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The 2005 hurricane season made it into the record books from the outset, taking the title for the greatest number of tropical storms at the start of a season, four in the first month. It also set the record for the most intense start. Dennis, at the beginning of July, was the earliest Category 4 hurricane since measurements were introduced; normally, storms of that intensity strike much later.

And now "much later" has come much sooner than expected in the devastating guise of Katrina -- a Category 5 hurricane, the most intense of all. It is the deadliest hurricane to hit the United States since Camille in 1969 and very likely will be the most destructive and costly of all time.

What's ahead will probably set even more records. The U.S. National Oceanic and Atmospheric Administration predicts double the number of tropical storms (as many as 21 compared with the average of 10) and as many as 11 hurricanes (almost double the average of six).

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The extraordinary meteorological fury of the 2004 season, followed by this year's unprecedented destruction, makes up what the NOAA calls "above normal" activity. This is not understatement. In fact, we are simply in a cyclical period of increased hurricane activity that began in 1995.

What we are experiencing is the result of a change in ocean and atmospheric conditions called the Atlantic Multidecadal Mode. This is "a North Atlantic and Caribbean sea surface temperature shift between warm and cool phases that lasts 25 to 40 years each," the NOAA says.

The last cycle of "above normal" activity occurred in late 1920s to the late 1960s. It was during this period that legendary Hurricane Hazel swept into Ontario, devastating Toronto and causing the deaths of 81 people.

But what's normal for a hurricane season? And what's normal for a hurricane? These cyclopean pinwheels vent such enormous quantities of energy that the only way to get a grip on what's normal is to turn to what's not normal. Arguably, the best measure available is the megaton weaponry of nuclear warfare.

It is just past 60 years since the atomic bomb dubbed Little Boy was detonated over the Japanese city of Hiroshima. So devastating was the explosion that it stands as the benchmark of humanity's horrific potential for destruction.

The bomb left a 150-metre-wide crater below the point of detonation and destroyed about 60,000 buildings over six square kilometres. Eighty-five thousand lives were instantly obliterated; 55,000 others were lost after months of suffering the effects of the atomic blast.

That explosion was the equivalent of 12.5 kilotons of TNT, enough to fill 200 freight cars stretching down two kilometres of railway track.

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However, in terms of the devastating power of hurricanes, Little Boy was as nothing at all.

Of course, hurricanes don't pack all of their destructive force into a bomb's narrow confines of space and time. They mete it out over an area that is, typically, one million square kilometres -- the size of Ontario or South Africa -- and over the course of days.

Still, the most intense activity is concentrated at the wall that rings the eye of the hurricane, which can be 20 to 100 kilometres across.

From above, torrential rains pelt down as much as 2.5 metres of water in a single day. From below, a storm surge whipped by violent waves can raise the height of bodies of water by as much as nine metres and pull the sea inland by 16 kilometres. In between, winds tear at everything in their path at speeds of up to 289 kilometres per hour.

Farther out from the eye, conditions are less intense, but violent nonetheless and capable of enormous destruction even at the outer fringes of a fierce storm.

Taken over the entire cloud shield of an "average" hurricane, the energy released daily in the form of rain and wind is the equivalent of about 13,000 megatons -- almost equal to the destructive potential of all the weapons in the Cold War missile silos in the United States and the former Soviet Union. That's a million Hiroshima bombs exploded at a rate of more than 10 a second -- 20 Little Boys for each of the 50,000 (estimated) cities on the planet.

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The source of all this energy is truly unremarkable and innocuous. It's what causes water ring stains on the cherished coffee table: evaporation.

Essentially, hurricanes are colossal heat engines that balance the heat contained in the atmosphere with that in the ocean. They do this, simply enough, by taking up energy in the form of water vapour from tropical surface waters and making the air hotter and more humid.

If it seems an extraordinary thing that something as ordinary as the evaporation of water could drive anything as powerful as hurricanes, consider the amount of energy that is involved in the evaporation of even small volumes of water.

To raise the temperature of a 1.5-litre bottle of water by just one degree Celsius requires little enough -- the energy used by a 100-watt bulb in just one minute will do it. This is so even when the rise in temperature is from 99 degrees to the boiling point.

However, taking water at 100 degrees from liquid form to vapour takes a lot more energy -- 6,000 times more. It takes so much more, in fact, that the energy needed to evaporate 1.5 litres is as much as the energy released by one kilogram of TNT.

The average rain cloud holds about 300,000 tons or the equivalent of 300 million litres of water. The amount of energy required to evaporate all this water is equal to almost 200 kilotons of TNT, or 15 Hiroshima bombs.

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The formation of a hurricane requires about 100 million tons of water vapour (100 billion litres of liquid water) per hour. It takes the equivalent of the energy released by 50 megatons of TNT to evaporate that much water. That's per hour, mind you. In a day, trillions of litres of water are taken up into the atmosphere as vapour, requiring the equivalent of more than a thousand megatons of energy.

If what goes up must come down, then all the energy taken up through the evaporation of water must come down again. And it does -- as the violent winds and torrential rains that make hurricanes so massively destructive.

In the course of a season, over the Atlantic, Pacific and Indian Oceans, the destructive energy released by hurricanes is staggering. On average, there are 45 hurricanes a year for a total of about 450 hurricane days. Together, they amount to the equivalent of almost five million megatons of TNT. More than 3,000 times the Cold War nuclear arsenal. Almost 400 million Hiroshima bombs. Annually.

But a hurricane's destructiveness is not about energy alone. In fact, the most destructive hurricanes are not necessarily the most intense. Of the five costliest hurricanes of all time in the Atlantic, only one was classified as Category 5.

More important is where a hurricane makes landfall, that is, where the eye of the hurricane crosses from water onto land. When this happens in populated and developed areas, economic damage soars. Last year, Hurricanes Charley, Ivan, Frances and Jeanne inflicted a collective economic toll in excess of $40-billion (U.S.).

However, the deadliest of hurricanes have not struck the United States. Of the 45 hurricanes that occur in the average year, only six take place over the Atlantic. The majority strike the Indian and Pacific Oceans.

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The worst Atlantic death toll was the Great Hurricane of 1780, which caused as many as 30,000 deaths in the Caribbean.

This figure is overwhelmed, all too often, by the number of deaths in the Pacific and Indian Oceans. Over these bodies of water, hurricanes (referred to as cyclones) that make landfall in highly populated areas have caused hundreds of thousands of deaths. Most catastrophic of all was a 1970 cyclone that killed 500,000 people in Bangladesh.

The present hurricane season has really only just begun. The most intense part is still to come.

Siegfried Betterman, a freelance journalist and former professor of mathematics at George Brown College in Toronto, is writing a book on the extremes of magnitude entitled Taking Measure.

Costly and deadly

The five costliest hurricanes of the Atlantic Ocean (in 2004 U.S.

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dollars):

1. Great Miami Hurricane (1926, Category 4),

$70-billion

2. Andrew (1992, Category 5),

$26.5-billion

3. Charley (2004, Category 4),

$15-billion

4. Ivan (2004, Category 3),

$14.2-billion

5. Frances (2004, Category 2),

$9-billion

Five deadliest hurricanes:

1. Bangladesh (1970),

500,000 deaths

2. China (1881), 300,000

3. Haiphong, Vietnam (1881), 300,000

4. Bengal, India (1737), 300,000

5. Bangladesh (1876), 250,000

Information for damage figures from the National Hurricane Center and the Hurricane Research Division of Atlantic Oceanographic and Meteorological Laboratory.

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