The origin of hurricanes and predictability of hurricane tracks

Low pressure systems form at mid latitude temperate zones as a result of the convergence of warm and cold air masses. Low pressure systems generally form over the polar front where the polar maritime air (a cold air mass) and tropical maritime air (a warm air mass) meet. Air currents in this area within the Northern Hemisphere will flow counter-clockwise due to the rotation of the earth and surface friction. Air flows accumulate at the centre of the system, and the warm air rises because it is less dense, leading to low pressure at the surface.

It contains more moisture than the polar maritime air and as a result, when it ascends, it condenses and produces clouds and rain. Cirrus type clouds are the first clouds that are usually created at this point. A circulation of air rotates in an anticlockwise motion due to the coriolis effect. The tropical maritime air swirls around the polar front, the system matures and eventually a warm and cold front is created.

At the warm front, the warm air rises over the cold air, the cirrus clouds develop to be cirrostratus, altostratus and nimbostratus clouds which eventually will lead to heavy rain because of the condensation of the warm air. Towards the edge of the warm front, conditions are more stable and pressure stops decreasing as much. No longer is there so much condensation and therefore there is less rain. Polar maritime air is fast, dense, and strong. Eventually the air pushes in and forces the warm air off the ground, creating instability.

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The pressure increases, as the air ascends rapidly, cumulonimbus clouds form bringing in heavy rain and storms[1]. The cold front consists of heavier and denser air and displaces the warmer and lighter air, because of this, it moves faster than the warm front and it will ultimately catch up with it. Warm air is forced off the ground by the cold air, and once this happens, an occlusion is created. Uniform air begins to fill the gap between the warm front and the cold front and this is where the low pressure system begins to die away.

At tropical latitudes, if sea surface temperatures are above 27 degrees c[2], then the low pressure system will grow. If the conditions are right, tropical thunderstorms may develop to become a hurricane. Low pressure systems often begin to rotate around a central area of low pressure. This is known as a tropical depression, if the depression increases in intensity so that winds reach at least 39 mph, it's categorised as a tropical storm. If wind speeds reach and average of 74 mph[3], it known as tropical cyclone or hurricane.

Hurricanes/ tropical cyclones mainly develop in the region between 10 and 20 degrees North of the equator (Goldenberg, 2001). When a storm grows to become a hurricane/tropical cyclone, it is described as a non-frontal low pressure system[4] and can reach up to 340 miles across in diameter[5]. Hurricanes absorb energy from the warm water of the ocean, and a thunderstorm will continue to grow so long as there is a fuel source i. e a supply of moist air and heat.

This source is normally found above the water in tropical waters. When the heat supply is cut off i. when the hurricane begins to migrate northwards (or southwards), over colder water, it will weaken and die away. Heat from the oceans is the primary source of energy for hurricanes, [6]and so, the greater the heat of the SST [7]the more intense and frequent hurricanes in that area will be (Goldenberg, et al, 2001).

Hurricanes don't usually develop far inland due to the lack of moisture. [8] If there is no moisture, then clouds are not likely to form. Cloud formation results in the generation of latent heat. Latent heat is the heat needed to initiate a change of phase i. to a state of higher energy, e. g. from solid to liquid, or liquid to gas, in this context though, it's from liquid to a gas. As clouds are not generated as much, then not much latent heat is released. the majority of hurricanes originate and stay within the oceans, though they do occasionally travel inland, and the effects they can have on the environment, society and on the economy of the affected area are potentially devastating. The coriolis effect, which is a product of the earth's rotation is the reason that storms rotate and why a hurricane has a typical swirling formation.

The rotation of the storm causes air to be drawn into the extreme low pressure at the centre (eye) of the storm. As the air rotates, the air ascends. The rising air is very moist, the higher the altitude, the colder the temperature, and so, it condenses forming clouds. Hurricanes aren't found within 0-5 degrees north and south of the equator ((300 miles (500 kilometers)) of the equator because the coriolis effect is at its weakest at this point, so the storm doesn't have enough spin, and there isn't enough force to maintain low pressure in the centre of the system.

Meteorologists can predict hurricanes in two main ways : through the use of seasonal probabilities and tracking of hurricanes that are in existence at a current point of time using modelling techniques[9]. Annually, scientists work out how many storms are likely to develop into hurricanes/tropical storms and they also calculate how many are likely to make landfall. Using statistical techniques such as CLIPER[10], past data, and by sending aeroplanes into the centre of storms they can determine wind speeds, temperatures and can predict the intensity of a hurricane, and how many people it is likely to affect.

Many scientists try and determine the paths of hurricanes, and it's a difficult duty because not all hurricanes have defined paths, however; the typical characteristics and properties of the weather and ocean in a specific area allow scientists to have a rough idea to which path a hurricane is likely to follow. If the path is predicted then warning and protection can be provided for those that could potentially be affected and this is the best way to prevent a social, economic and environmental disaster from happening.

Hurricanes form in various areas depending on the various times of the hurricane season (Reading, 1990). Tracks can be predicted efficiently however, accuracy seems to be an issue in many cases. Models have become more accurate (NOAA,2004) and prediction techniques have improved (Aberson,2001), however there is still a large uncertainty and error is still an issue. It is easier to predict exactly where a hurricane is going to make landfall the closer to landfall the storm is.

So the further the hurricane is away from land, the more error there is when trying to work out its path (NOAA,2004). This is mainly due to natural changes in the storms physical characteristics. It has been determined by NOAA, that, 5 days before landfall there is an average of 350 miles of error, and one day before landfall there is a 100 mile error, [11]which is a major problem because a difference of that mileage could determine whether or not whole cities or villages need to be evacuated or not, and if there is an error, it could be devastating.

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