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HURRICANES: low-pressure systems that pack more power than the energy released by the atomic bomb that levelled Hiroshima.
Paris, France | AFP | No matter what they are called — hurricanes, cyclones or typhoons — the giant tropical storms that form in oceans near the Americas and Asia can be deadly, destructive and terrifyingly capricious.
Even as a waterlogged Houston struggles to recover from Hurricane Harvey, another ferocious storm, Irma, is ripped across the Caribbean towards Florida.
At full throttle, these low-pressure systems pack more power than the energy released by the atomic bomb that levelled Hiroshima.
In the Atlantic and northeast Pacific, they are known as hurricanes, while typhoon is the term used in Pacific Asia. The same weather phenomenon in the South Pacific and Indian Ocean is a cyclone.
From outer space, they look like a smoke-enshrouded firework pinwheel, or what astronomers may imagine as the swirling vortex around a black hole.
Meteorologists call them all “tropical cyclones,” and grade them according to intensity, taking into account maximum sustained wind force and potential damage.
Hurricane Irma, gusting at up to 294 kilometres per hour (185 miles per hour), is a top-level Category 5 storm on the Saffir-Simpson scale. It is one of the strongest to ever be recorded in the Atlantic.
Another Category 5 storm, Hurricane Katrina, killed over 1,800 people across the US Gulf Coast when it struck in 2005.
Category 5 storms have sustained winds — at least a minute — of 252 kph (157 mph) or higher, while the range for Category 4 cyclones is 209-251 kph.
At the lowest level on the scale, Category 1 winds blow at 119-153 kph. Below this threshold, the phenomenon is rated as a tropical storm.
Cyclones are formed from simple thunderstorms at certain times of the year when the sea temperature is more than 26 degrees Celsius (79 degrees Fahrenheit) down to a depth of 60 metres (200 feet).
They suck up vast quantities of water through evaporation, which is dumped as torrential rain. Flooding, property damage and loss of life can result.
Late afternoon sun casting a shadow on Hurricane #Irma‘s eye. #GOES16 pic.twitter.com/CQaSgHzVN6
— NASA SPoRT (@NASA_SPoRT) September 5, 2017
– Storms ‘to get stronger’ –
Scientists have long predicted that global warming will make cyclones more destructive, and some say the evidence for this may already be visible.
Warmer oceans add to the raw fuel on which cyclones feed, and higher sea levels boost storm surges that may overcome coastal defences.
“We know that the strongest storms are going to get stronger as the climate warms,” said James Elsner, a professor at Florida State University and an expert on hurricanes.
“The ocean fuels these storms with warm water — the warmer the waters, the stronger the storm can get,” he told AFP.
Sea level rise — roughly predicted to top a metre (3.25 feet) by century’s end — is already contributing to more devastating storm surges, he added.
“Finally, as the atmosphere warms, it holds more water — that makes the storm produce more rain, which can lead to flooding.”
At the same time, cyclones may become less frequent, he added.
How much damage do Category 5 winds do compared to Category 1 winds? There’s a visualization for that (and it’s not pretty) pic.twitter.com/GaO3jbp2QE
— Brian L Kahn (@blkahn) September 5, 2017
And how destructive a storm is depends greatly on vulnerability — sprawling urban development in at-risk areas, for instance — and emergency preparedness, experts add.
Hurricanes and typhoons can trigger large swells that move faster than the storm, travelling 1,000 kilometres (more than 600 miles) beyond its confines.
The storms themselves — with a calm “eye” at their centre — measure up to 1,000 km across.
They weaken rapidly when they travel over land or colder ocean waters.
Cyclones are closely monitored by satellites, and specialised centres around the world — in Miami, Tokyo, Honolulu and New Delhi — track the super storms’ trajectories under the coordination of the World Meteorological Organization (WMO).
Some experts have criticised the Saffir-Simpson scale as being too narrowly focused on wind speed.
“The irony is that hurricanes are known for wind, yet wind is third on the list of lethal aspects,” after storm surges and flooding caused by rain, noted Kerry Emmanuel, a professor of atmospheric science at the Massachusetts Institute of Technology (MIT) in Boston.
