By John Pickrell
A hurricane is a fiercely powerful rotating tropical storm that is 200 to 2000 kilometres across. Hurricanes have a calm central region of low pressure between 20 and 100 kilometres across, known as the eye. They occur in tropical regions around the world, and are called typhoons in south-east Asia and the Pacific, and cyclones in the Indian Ocean and Australia.
The term hurricane is derived from Huracan, the name of a Mayan storm god. Over its lifetime, one of these massive storms can release as much energy as a million Hiroshima nuclear bombs.
The brutality of each annual hurricane season shifts according to decadal variation, but there is now evidence to suggest global warming could be making hurricanes more frequent and unpredictable. As populations and economies grow along coastal regions, the damage from these storm is increasing.
In 2005, hurricane Katrina had a devastating impact on the Gulf Coast of the US, leaving a disaster zone of 230,000 square kilometres in its wake and causing damage estimated at $75 billion. Katrina whipped up sustained winds of 225 kilometres per hour and a 5- to 9-metre-high “storm surge” of sea water. The maelstrom tore through New Orleans’ flood defences and flooded 80% of the city. Thousands were killed or injured and more than half a million people were displaced in a humanitarian crisis not seen in the US since the great depression.
Forming a storm
The seed for hurricane formation is a cluster of thunderstorms over warm tropical waters. Hurricanes can only form and grow when sea-surface temperatures exceed 27°C and the surrounding atmosphere is calm. These requirements are met between June and November in the northern hemisphere. In Australia and the southern hemisphere, the cyclone season runs from October to May.
Under these conditions, large quantities of water evaporate and condense into clouds and rain – releasing heat in the process. It is this heat energy, combined with the rotation of the Earth, that drives a hurricane.
When the warm column of air from the sea surface first begins to rise, it causes an area of low pressure. This in turn creates wind as air is drawn into the area. This wind drags up more moisture-laden air from the sea surface in a positive feedback process that swells the storm. Cold air falls back to the ocean surface on the outside of the storm.
When wind speeds hit 37 kilometres per hour, these mild, wet and grey weather systems are called as tropical depressions. Hurricane Katrina formed in this way over the south-eastern Bahamas on 23 August 2005 and was labelled tropical depression 12 of that year.
If this process happens far enough from the equator – typically at latitudes of more than 10° – then the Coriolis force associated with the rotation of the Earth sets the massive weather system spinning, with winds spiralling in towards the centre.
The weather systems spin anticlockwise in the northern hemisphere and clockwise in the southern. The rotation of the Earth also determines the direction in which these storms travel, so hurricanes in the north Atlantic move west towards Central and North America.
As winds increase to a sustained speed of 63 kilometres per hour, a tropical depression becomes a tropical storm. Such storms are then given a name – such as Charley, Frances, Gilbert, Isabel, Ivan, Jeanne or Mitch – which cycle through the alphabet each year.
A tropical storm becomes a category 1 hurricane (or cyclone or typhoon) when winds reach sustained speeds of 120kph. A hurricane becomes category 2 when sustained winds hit 154kph, category 3 at 179kph, category 4 at 210 kph, and finally the most devastating variety, category 5, when wind speeds hit 250kph.
Category 5 hurricanes rarely make it to land – 2005’s Katrina briefly achieved category 5 status, but diminished slightly before landfall. This University of Wisconsin-Madison animation of infrared satellite images shows how hurricane Katrina grew in power as it crossed the Gulf of Mexico.
Hurricanes are characterised by high winds, torrential rain and sometimes tornadoes. These conditions cause massive damage to buildings and infrastructure and create flash floods, mudslides and storm surges of sea water. Storm surges are huge tsunami-like waves resulting from the acutely low pressure at the eye of the hurricane. The low pressure draws in water, raising sea levels by up to 4 metres. Hurricanes may even trigger weak earthquakes and set the Earth vibrating.
Hurricanes can also contain very tall clouds called hot towers that stretch up to the ceiling of the troposphere. Hurricanes can continue to grow in size and ferocity as long as they are over warm waters but in reality they rarely persist for more than a few weeks. As soon as they travel inland, their power diminishes rapidly, as they can no longer draw up warm, damp air from the sea – they run out of steam, or water vapour at least.
The storm season of 2005 was the busiest since records began in 1928: 26 tropical storms formed in the north Atlantic, 12 more than 2004.
Fourteen of these – including Beta, Dennis, Emily, Epsilon, Ophelia, Rita and Stan – became hurricanes, setting another record as three developed to category 5. Wilma was the most intense hurricane ever recorded in the north Atlantic.
The hurricane season in 2005 was remarkable in other ways too. An unusual south Pacific hurricane battered the coasts of Guatemala and El Salvador in May – the first ever to strike from the direction of the Pacific. More of a surprise was hurricane Catarina, the first ever known to come from the southern Atlantic – a place the textbooks claim is too cool for hurricanes to form.
Hurricane activity was predicted to be above average in 2006 as well.
This burst of activity has fuelled speculation that climate change may be responsible. There has been a string of studies suggesting that hurricanes have become more intense, powerful and unpredictable in the last 30 years – and that this is not just down to normal decadal variation in storm activity.
One controversial study argues that the number of intense hurricanes worldwide has almost doubled over the past 35 years and it pins the blame on rising sea-surface temperatures. As the world warms, the amount of sea surface that exceeds the 27°C required to spawn a hurricane will increase – but a lot of other factors are also required for hurricanes to form, so experts are not sure what the future holds.
If hurricanes do become more frequent and intense, we might be able to find ways to mitigate their effects or even modify the weather. Possible methods include sending them off course, using water-absorbent powder to mop up clouds, coating the sea surface with fish oil or surfactants to stop warm water evaporating, and even zapping the colder streams of air in hurricanes and tornadoes with microwaves to disrupt their structure.
More on these topics:
And their effect on the built environment They are known as Hurricanes in the Caribbean, Typhoons in the China Sea, Cyclones in the Indian Ocean and most commonly called Tropical Cyclones. You can call them what you want but they all have one thing in common: they are extremely destructive when they hit cities and towns.
A cyclone is a huge mass of air moving in a spiral with winds that can reach up to 250 km/h. A cyclone starts off in the sea where the winds are calm and the water temperature is at least 240. Warm air pushes upwards at a fast rate while cold air is sucked in below. The warm air gains moisture and starts producing thunderclouds. Spinning in a clockwise direction (anticlockwise in the Northern Hemisphere), the cyclone gradually picks up speed. Driven by the fast winds, the Cyclone can move at up to 50km/h. While all this is happening a distinct "eye" is forming in the centre of the Cyclone. This "eye" unlike the rest of the Cyclone is completely calm and is not windy at all.
Cyclone Tracy was the most destructive cyclone recorded to hit Australia since white settlement in 1788 (the Bathurst Bay cyclones killed 300 people but didn't reach land). Cyclone Tracy hit the coastal city of Darwin on Christmas Day 1974. Although people had warning on New Year's Day about Cyclone Tracy, they were busy preparing for Christmas and thought that the cyclone would pass away just like Cyclone Selma had...
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