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How Do Hurricanes Form

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Published: 04Mar2009
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This article explains what hurricanes are, and how they form, and their relevance to global warming. Hurricane and typhoon are names given to a strong tropical cyclone. A 'tropical cyclone' is a generic term for a low-pressure system that has a definitive cyclonic surface-wind circulation.

Before considering the effects global warming may have on these weather systems, we will look at a few hurricane facts and figures. Depending where a cyclone occurs will determine whether it's called a hurricane, typhoon or tropical cyclone. Hurricanes form in the North Atlantic and Northeast Pacific Oceans. Typhoons form in the Northwest Pacific area to the east of 160° longitude. Cyclones form in the Southwest Pacific Ocean, and the North and Southwest Indian Ocean. The Atlantic hurricane season, which officially begins on 1st June and continues to the end of November each year. However, hurricanes do of course occur outside this time period. The seasons are different for Pacific and Indian Ocean areas, particularly the Northwest Pacific basin, where cyclones can occur all year round.

According to NOAA, hurricanes rotate in a counter-clockwise direction around a central 'eye', and a tropical storm will be classed as a hurricane only when wind speeds reach 74 mph (119 km/h) or more. Hurricanes can of course cause immense damage, especially if they hit land, where heavy rain, strong winds and especially strong waves - called the storm surge - can wreak destruction, as the unfortunate people of New Orleans found out during August 2005, when Hurricane Katrina hit with devastating consequences.

Hurricane strength is measured using the Saffir-Simpson scale, named after two engineers from the US National Hurricane Center, who developed it in 1969. The scale is used only to describe hurricanes that form in the Atlantic and Northeast Pacific basins. The scale has five intensities depending on wind speed. For example, a category one hurricane has a wind speed of 74 mph (118 km/h) and a category five hurricane 156+ mph (251 km/h). Hurricane Katrina, mentioned above, was probably the most devastating storm in US history, causing an estimated $100,000,000,000-worth of damage. Katrina was a category five storm, but dropped to a three when it hit land on the eastern seaboard of the USA, in August 2005.

For hurricanes to develop, certain environmental conditions must be present, such as warm ocean water, high humidity and favourable atmospheric and upward spiralling wind patterns off the ocean surface. Atlantic hurricanes usually start off as a weak tropical disturbance off the West African coast, and intensify into rotating storms with weak winds called tropical depressions. It's only when wind speeds reach at least 74 mph (118 km/h) that they are classified as hurricanes. NASA scientists and NOAA have been studying how winds and dust conditions from Africa influence the birth of hurricanes in the Atlantic Ocean, using an armoury of NASA's Earth observing satellites. It seems that NASA is on course to establish whether or not global warming is indeed influencing the weather. NASA's AIRS instrument on board the Aqua satellite, which is short for Atmospheric Infrared Sounder, can measure very subtle changes in the Earth's climate. Scientists from NASA and NOAA, as well as other scientists, have already demonstrated that AIRS data can lead to better forecasts about the location and intensity of 'extratropical cyclones', which are mid-latitude storms, often striking the east coast of the USA. This may enable the team to test the climate-weather hypothesis once more data is available. AIRS will also have the ability to test the hypothesis that climate change may be causing the water (hydrological) cycle to accelerate, by measuring the humidity distribution within the atmosphere.This will show with sufficient accuracy whether the water cycle is indeed speeding up. If so, as is suspected in a warmer world, there will be more water vapour and clouds in the atmosphere resulting in more rainfall. If so, AIRS will be able to establish a link between global warming and the weather, as a faster water cycle causes greater rainfall as a result of an accelerated hydrological cycle.

2005 was the most active hurricane season since reliable records began, with fifteen hurricanes, seven of which were major ones.

The above extract has been taken from The A-Z of Global Warming, published in September 2008 by Schmall World Publishing. Visit the following website http://www.a-zofglobalwarming.com for further information, or search for the book on Amazon.

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