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__Formation, characteristics and dissipation of Hurricanes. __ Our Wiki pages are focused on the formation and characteristics of Hurricanes. In our reseaech we intend to discuss the formation and the characteristics which help form these tropical storm phenomina. We will research the reasons why and how a hurricane is formed, and go onto the structure and locations hurricanes occur in. Finally we will finish with the dissipation and how the storm finally comesto its 'death'.

__Hurricane Location __ There are many different names for hurricanes depending on where they are situated (typhoons, cyclones). Hurricanes occur in the tropics, a region 23.5 degrees north and south of the equator (tropic of Cancer and Capricorn). In this region the temperature does not vary greatly between each season, but instead is defined by levels of precipitation. Heaviest precipitation occurs during the high sun period when the ITCZ (Inter-tropic convergence zone) moves into the region. Two reasons why a hurricane forms in the tropical region is because the air is warm, and there is a sufficient spin from the earth. The region with the most number of hurricanes is the west part of the north pacific, with an average of 17 a year. North of the equator hurricanes generally occur between August-September, where as south of the equator they occur between January-March.  ﻿[]

__Hurricane Characteristics __ Of all storms, hurricanes are the most powerful, although the wind speed is much less than tornados, hurricanes are bigger and have a longer life span. The average peak wind speed is around 90mph, but the most extreme speed measured was 210mph. Hurricanes form around areas of low sea level pressure, the average sea level pressure in the centre of a hurricane is 950mb, but pressures as low as 870mb have been recorded, and this makes the hurricane much more powerful and thus dangerous. Hurricanes range in size, but the average is 350 miles wide. A hurricanes life span can vary between several days, a week or even longer.

__Hurricane structure __ Hurricanes are a large number of thunderstorms arranged in a pinwheel formation, thick clouds spiral outwards from the storms centre. The pressure gradient, speed of winds and levels of precipitation increase closer towards the eye wall of the storm. The increase is gradual, but towards the eye wall the increase is more rapid. In the upper regions of the storm, ice crystals can form, due to lowered temperatures. These crystals then create a layer of cirrostratus clouds that obscure the structure of the hurricane. __Eye and Eye Wall __ The eye is situated in the centre of the hurricane, it has relatively clear skies and light winds. The average diameter of the eye is 20 miles, but it varies heavily with some as little as 3.5 miles, whilst others as large as 60 miles. The eye changes in size over time, this gives an indication whether the storm is intensifying or weakening, a shrinking eye means that the storm is intensifying. Surrounding the eye is the eye wall, where the storm is at its most intense, with fast winds, deep clouds and heavy precipitation with levels of 2500mm of rainfall per day. The eye wall reaches up to heights of 49,000ft above sea level. Large waves of over 10 meters high are experienced due to extreme winds. There is a huge difference in weather between the eye and its wall. As the centre of the hurricane approaches, wind and precipitation increases in intensity, until the eye is hit when the weather turns calm, this calm lasts between 1-2 hours, until the eye wall is once again reached. Portions of the eye wall contain hot towers, which are parts of the wall which rise to a greater height than the rest of the wall, when this happens it means that the storm will intensify within the next 6 hours. <span style="font-family: 'Comic Sans MS',cursive;">The temperature within the eye is several degrees warmer than outside the rest of the hurricane. The air is also dryer, because the warming of the unsaturated air lowers humidity. Although the weather within the eye is calm it is not cloud free, cumulus clouds are scattered throughout. <span style="font-family: 'Comic Sans MS',cursive;"> __<span style="font-family: 'Comic Sans MS',cursive;">Sea Temperature and Depth __ <span style="font-family: 'Comic Sans MS',cursive;">The release of latent heat from tropical oceans plays a vital role in not only the formation of hurricanes. Also the intensity, precipitation and the dissipation of hurricanes is controlled by the ocean’s surface temperatures. <span style="font-family: 'Comic Sans MS',cursive;">The early stages of hurricane formation, particularly those affecting the Gulf of Mexico, occur on the west coast of Africa. Small tropical disturbances pass over the cold Canary current, where the lack of latent heat can stabilise the air and stop any chance of it developing into a tropical storm. Should any of these disturbances make it past the cold ocean current, they will pass over warmer waters. <span style="font-family: 'Comic Sans MS',cursive;">Eastern seaboards of continents tend to have higher average sea temperatures than their western counterparts, due to cold ocean currents.

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<span style="font-family: 'Comic Sans MS',cursive;">Huge amounts of heat energy are needed to fuel a full scale hurricane; the main fuel store for this energy is heat from oceans. High amounts of evaporation are needed to produce enough latent heat to give the hurricane the power to continue, without this, the storm would dissipate before becoming a tropical storm, and then a hurricane. The required amount of heat can only be found from an ocean with a deep surface layer (at least 10 meters) and with an average temperature of 27oc. As the Coriolis Effect prevents formation of hurricanes between 0 and 5o latitude, the need for warm oceans stops hurricanes, in most cases, from forming north of 200. The reason for the so called ‘hurricane season’ being in late summer and early autumn, is because that is when tropical waters are at there warmest.

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<span style="font-family: 'Comic Sans MS',cursive;">The unequal distribution of air across the globe establishes the horizontal pressure gradients that cause the movement of air as wind. If no other forces were involved the wind would always flow in the direction of the pressure gradient force. However, the situation is also influenced by the coriolis force which arises from planetary rotation. ===== <span style="font-family: 'Comic Sans MS',cursive;">All points on the earth except the equator undergo some rotation, all moving objects experience this displacement which is the result of the coriolis force. As well as objects, the force acts on the movement of the atmosphere as well. In between the equator and the poles there is a gradual increase in the strength of the coriolis force with increasing latitude. No displacement occurs at the equator as there is no twisting motion. Instead, any point on the surface of the equator travels the full circumference of the earth (40,000 km) in 24 hours. <span style="font-family: 'Comic Sans MS',cursive;">The coriolus force can be described using the north pole as an example. As the northern hemisphere rotates in a counterclockwise direction, any object or weather system in this case moving across the pole would appear to be turning to the right. The same relationship between latitude and magnitude of the coriolis force can also be seen in the southern hemisphere. The only difference is that the deflection is to the left as opposed to the right, this is because the southern hemisphere rotates in a clockwise direction.

<span style="font-family: 'Comic Sans MS',cursive;"> <span style="font-family: 'Comic Sans MS',cursive;">[|www.google.co.uk/images]

<span style="font-family: 'Comic Sans MS',cursive;">The coriolis force changes the direction of a moving object, never its speed. To produce detectable effects the coriolis force must act over relatively long periods of time. It is seen mainly for the for the motion of objects travelling long distances including the air circulating around a hurricane. <span style="font-family: 'Comic Sans MS',cursive;">In the northern hemisphere the coriolis force turns winds to the right, however, hurricanes move in an anticlockwise direction. This happens because coriolis is only one of several forces acting on the hurricane. While the coriolis force is pushing the winds towards the right, the pressure gradient force caused by air pressure differences pushes the air towards the center of the low pressure system. This pressure gradient is stronger than the coriolis force and so results in an anticlockwise movement of air round the area of low pressure system. <span style="font-family: 'Comic Sans MS',cursive;">The winds in the poleward portion of a hurricane are pulled slightly poleward by the coriolis force. However, the equator ward portion of the hurricane contains winds which are pulled slightly towards the equator. However, because the coriolis force weakens towards the equator, the net drag of the hurricane is pole ward. Therefore, hurricanes in the northern hemisphere usually turn north and tropical cyclones in the southern hemisphere usually turn south. <span style="font-family: 'Comic Sans MS',cursive;">Air masses are large enough for the coriolis force to act on, however, small objects such as boats are far too small to experience significant deflections in direction.

__<span style="font-family: 'Comic Sans MS',cursive;">What causes hurricanes to dissipate? __ <span style="font-family: 'Comic Sans MS',cursive;">A Hurricane/ tropical storm can last days or just hours depending on its factors and sources of energy to support the storm. The dissipation of a hurricane is mainly a result from natural causes.

<span style="font-family: 'Comic Sans MS',cursive;">· Movement from water to land- For a hurricane to move there must be a source of heat/energy continuously feeding it which maintains the rising currents of air up the storm. <span style="font-family: 'Comic Sans MS',cursive;">There are many factors which occur in the dispersion of a hurricane, especially when their path crosses land areas. Hurricanes form over warm ocean waters because they are powered by latent heat energy which is released from condensation; this feeds energy to the hurricane, they need this supply of heat energy constantly to survive. As a hurricane moves onto land from the ocean its energy source from the currents and the evaporation it depends on for energy to feed it are lost, also if the storm moves over cold water energy is lost and the hurricane slowly dissipates. This is the same if a hurricane stays over the same area of water it will force up colder water restricting the energy avaliable to it and eventually forcing it to dissipate. Once the hurricane reaches land friction causes it to lose its energy, this affects the strength of the hurricane which then rapidly demises it and causes it to slowly die. Once the storm has slowly decreased over land and almost dispersed, the hurricane can cause large masses of rain and potentially flood an area. However when crossing over land if the storm finds water it can absorb it and more energy strengthens the hurricane which then allows it to continue on its path of destruction across land. <span style="font-family: 'Comic Sans MS',cursive;">[online]http://www.eosnap.com/tag/paloma/ Google images- Hurricane dissipation, Earth snapshot, Hurricane Paloma (2008)

<span style="font-family: 'Comic Sans MS',cursive;">· One famous example is Hurricane Katrina in 2005, the storm moved across the land reaching Tennessee were it was downgrading because of its loss of energy through dispersion, however finding the Eastern Great Lakes which it crossed it absorbed the water and energy was recreated, this allowed the hurricane to expand and become very strong creating mass damage and deaths.

<span style="font-family: 'Comic Sans MS',cursive;">· Wind shearing- This is also a natural cause of dispersion of hurricanes. This is a difference in wind speed and direction over a small area across the atmosphere which affects the hurricane path and direction. 1000’s of feet above the surface of the storm the wind shear in the hurricane can kill the energy by taking heat away and slowing the storm down eventually dispersing it. Because it is a combination of winds in direction and speed it can destroy a hurricane by splitting it and sending it in different directions. With increasing wind power from the ‘wind shear’ this sucks the moisture out of the hurricane and as these tropical storms need humid moist air to feed its energy off, then the dry air ‘chokes’ the storm.

<span style="font-family: 'Comic Sans MS',cursive;">· Dispersion of hurricanes over water- A hurricane that stay’s in the place of its origin in the ocean usually disperses itself however it lasts for much longer. In 1992 Hurricane Tina was the second longest lived storm in the south Pacific, it was active for 24days. Hurricanes which do not move will eventually disperse itself. Because it stays over the same area of water for its lifespan, it continues to evaporate and absorb the same warm waters which will continue in a cycle absorbing the same water, as a result of this the water will eventually get cooler and cooler, and as the storm needs heat from the ocean to feed it the continuous cycle of the same cooler water would create less and less energy for the hurricane eventually leading to dispersion of the storm.

<span style="font-family: 'Comic Sans MS',cursive;">There has been attempts to artificially dissipate hurricanes over the past century by governments concerned on the impacts on their cities, the most notable of which is the U.S. Government. This involved 'seeding' or dropping small parts of silver iodide into the hurricane to cause the storm to lose it's damaging power, as the silver iodide would join with ice particles in the clouds to produce rain before the hurricane hit the coast. There has been other hypothesis to try and stop hurricanes developing such as dropping large quantities of ice in the storm early development, however these have been mostly dismissed as there would simply not be enough to stop something so large and short-lived.

__<span style="font-family: 'Comic Sans MS',cursive;">Case Study-Hurricane Katrina __ <span style="font-family: 'Comic Sans MS',cursive;">Hurricane Katrina began forming in the later part of August 2005. On the 25th of August began to near the coast of Florida, strengthening as it moved towards Louisiana. It touched ground on the 29th of August jus 90km from New Orleans.

<span style="font-family: 'Comic Sans MS',cursive;">The hurricane had formed from the remnants of other tropical storms that dissipated. This hurricane was so deadly as it changed its predicted path, to an area of dense unprepared population.

<span style="font-family: 'Comic Sans MS',cursive;">Source-Google Images

__//<span style="font-family: 'Comic Sans MS',cursive;">References: //__

<span style="font-family: 'Comic Sans MS',cursive;">-David Waugh (2000), Geography, An integrated approach, Third edition. <span style="font-family: 'Comic Sans MS',cursive;">-Donald Athrens (2007) Meteorology Today, Eighth edition -<span style="font-family: 'Comic Sans MS',cursive;">Edward Aguado, James E. Burt (2007) Understanding weather and climate, fourth edition. <span style="font-family: 'Comic Sans MS',cursive;">-John.H.Seinfield, Spyros.N.Pandis (1998) Atmospheric chemistry and physics, from air pollution to climate change. <span style="font-family: 'Comic Sans MS',cursive;">Internet sources: - [|www.physicalgeography.net], tropical weather and hurricanes <span style="font-family: 'Comic Sans MS',cursive;">- [online] For the people - [] <span style="font-family: 'Comic Sans MS',cursive;">- [|www.servinghistory.com/topics/tropical_cyclones], Tropical Cyclones: Dissipation

<span style="font-family: 'Comic Sans MS',cursive; font-size: 110%;">Created by; <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%;">Oliver Hemsley <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%;">Donna Kirk <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%;">Gemma Lownsbrough <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%;">Heath Mansell <span style="font-family: 'Comic Sans MS',cursive; font-size: 110%;">Blythe Smith