A tropical storm becomes a category one hurricane when winds reach sustained speeds of 74 miles per hour. Such storms are then given a name, such as Gilbert, Ivan, Katrina or Ophelia, which cycle through the alphabet. 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.Īs winds increase to a sustained speed of 39 miles per hour, a tropical depression becomes a tropical storm. The weather systems spin anti-clockwise in the northern hemisphere, and clockwise in the southern hemisphere. If this process happens a little way from the equator – typically at latitudes of more than 10° north – then the Coriolis force associated with the rotation of the Earth sets the massive weather system spinning, with winds spiralling in towards the centre. Hurricane Katrina formed in this way over the south-eastern Bahamas on 23 August 2005 and was labelled Tropical Depression 12. Initially, when wind speeds reach 23 miles per hour, these mild, wet and grey weather systems are know as depressions. This spinning wind drags up more moisture-laden air from the sea surface in a self-enhancing feedback process that swells the storm.Ĭold air falls back to the ocean surface through the eye and on the outside of the storm. This in turn creates wind as air is drawn into the area. When the warm column of air from the sea surface first begins to rise, it causes an area of low pressure. ![]() It is this heat energy, combined with the rotation of the Earth, that drives a hurricane. Under these conditions, large quantities of water evaporate and condense into clouds and rain – releasing heat in the process. ![]() These requirements are met between June and November in the northern hemisphere. Hurricanes can only form and be fed when the sea-surface temperature exceeds 27 ☌ and the surrounding atmosphere is calm. The seed for hurricane formation is a cluster of thunderstorms over warm tropical waters. Over its lifetime, one of these storms can release as much energy as 10,000 nuclear bombs. They occur in tropical regions and are called typhoons in South-East Asia and the Pacific, and cyclones in the Indian Ocean. Hurricanes are characterised by a calm central region of low pressure between 12 to 60 miles in diameter, known as the eye. The term hurricane is derived from Hurican, the name of a native American storm god. ![]() How hurricanes are bornĪ hurricane is a fiercely powerful, rotating form of tropical storm that can be 124 to 1240 miles in diameter. Here, New Scientist explains how hurricanes form and why Hurricane Katrina was so destructive. The cost of the damage may top $100 billion. Thousands have been killed or injured and more than half a million people have been displaced in a humanitarian crisis of a scale not seen in the US since the great depression. Katrina has left a disaster zone of 90,000 square miles in its wake – almost the size of the UK. The maelstrom tore through flood defences protecting low-lying New Orleans from neighbouring Lake Pontchartrain, and flooded 80% of the city to a depth of up to 8 metres. It whipped up sustained winds of 140 miles per hour (225 kilometres per hour), and a 5 to 9 metre high “storm surge” of sea water when it made landfall at 0610 local time on 29 August 2005. Hurricane Katrina has had a devastating impact on the Gulf Coast of the US.
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