The international term used to describe a severe storm at sea is a tropical cyclone. The most powerful tropical cyclones are called hurricanes in the Atlantic Ocean and near North America in the Pacific Ocean. In the western Pacific they are called typhoons. Before a storm attains hurricane status, it passes through the tropical storm stage, with winds ranging between 65 and 120 km/h, while hurricane winds exceed 120km/h.
Hurricanes form over warm oceans where there is enough evaporation to provide the storm's energy. They also need to be far enough away from the Equator for the Earth's rotation to provide the impetus for spiral motion. The rotation results in winds circulating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
The right conditions to form hurricanes and tropical storms occur during June through to November. The ocean water must be at least 26.5 degrees Celsius or higher for this natural hazard, therefore August to October are prime hurricane months. Hurricanes can sustain themselves for as long as a few weeks, however upon reaching cooler water or land they rapidly lose their intensity.
A distinctive feature of a hurricane is the dark centre core called the eye. Surrounding the eye is a region of the most intense winds and rainfall. The eye of the storm is actually the calmest section of any hurricane. This recognizable feature allows these devastating storms to be easily spotted by satellite or radar observing stations. Being able to detect these storms, predict their evolution and track them, allows for advance warning to be given to citizens potentially affected by the storm and for emergency preparedness activities to be undertaken. This has significantly reduced hurricane fatalities and injuries in developed countries like Canada and the United States. However there is still a large toll of lives lost in countries without adequate warning systems.
Hurricanes have a devastating impact on the affected area, ranging from enormous waves on the high seas to heavy rains on land. Storm surges, a dome of ocean water that can be 5.5 m at its peak and 100 to 200 kms wide, can devastate coastal communities. Nine out of 10 hurricane fatalities are attributable to storm surges.
The Saffir-Simpson scale is based on the windspeeds of tropical cyclones using a five-point ranking system - category 1 (least dangerous) to category 5 (extremely dangerous). A category five hurricane has wind speeds exceeding 250 km/h with a 5.5 metre high storm surge (the height of the ocean water above normal tide). This type of storm is rare and to date has never been recorded in Canada. However the United States has experienced two category 5 hurricanes over the past seven decades.
