The latter force causes the storms to eventually start turning to the right in the northern hemisphere and to the left in the southern hemisphere. Those that begin to form near the coast of Africa are often referred to as "Cape Verde" hurricanes, because the area in which they develop is near the Cape Verde Islands.

Overview, Solar Energy, Pressure, & Wind Belts

It is notable that essentially no tropical cyclones develop south of the Equator in the Atlantic Ocean, although one occurred off the coast of Brazil in March of After Abbott Since the main source of energy for the storm is the heat contained in the warm tropical and subtropical oceans, if the storm moves over the land, it is cut off from its source of heat and will rapidly dissipate. Winds spiraling counterclockwise in the northern hemisphere into the eye of the hurricane achieve high velocities as they approach the low pressure of the eye.

The velocity of these winds is called the hurricane-wind velocity. The central low pressure center of the eye also moves across the surface of the Earth as it is pushed by regional winds. The velocity at which the eye moves across the surface is called the storm center velocity.

Thus, when we consider the velocity of winds around the hurricane we must take into account both the wind velocity and the storm center velocity. Depending on the side of the hurricane, these velocities can either add or subtract.

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On the left hand side of the storm, however, the wind is blowing to the south. This is an important point. Winds are always stronger on the right side of a moving hurricane in the northern hemisphere. The opposite is true in the southern hemisphere, since winds circulate in a clockwise direction, the winds are stronger on the left-hand side of the storm in the southern hemisphere. Tropical Cyclone Size Since winds spiral inward toward the central low pressure area in the eye of a hurricane, hurricane-wind velocity increases toward the eye.

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The distance outward from the eye to which hurricane strength winds occur determines the size of the hurricane. Winds in the eye wall itself have the highest velocity and this zone can extend outward from the center to distances of 16 to 40 km. Hurricane Katrina, in , was a large hurricane with tropical storm force winds extending outward from the eye about km. The smallest, Cyclone Tracy, had gale force winds that only extended 50 km in radius when it struck Darwin, Australia, in There is very little association between hurricane intensity either measured by maximum sustained winds or by central pressure and size.

Hurricane Andrew is a good example of a very intense tropical cyclone of small size. The numbers in this column should be used with caution in predicting storm surge levels. Note that beginning in , the National Hurricane Center no longer assoicates the storm category based on wind speed with the height of the storm surge because the relationship is not well established.

Monitoring and Tracking Hurricanes In the United States, hurricanes and developing tropical disturbances are monitored very closely. Such monitoring has drastically reduced the number of deaths from hurricanes in the last 50 years. Monitoring is conducted using several methods: Ships at sea transmit weather reports that help meteorologists locate centers of low pressure that may develop into tropical disturbances.

Images from weather satellites, which are collected every 30 minutes, are then scanned to look for any development or growth of the disturbance. In particular, the images are examined to detect any rotational development of the storm, an indication that it may be approaching tropical storm strength. If a tropical storm or hurricane is detected and appears to pose a threat to land areas, observation airplanes are sent to examine the storm.

Such planes fly into the storm at an altitude of about 3, meters or lower. The plane collects data on wind speed, air pressure, and moisture content by dropping devices called dropsondes into the storm. These dropsondes transmit the meteorological data continuously has they fall to the ocean surface, and thus provide information on the vertical structure of the storm. In addition, radar devices carried on the plane collect data about the intensity of the rainfall and wind velocities. The planes fly completely through the storm, passing though the eye, sometimes making several passes.

The data collected give meteorologists a 3 dimensional picture of the structure of the storm. Satellite images and radar from land based stations allow scientists to track the position of the storm and report it to all agencies that may be affected if the storm makes landfall. Changes in Hurricane Tracks and Intensities Because hurricanes are influenced by large-scale air masses, they sometimes move along rather erratic paths.

Hurricanes are especially influenced by the strength and direction of upper level winds. As noted above, strong upper level winds create a vertical wind shear that cause the top of the hurricane to be sheared off and result in the loss of strength of the storm. The erratic nature of a hurricane's path often makes it difficult to predict where and when it will make landfall prior to several hours before it actually does make landfall.

This increase in storm center velocity usually results from the interaction of the storm with other air masses.

Tropical Cyclones

Off the eastern coast of the United States there is an area of semi-permanent high pressure, known as the Bermuda High. Other high pressure centers are continually moving eastward off of North America. If the hurricane encounters a low pressure trough between two high pressure centers, it is steered into the trough and follows it along a northeastward trend, increasing its velocity as it does so.

Interaction with the land and other air masses are most responsible for changes in hurricane tracks and intensities. Some examples are shown on the map below. Two of the most erratic hurricane paths recorded are shown by Hurricane Betsy, in and Hurricane Elena in Hurricane Betsy, a category 3 storm, took a northwestward track from the Caribbean Islands, but then turned abruptly west as it passed north of Puerto Rico.

It then took a northwest track again, looking like it would hit along the coast of Georgia or South Carolina. Suddenly, however, it looped back to the south, passed over the southern tip of Florida, crossed the Gulf of Mexico and hit just east of New Orleans. Because the storm surge occurs ahead of the eye of the storm, the surge will reach coastal areas long before the hurricane makes landfall.

This is an important point to remember because flooding caused by the surge can destroy roads and bridges making evacuation before the storm impossible. Since thunderstorms accompany hurricanes, and these storms can strike inland areas long before the hurricane arrives, water draining from the land in streams and estuaries may be impeded by the storm surge that has pushed water up the streams and estuaries. It is also important to remember that water that is pushed onto the land by the approaching storm the flood surge will have to drain off after the storm has passed.

Furthermore after passage of the storm the winds typically change direction and push the water in the opposite direction. Damage can also be caused by the retreating surge, called the ebb surge. Along coastal areas with barrier islands offshore, the surge may first destroy any bridges leading to the islands, and then cause water to overflow the islands. Barrier islands are not very safe places to be during an approaching hurricane! Hurricane Damage Hurricanes cause damage as a result of the high winds, the storm surge, heavy rain, and tornadoes that are often generated from the thunderstorms as they cross land areas.

Strong winds can cause damage to structures, vegetation, and crops, as described in the Saffir-Simpson scale discussed previously. The collapse of structures can cause death. The storm surge and associated flooding, however, is what is most responsible for casualties. Extreme cases of storm surge casualties have occurred as recently as and in Bangladesh and in Myanmar.. In a cyclone struck Bangladesh during the highest high tides full moon. The storm surge was 7 m 23 ft. Another cyclone in created a storm surge 6 m high and resulted in , deaths.

The May cyclone in Myanmar is estimated to have killed , The amount of damage caused by a tropical cyclone is directly related to the intensity of the storm, the duration of the storm related to its storm-center velocity, as discussed above , the angle at which it approaches the land, and the population density along the coastline. The table below shows how damages are expected to increase with increasing tropical storm category. Like the Richter scale for earthquakes, damage does not increase linearly with increasing hurricane category.

Prediction of hurricane intensity wind speed is more problematic as too many factors are involved. Hurricanes are continually changing their intensity as they evolve and move into different environments. Without the ability to know which environmental factors are going to change, it is very difficult to expect improvement on intensity forecasting. Hurricane Katrina was expected to loose intensity as moved out of the warmer waters of the Gulf of Mexico. But, it showed a more rapid drop in intensity just before landfall because a mass of cooler dry air was pulled in from the northwest.

Some progress has been made in predicting the number and intensity of storms for the Atlantic Ocean by Dr. William Gray of Colorado State University. He has shown that there is a correlation between the frequency of intense Atlantic hurricanes with the amount of rainfall in western Africa in the preceding year. This correlation has allowed fairly accurate forecasts of the number of storms of a given intensity that will form each year.

Nevertheless, Dr. Gray's predictions are closely watched, and have been otherwise fairly accurate.

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Reducing Hurricane Damage There is plenty of historical data on hurricane damage in the United States so that it is not difficult to see ways that damage from hurricanes can be reduced. In terms of protection of human life, the best possible solution is to evacuate areas before a hurricane and its associated storm surge reaches coastal areas.

Other measures can be undertaken to reduce hurricane damage as well.

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The problem, however, is that it may not always be possible to issue such a warning in time for adequate evacuation of these areas. Because the storm surge and even gale force winds can reach an area many hours before the center of the storm, warnings must be issued long enough before the storm strikes that the surge and winds do not hinder the evacuation process.

The effectiveness of the warning systems also depends on the populace to heed the warning and evacuate the area rather than ride out the storm, and the state of preparedness of local government agencies in terms of evacuation and disaster planning.

New Orleans is a particularly notable example. Since most of the city is at or below sea level, a storm surge of 6 meters 20 feet from a category 4 or 5 hurricane would most certainly flood the city and choke all evacuation routes.

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Even with 24 hours notice of the approaching surge which would mean as soon as the storm entered the Gulf of Mexico it would be difficult to evacuate or convince people to evacuate within that 24 hour period. A hurricane approaching New Orleans was a disaster waiting to happen as we can all testify.

Hurricane Donna in shows the effects of the land decreasing the intensity of a hurricane.