SOME NOTES ON THE BIRTH AND DEATH OF HURRICANES BACK TO WEATHER-BLOG MENU New! Fine Art Prints & digital images for sale- Welsh Weather & Dyfi Valley landscapes Slide-Library - Click HERE

A number of people have recently asked me about the effects ex-hurricanes have on our weather, so here's an explanation! The Atlantic hurricane season begins in late Summer, when disturbances in the shape of thunderstorm clumps start to migrate westwards from the West African countries out into the Atlantic. These phenomena, known as Easterly Waves, are how most hurricanes in the Atlantic begin their lives. Under the right circumstances, these clumps of thunderstorms become more organised. Convective clouds continue to form, the condensation of the water vapour causing a fall in atmospheric pressure. When water vapour condenses it releases heat, thus warming the surrounding air, which then rises, expands and cools, causing more condensation and further warming, thus allowing more air to warm and rise in turn. In the storm's centre, where the warming is greatest, the low pressure pulls in more surface air from its surroundings. Meanwhile, the Coriolis Effect (which is what makes cyclones spin anticlockwise in the Northern Hemisphere and the reverse in the Southern) gets the whole thing spinning, the clouds in the inflow-bands showing up on satellite images like a giant catherine-wheel. By now, if these processes have been allowed to run unchecked (and there are many things that can stop them), winds may be as high as 39 miles per hour. Once they reach this strength the disturbance is classified as a tropical storm. Aloft, over the top of the storm, the ascending warm air begins to flow outwards. Think of smoke going up a chimney. The chimney needs to have a good "draw" to it or the fire goes out and you get a room full of smoke. It's the same for tropical storm systems - they need to vent if they are to grow. Venting is encouraged by a number of factors, such as the proximity of high-velocity upper winds, which carry away the outflow and let the surface inflow continue to pile on in. And a well-vented tropical storm, so long as it has warmth and moisture in ready supply, will continue to grow as the bands of thunderstorms circulate around the eye. When the winds exceed 74 miles per hour it is officially a hurricane. Steered north and west by nearby ridges of high pressure, hurricanes find their way into the Caribbean area and onto the Eastern seaboard of the USA where they wreak the havoc with which we are familiar in the news. At the time of writing hurricane Ivan, a dangerous hurricane which has reached Category 5 status (the top of the scale), has been crossing the Caribbean, with sustained winds to 160mph and gusts as high as 200mph. It is currently thought to be heading for the Southern States of the USA and will most likely dissipate in time over the North American Continent, but not before more chaos has been caused. Some hurricanes, however, veer back out into the Atlantic. The cooler northern waters cannot sustain a hurricane and so it rapidly weakens to become an Atlantic cyclone - like the dozens of windy, rainy depressions we see in the UK every year in many ways. The difference however is the amount of warm, moist air still entrapped. Hence the very heavy rainfall that the remains of such systems can bring along with them. Although hurricanes by definition only occur over warm tropical seas, some of these Atlantic depressions still pack quite a punch, with occasional ones giving 75mph+ sustained winds and gusts over 100mph. October 1987 was such an occasion. These however are NOT hurricanes but Atlantic cyclones with winds up to hurricane force. Still dangerous but nothing compared to a beastie that gives 200mph gusts and drops 12 inches or more of rain!

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