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Hurricane Earl is seen as it moves up the U.S. East Coast on Sept. 3, 2010. ((NOAA/Associated Press))

Storms that originate over tropical waters are known by different names in different parts of the world. They're called hurricanes in the north Atlantic and Pacific oceans, typhoons in the Asian Pacific and cyclones in the Indian Ocean and elsewhere.

All are powerful storms that can cause damage on a massive scale. Witness the Bhola cyclone, which killed more than 300,000 people in Bangladesh in 1970. Or Hurricane Andrew, which caused more than $30 billion US in property damage as it cut a swath across Florida in 1992.

The dangers come from a number of directions. Heavy rains can bring floods, as can storm surges — walls of water that can overwhelm coast areas. Winds, ranging from 120 kilometres a hour to more than 250 km/h, can flatten buildings and devastate entire cities.

Birth of a tropical storm

A number of ingredients must be in the mix to create a tropical storm.

First, the storm needs a relatively deep ocean — at least 50 metres of warm water, with temperatures upwards of 27 or 28 Celsius. An existing weather disturbance, or low-pressure area, is also required, but the upper atmosphere must be fairly calm.

Winds near the ocean's surface spiral into the low pressure area of the disturbance, as the warm ocean waters provide the rising air with moisture and heat. The moisture then condenses into drops, releasing more energy and powering the circular winds.

Bands of thunderstorms form if the conditions continue to be right, with the cloud tops rising further and further into the atmosphere. If winds at the top remain light, the storms can stay intact and strengthen.

The storm can then become a tropical depression, an organized system of thunderstorms with an overall circular motion and maximum sustained winds of less than 62 km/h. When a depression becomes severe enough and the winds pick up to more than 62 km/h, it is designated a tropical storm. At that point, the storm is given a name.

When the winds reach 119 km/h, the tropical storm is upgraded to hurricane, typhoon or cyclone status.

Eye of the storm

How are storms named?

An international committee has drawn up a list of 126 names — half male and half female — for North Atlantic hurricanes. The gender alternates, so Bill is followed by Claudette, then Danny, then Erika, and so on. There are 21 names for a given year, and these are repeated after a six-year cycle.

Each name corresponds to a letter of the alphabet. The letters Q, U, X, Y and Z are skipped. If the list of names is exhausted in a given year, subsequent storms are named after the letters of the Greek alphabet: Alpha, Beta, Gamma and so on.

Hurricane names are taken off the list only if they prove to be so deadly or costly it would be insensitive to re-use them in the future. The devastating Andrew, for example, has been replaced on the list by Alex.

Pacific and Indian ocean regions have similar naming conventions for hurricanes, cyclones and typhoons.

By this point, the cloud-free eye of the storm has typically formed. It's an area, up to 60 kilometres in diameter, caused by rapidly sinking air that warms and dries the centre. But the calm conditions and temporary clearing skies are an illusion — the strongest winds are found just outside the eye at ground level.

Besides strong winds and heavy rain, these intense weather systems also create a "storm surge," a massive wave beneath the centre of the storm.

In the eye, air is sucked upward faster than it can rush in at the bottom. This lowers the atmospheric pressure under the eye of the storm. As a result, the eye tries to pull at the ocean itself, creating a bulge of water as much as six metres high that moves together with the storm.

Atlantic hurricanes

The word "hurricane" itself comes from the Carib Indian word "hurican," which referred to the tribe's god of evil. This may in turn derive from the name of a Mayan god who created the world with his breath, blowing on the oceans to create dry land.

Hurricane season in the Atlantic Ocean officially runs from June 1 to Nov. 30, but the most intense storms mainly occur between mid-August and mid-October as the water remains warm but the air starts to cool.

Tropical storms that get strong enough to be classified as hurricanes are categorized by the intensity of their wind speeds using the Saffir-Simpson scale. These range in strength from a Category 1 up to a Category 5.

 Hurricane strength  Wind speed (km/h)  Damage description  Storm surge (metres)
 Category 1  119-153  minimal  1.2-1.6
 Category 2  154-177  moderate  1.7-2.5
 Category 3  178-209  extensive  2.6-3.8
 Category 4  210-250  extreme  3.9-5.5
 Category 5  250 and greater  catastrophic  5.6 or greater

These rankings are relative. A lower-category storm can cause more damage than one in a higher category, depending on where it strikes and the particular hazards it brings.

But at the top level of classification, the storms (also known as "super typhoons" or "very severe cyclonic storms" in other regions) are catastrophic events. A Category 5 storm can blow down several small buildings and clear entire streets of trees and hydro poles.

There have been eight Category 5 hurricanes in the Atlantic basin since 2000, and more than 20 since since 1924.

Hurricanes in Canada

Canadians are somewhat fortunate when it comes to hurricanes. Although these storms often end up over Canadian land and waters, they have usually lost most of their strength by the time they get here. Hurricanes need a supply of warm, moist air, and this supply is cut off when a hurricane crosses land or moves too far north.

"Most of the storms, by the time they reach Canadian areas, have weakened to tropical storm status," says Ken Kirkwood, a meteorologist at the Canadian Hurricane Centre in Dartmouth, N.S.

Still, there have been some notable exceptions.

One of the deadliest storms in the history of Atlantic hurricanes struck Newfoundland in 1775, killing an estimated 4,000 British and Irish sailors.

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Residents in central Halifax walk past trees downed by Hurricane Juan in September 2003. (Paul Darrow/Reuters)

Then, in September 2003, Hurricane Juan slammed into the eastern shores of Nova Scotia, bringing winds of more than 140 km/h and a record storm surge causing coastal flooding. A Category 2 storm, Juan tore through the centre of the province, including Halifax, and caused damage on Prince Edward Island.

The storm killed eight people and left about 300,000 homes and businesses without power.

About 70 per cent of the trees in Halifax's 186-acre Point Pleasant Park were destroyed. In 90 minutes, the storm knocked over as many trees in that one park as the Nova Scotia logging industry cuts down in a year.

A year after the storm, provincial officials estimated the cleanup costs at $150 million.

Several other storms — including Kyle (2008), Hortense (1996), Gustav (2002) and Michael (2000) — have landed at hurricane force along Canada's east coast in recent years.

Tropical storms have reached as far inland as Ontario. Hurricane Hazel in 1954 was the storm of a lifetime — and one of the few to inflict significant damage in central Canada. 

The storm formed in early October and crossed the Caribbean and the eastern U.S. before entering southern Ontario, where 81 people were killed in the Toronto region.

Forecasting tropical storms

The ability of scientists to predict the path and destructive power of a tropical storm has improved greatly since Hurricane Hazel. The greatest boost has come from weather satellites that show a storm's precise location and allow tracking at 15-minute intervals.

Meteorologists are usually able to give several days warning before a storm strikes land. (By comparison, the devastating hurricane of 1900 took the city of Galveston, Tex., completely by surprise and killed 8,000 people).

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A large part of the city of Galveston, Texas, is reduced to rubble after being hit by a surprise hurricane, Sept. 8, 1900. (Associated Press)

Another difference is the ability of scientists to collect data from within the storm itself. One tool they use is the "dropwindsonde," an instrument dropped from an airplane and used to measure wind speed, air pressure and humidity within the storm. The data from satellites, dropwindsonde, and other sources are fed into powerful computers that use mathematical models to simulate a storm's development. Forecasters then study the computer's output to plot a hurricane's expected path.

Canadian technology has contributed the RADARSAT, a satellite that orbits the Earth bouncing radar signals off the surface of the planet. For a hurricane that's not yet over land, it can create a high-resolution picture of the sea surface, penetrating cloud cover, which picture-taking optical satellites can't do.

The end result is that progress is being made: The average tracking error has been halved since the 1970s when a typical three-day forecast would be off by 650 to 720 km; that error is now between 325 and 400 km.

"We've made significant improvements," says Dan Petersen, a meteorologist with the U.S. National Weather Service in Silver Spring, Md. "The average track errors have improved."

But that's only half the battle, he says. Because of the dramatic rise in population along coastal areas (and in South Florida in particular), the region needs earlier warnings than in the past. "Our population has increased so much along our coastline that it takes people longer to leave if they have to evacuate. And so they need longer lead times in order to make a decision, or to leave the area."

Global climate and the future

While tropical storms seem to follow certain natural cycles, scientists are concerned about the effects of global warming and long-term climate change. On Sept. 15, 1999, the United Nations issued a report predicting that global warning will cause more frequent and more severe tropical storms, floods and tornadoes in the coming century.

"We do know that hurricane intensity is directly correlated to how warm the ocean waters are," says Petersen. "And if global warming continues to occur, and this results in warmer water temperatures, then we'll see an undeniable signal of stronger hurricanes."

But scientists have a difficult time disentangling the effects of man's activity from the various natural cycles also known to be present.

For example, when there's an El Nino — with warm surface waters building up in the eastern Pacific Ocean — high-altitude wind patterns are affected, and the number of Atlantic storms is low. Hurricane formation is also linked to wind patterns in the stratosphere, ocean temperatures in different regions, and rainfall in West Africa.