Tropical cyclone is the general term for an intense low-pressure system in the tropics. Its features are revolving storms and cloud bands around a centre of very low atmospheric pressure. In the Americas its name is hurricane, in Asia typhoon and in Australia simply cyclone.
Tropical cyclones develop from low-pressure systems. Warm and moist air lifts over a large area and draws air from the surroundings, creating wind. The deflection of the earth’s rotation, the Coriolis effect, forces the wind to rotate around the calm center A circle of severe thunderstorms forms and draws an ever-increasing amount of air. The air pressure in the center falls steadily. If the conditions are right, the system becomes self-generating. Finally, when the wind speed exceeds 118km/h a tropical cyclone is born.
A mature tropical cyclone shows a distinct cloudless eye, surrounded by very strong winds and bands of severe thunderstorms. The system remains alive until it moves over a region with lower water temperatures or over land. In either case, the supply of warm and moist air stops. Before it dies, however, the system dumps a large amount of rain and may cause widespread flooding.
Weather services around the world agree on one particular scale to categorize tropical cyclones. This scale, the Saffir Simpson scale, uses five wind speed ranges as indicators of a cyclone’s power.
Satellites, weather radar and reconnaissance flights allow the meteorologist to keep track of a tropical cyclone. The difficult part is to predict its next move. Cyclone movements are very erratic and sometimes they don’t move at all. The strength varies constantly and even a ‘dead’ cyclone may come back to life. The forecaster has no option other than warning the population of a large section of coastline. A large percentage of the population is, therefore, only moderately affected by the storm and may become complacent the next time.
Will a global climate change bring us more and stronger tropical cyclones? Very likely. Scientists estimate that a small water temperature rise of less than one degree could extend the cyclone season by 20 days. Tropical cyclones need water temperatures of at least 26.5°C to develop and survive, so rising water temperatures will expand the area of activity. The Americas have another reason to fear imminent bad seasons. Caribbean hurricanes follow a 20-year cycle of changing activity levels. US meteorologists warn that the hurricanes are entering a phase of increased activity.
Around 83% of the Australian population live within 50km of the coastline (1996 census). The US expects 73 million people will make the coastline and a warm climate their home by the year 2010. But you don’t have to look to the future for population growth. Tropical Queensland is the fastest growing state in Australia. And between 1988 and 1993, the population of Florida increased by more than one third.
The population growth along the coast ensures an ever-increasing damage bill. If the predictions of increased cyclone activity due to global warming hold true, you don’t have to be a seer to predict a ‘big’ one.
A tropical cyclone causes damage in three ways:
Rising water levels and large waves are the heralds of worse to come. Low atmospheric pressure in the center of the cyclone raises the sea level, as if it were ‘sucking up’ the ocean. The rise can be anything up to 1.5 m. In addition, strong winds around the eye pound the seas and whip up mountainous waves.
If the raised water and the waves reach the shallows of the coast and encounter a sloping beach, they grow even higher. And if this combination, called a storm surge, is trapped in a bay, the sea level can gain the height of a two-storey building: around 5 m. Many major harbor cities are well below this mark. Some are even below mean sea level, only protected by embankments.
In 1969, the ‘ifs’ came together in the Bay of Bengal and caused the worst weather-related disaster in history. Over 250,000 citizens of Bangladesh lost their lives. The 1900 storm surge ahead of an unnamed hurricane that swept across Galveston, US, killed more than 8,000 people, and showed that water can be more devastating than wind.
These situations are rare, but even waves half the size that last several hours can cause enough trouble. Then you find out if your builder has done a good job on your house’s foundations: will it still stand after all the sand or soil has been washed away, or will it float like a boat and be driven away by strong winds?
Very strong winds, sometimes lasting for several hours, blow across communities, causing immense damage to housing and infrastructure. While the wind doesn’t exact a high death toll – the average annual death toll from hurricanes in the US is below 100 – the economic costs can reach many billions of dollars.
When hurricane Hugo struck South Carolina in 1989, it left a damage bill of almost A$20 billion. Three years later, in 1992, hurricane Andrew surpassed this mark with damage to insured properties in excess of A$30 billion. Experts believe that the bill is likely more than double the amount if they could add the uncalculated damage to uninsured properties.
Several cyclones cross the Australian coast every year. In 1974, tropical cyclone Tracy destroyed Darwin with winds of up to 250 km/h. Of 11,200 homes, only 400 remained in reasonable condition. To avoid an outbreak of diseases, over 35,000 residents were evacuated. The cyclone killed 65 people.
The winds of a tropical cyclone are generally not quite as strong as the swirling air inside a tornado, but size is an issue. Tropical cyclones are often over 500 km in diameter. Large storms in the Philippines and Japan occasionally reach a size of 1,600 km. The zone of destructive winds, sometimes in excess of 250 km/h, covers an area of over 50 km. If this isn’t bad enough, the system also takes its time. The destructive winds can blow for several hours before they pass a particular point. The tropical cyclone itself may last for a week or more.
The wind itself is often only indirectly responsible for building collapses. Windborne debris, such as tree branches, roof tiles, metal sheeting etc. destroy roofs, windows and walls. Strong winds can then go to work and disassemble the rest. Flying debris inflicted most damage to homes in Darwin when tropical cyclone Tracy hit the town.
Of course, a punctured roof or broken window can’t keep out the third burden: rain. Quite often, the content of a factory or home is more valuable than the building itself. The building may withstand the fury of the wind, but the hole in the roof or the broken window is enough to destroy your million-dollar machinery or stamp collection.
A tropical cyclone carries a large amount of moisture in the cloud bands around the eye. In 1998, hurricane Mitch devastated communities in Central America, in particular Honduras. Torrential rain and hurricane flooding caused heavy local floods (flash floods), river floods and mud slides, which killed more than 10,000 Hondurans, left several million people homeless and destroyed the economies of struggling nations.