Boyles says those Cape Verde storms can pack a punch. Some of them were the biggest storms of the century - Fran, Floyd, Hugo, Ivan… and now Ike could join their deadly ranks.

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