TROPICAL STORMS & HURRICANES

CBC News Online Updated October 19, 2005

WHAT ARE TROPICAL STORMS?

Severe storms spawned in the tropics are known by different names in different parts of the world: hurricanes in the Atlantic and east Pacific; typhoons in the northwest Pacific and cyclones in the southwest Pacific and Indian Ocean. These storms originate over tropical waters, close to the equator. If the atmosphere is calm and the water is warmer than about 27 C, evaporation forces large amounts of moisture into the air, creating a low-pressure system. When this water vapour condenses, it releases heat that powers the circular winds that characterize these storms. Rainfall in the developing storm releases more heat, triggering a convection process that pulls more moisture-laden air up through the centre of the system. The storm grows via this feedback mechanism. The strongest winds are found immediately outside the centre, or "eye," of the hurricane at ground level.

Every one of these systems begins as a tropical depression, a system of thunderstorms with an overall circular motion and maximum sustained winds less than 62 km/h. When a storm becomes severe enough and the winds pick up to more than 62 km/h, it is designated a tropical storm. When the winds reach 119 km/h, the system is called a hurricane, typhoon or cyclone. An average of 10 tropical storms develop over the Atlantic Ocean each year, of which about six become severe enough to be called hurricanes.

Besides strong winds and heavy rain, these hurricanes also create a "storm surge," a massive wave beneath the centre of the storm. In the eye of the hurricane, 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.

WHEN IS HURRICANE SEASON?

Hurricane season officially runs from June 1 to November 30, but the most intense storms mainly occur between mid–August and mid–October.

HOW ARE HURRICANES CLASSIFIED?

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.

Only three Category 5 hurricanes have made landfall in the U.S. in the past century – one that struck the Florida Keys in 1935, Hurricane Camille in 1969, and Hurricane Andrew in 1992.

HOW ARE TROPICAL STORMS NAMED?

A tropical storm is given a name if its winds reach a speed of 62 kilometres per hour. An international committee has drawn up a list of 126 names - half male and half female - which are repeated after a six-year cycle. There are 21 names on the list for a given year, with each name corresponding to a letter of the alphabet. The letters Q, U, X, Y and Z are skipped. If the list of names is exhausted for a given year, which may occur in 2005 with the formation of tropical storm Wilma in October, subsequent storms are named after the letters of the Greek alphabet: Alpha, Beta, Gamma and so on. However, if a hurricane causes extensive damage, its name is retired from use. So far, more than 50 names have been retired, from Hazel in 1954 to Juan in 2003. 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.


MEMORABLE HURRICANES

Andrew

In August and September 1992, Hurricane Andrew wrought havoc across the Caribbean and Florida. Andrew was an unusual storm - after brewing for several days at low latitudes, it weakened and nearly vanished about 600 km east of Puerto Rico. But the storm regained its strength and moved northward with devastating results. Andrew left 17,000 people homeless in Florida alone and destroyed or badly damaged 85,000 homes. The storm caused a record $26.5 billion US in property damage. In 2002 Andrew, initially classified a Force 4, was upgraded to have actually been a Force 5 storm.

Mitch

In November 1998, Hurricane Mitch tore through Central America, killing as many as 10,000 people and leaving two million homeless in Nicaragua and Honduras. The storm's 300 km/h winds and heavy rains caused more than $3 billion US in damage – more than half the combined gross domestic product of those two countries. Mitch also unleashed deadly landslides, and caused the worst flooding in the region in 200 years. In the storm's wake, with roads and infrastructure wiped out, thousands of people developed illnesses such as dengue fever, cholera and malaria.

Hugo

Hurricane Hugo swept across the Caribbean and the southeast U.S. in September 1989, leaving a 3,700-kilometre-long trail of destruction from Guadeloupe to the Carolinas. The storm killed at least 28 people in the Caribbean, left up to 80,000 homeless and caused $2 billion US in damage. In the U.S., it killed another 11 people and caused more than $750 million in damage. The storm sent giant waves crashing onto the U.S. Eastern Seaboard, with a wall of water some five metres in height washing over Charleston, S.C.

HURRICANES IN CANADA

Canadians are lucky when it comes to hurricanes. Although these storms often end up over Canadian land and waters, they have usually lost most of their strength. "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. Hurricanes need a supply of warm, moist air, and this supply is cut off when a hurricane crosses land or moves too far north. However, a storm occasionally reaches eastern Canada while still packing hurricane-strength winds.

This happened in September 2003 when 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.Two people were killed, including a paramedic who died when a tree fell on his ambulance near a Halifax hospital. More than 100,000 people lost electricity, thousands of trees were uprooted and a section of a four-storey apartment building in Dartmouth collapsed.

Hurricane Hortense

In September 1996, Cape Breton Island, N.S., felt the wrath of Hurricane Hortense. The storm brought 120 km/h winds and caused $3 million in damages, mostly due to flooding, wind damage and power outages.Hurricane HazelFor Ontarians old enough to remember it, Hazel was the storm of a lifetime – and one of the few to inflict significant damage in central Canada. Hurricane Hazel formed in early October 1954 and crossed the Caribbean and the eastern U.S. before entering southern Ontario. The storm left as many as 1,000 dead in Haiti, six more in the Bahamas, another 95 in the U.S. and 81 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). Another difference is the ability of scientists to collect data from within the storm itself. One tool they use is the "dropwindsone," an instrument dropped from an airplane and used to measure wind speed, air pressure and humidity within the storm. The data from satellites, dropwindsone, 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. (In practice, several different models are used simultaneously and their results averaged together. As the storm progresses, the output from the most successful models is given greater "weight.")

And 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."

TROPICAL STORMS AND GLOBAL CLIMATE TRENDS

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.