HURRICANE Dorian reached category 5 and is now set to hit the coast of Florida – leaving locals terrified.
Here is our guide to how hurricanes form and an explanation of the different categories used to rate their destructive power.
What is a hurricane and how do they form?
A hurricane is another name for a tropical cyclone – a powerful storm that forms over warm ocean waters near the equator.
Those arising in the Atlantic or eastern Pacific are called hurricanes, while those in the western Pacific and Indian ocean are dubbed typhoons or cyclones.
North of the equator they spin anticlockwise because of the rotation of the earth.
They turn the opposite way in the southern hemisphere.
Cyclones are like giant weather engines fuelled by water vapour as it evaporates from the sea.
Warm, moist air rises away from the surface, creating a low pressure system that sucks in air from surrounding areas – which in turn is warmed by the ocean.
As the vapour rises it cools and condenses into swirling bands of cumulonimbus storm clouds.
The system grows and spins faster, sucking in more air and feeding off the energy in sea water that has been warmed by the sun.
At the centre, a calm “eye” of the storm is created where cooled air sinks towards the ultra-low pressure zone below, surrounded by spiralling winds of warm air rising.
The faster the wind, the lower the air pressure at the centre and the storm grows stronger and stronger.
Tropical cyclones usually weaken when they hit land as they are no longer fed by evaporation from the warm sea.
But they often move far inland – dumping huge amounts of rain and causing devastating wind damage – before the “fuel” runs out and the storm peters out.
Hurricanes can also cause storm surges when the low air pressure sucks the sea level higher than normal, swamping low-lying coasts.
What are the different hurricane categories on the Saffir-Simpson scale?
When wind speeds reach 39mph a storm is officially classed as a Tropical Storm.
Above 74mph and it is called a Category one hurricane.
A storm heading towards Britain in October 2017 – Hurricane Ophelia – was upgraded to a category one hurricane after its winds were measured at 75mph.
Further categories up to a maximum of five are available for very powerful storms, with those of Category three and above considered a “major hurricane”.
The Saffir-Simpson scale was devised in 1971 by civil engineer Herbert Saffir and meteorologist Robert Simpson, who was head of the US National Hurricane Centre.
It was introduced to the public in 1973 as a means of communicating the danger of upcoming storms so that populations could better prepare.
The scale was tweaked in 2009 to remove air pressure and storm surge ranges, transforming it into a pure measurement of wind scale.
It helps to paint a clear picture of the damage to buildings that can be expected from hurricane force winds.
But it does not take account of the potential devastation caused by monsoon downpours or tidal storm surges caused by the low air pressure.
The categories are as follows:
- Category one: Wind speeds 74-95mph — very dangerous winds will produce some damage
- Category two: Wind speeds 96-110mph — extremely dangerous winds will cause extensive damage
- Category three: Wind speeds 111-129mph — devastating damage will occur
- Category four: Wind speeds 130-156mph — catastrophic damage will occur
- Category five: Wind speeds 157+mph — areas will be completely razed
What is the strongest category of hurricane?
The strongest category of hurricane is a category 5, with winds of over 157mph.
Currently, Hurricane Dorian has hit category 5 after beginning as a tropical storm.
Hurricane Katrina, the single most catastrophic natural disaster in US history, was a category 5 hurricane.
It killed 1,833 and caused $108billion of damage in New Oreleans, Missippi and the neighbouring states in late August 2005.
Hurricane Irma in 2017 was also classed as a category five storm.
It was the most powerful Atlantic hurricane ever recorded with sustained winds over 185mph and gusts of 220mph.
Irma devastated a number of islands in the Caribbean before smashing into Florida on the US mainland.
It followed hot on the heels of category four Hurricane Harvey, which caused catastrophic flooding in Texas.
Hurricane Jose, following a similar path, reached category three with winds above 110mph.
And category one Hurricane Katia hit Mexico in September before dissipating.
Around 2,000 people are believed to have died when Hurricane Maria left “apocalyptic scenes” in Puerto Rico. That storm was also category five.
And category one Hurricane Nate left 48 dead in Central America in October 2017.
Harvey, Irma, Maria and Nate were all retired from the name lists following the devastation.
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When is hurricane season in the Atlantic?
Hurricanes form when the ocean is warm. In the Atlantic this means the summer and autumn.
Meteorologists class the Atlantic hurricane season as running from June 1 to November 30, although intense storms can happen outside these dates.
Forecasters said as early as last August that the 2017 hurricane season would be “above-normal,” with 14 to 19 named storms ahead of the peak season.
An average Atlantic hurricane season produces 12 named storms, of which six become hurricanes and three major hurricanes.
Some scientists have blamed the series of intense storms last year on the absence of the El Niño weather phenomenon in the Pacific.