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September/October 2005 issue	In this issue Table of Contents Feature: Ottawa Valley The Inside Story Editor’s Notebook Reverberations Explorer À la carte Mosaic Re:Sources

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Spin cycle
Hurricane season is in full force. What fuels these sometimes deadly Atlantic storms?
By Sarah Mayes and Steven Fick

People living in the Atlantic provinces have good reason to fear hurricane season, which runs between June and November every year. When Hurricane Juan pounded Nova Scotia in September 2003, it claimed eight lives, levelled 100 million trees and left 900,000 people without power. It was the worst hurricane to hit Atlantic Canada in more than a century. Forecasters predict another fierce season — Dennis and Emily had already roared through as we went to press — with as many as 15 tropical storms spawning up to nine fully developed hurricanes.

The number of big blows over the past decade has been higher than normal, which most meteorologists and climatologists say is typical of the natural 10-to-20-year hurricane cycles that have been recorded over more than a century. But scientists also suggest that rising sea temperatures brought on by climate change might fuel more severe storms in years to come.

A phenomenon most common to the western North Atlantic, a hurricane forms when high temperatures heat the air at the ocean surface. Hot air rises, carrying with it water vapour that condenses into storm clouds and rain droplets and releases heat into the atmosphere. This heat, in turn, warms more air, drawing it into the developing storm and forcing it upward. Winds coming in from different directions converge, and the Earth’s rotation gives the storm its distinctive spin.

But a hurricane quickly loses steam if it moves over cool water, which cuts off its supply of warm, humid air. In the case of Juan, warm seasonal air temperatures and an unusual movement of the Gulf Stream into Nova Scotian waters meant the ocean didn’t cool off as it normally would in September. Researchers at the Canadian Hurricane Centre in Dartmouth, N.S., say the storm likely would have caused 30 to 50 percent less damage had the ocean temperatures been even three to four degrees cooler.

Whirling toward Nova Scotia on Sept. 28, 2003 (satellite image, top), Hurricane Juan decimated lives and landscapes when it made landfall the next day. Juan was fuelled by warm ocean waters, packed more than 130-kilometre-per-hour winds and dumped more than 25 millimetres of rain in some areas (above).

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