During the summer and fall months, interest along the United States coastline, Mexican coastline, Central America, and the Caribbean need to carefully watch the waters of the Altanic Ocean as they might have one of the most dangerous weather systems developing and traveling to the west. The hurricane season runs from June 1st to November 30th with the peak of the hurricane season around September 10th. During this time frame, hurricanes develop in the Atlantic basin, travel westward on the tropical jet stream and could threaten any of the above mentioned areas or take the big right hand turn back out to sea. A hurricane is defined to be system that has winds greater then 74 miles per hour. The center of the hurricane is called the "eye" cause this is where the lowest barometric pressure occur in the storm. Some of the big named hurricanes that most people remember are storms like: Andrew (1992), Hugo (1989), Camile (1969), Carol and Diane (1955), and the Hurricane of 1938.

Stages of Development in Hurricane's Life Cycle

Before a hurricane can become a hurricane, the storm system must go through many different stages of development. These stages for development begin at a Tropical Low and work there way up the chain of becoming a hurricane. These stages of development are tropical low or wave, tropical depression, tropical storm, and hurricane. Everyone one of these stages must meet a certain criteria before moving onto the next stage. The National Hurricane Center in Miami, Florida, monitors the tropical waters of the Atlantic for any signs of development.

In order to have a hurricane develop, the system must start off as a cluster of thuderstorms that are associated with a tropical low. These thunderstorms die down as the day wears on, but refire up around or close to the center of the low. As the cluster of thunderstorms becomes larger and larger around the tropical low, the arial coverage becomes more extensive. As this is going on, the winds in the low pressure area begin to wrap around the center as the start to organize the system further.

With the increase of thunderstorms and arial coverage, NHC will then determine if the system has grown enough to be classified as a Tropical Depression. If the system is classified as a tropical depression, that means that minimum winds around the center of the storm are now estimated to be above 30 miles per hour. The stats of the storm are usually estimated because they are so far away from any land mass, NHC depicts the strength and motion of the storm by using satellite information. If the system continues to intensify, with more thunderstorms developing faster then dieing off around the center of low pressure, and if satellite information predicts the winds to be greater then 38 miles per hour, NHC will then classify the storm as a Tropical Storm.

A tropical storm is when the winds are between 39 and 73 miles per hour, the central pressure is falling off slowly, and banding of the clouds is evident on satellite images. When the system reaches this stage of development, NHC will then issue the system a name so that it is easier to track if there are more then one system in the open Atlantic at one time. The tropical storm will look rather rigid for the first couple of days of being upgraded, but as time goes on with more thunderstorms develop and the arial coverage continues to expand outward, banding features become visible on satellite images. Also, the tropial storm will have outflow developing from the center of the storm which will help the system to continue to develop and strengthen. If the system continues to develop, showing signs of really good outflow, a lot of high clouds rotating around the center of low pressure, and if satellite interpertations (if the system is still to far for reconissance planes to fly) show that the system has gone above 74 miles per hour, the storm system is then classified as a hurricane.

A hurricane is a storm system that has winds greater then 74 miles per hour, the central pressure still falling, and really good outflow. At this stage, the hurricane is then classified under the Saffir-Simpson scale which ranges from 1 to 5 with 1 being the weakest and 5 being the strongest. The hurricane is the pinnacle of the storm system. At this point, the storm will intensify until conditions start to become unfavorable to maintain the system or it has reached landfall. Once the hurricane has really begun to weaken, it does the process backwards going to back to a tropical storm then to a tropical depression and so on. But when the hurricane is at its strongest, the winds around the center of the storm could exceed 140 miles per hour and a barometric pressure lower then 955 millibars! When the hurricane has become fully mature, the eye of the storm becomes visible on satellite images. The eye of the storm is where the pressure is the lowest and all of the heat of the storm is generated that keeps the storm moving. If the storm is really powerful and strong, a second eye might develop inside the storm system, but that is usually the expection and not the rule. If the hurricane hits the land, the storm begins to interact with the land and begins to weaken, causing the storm to begin its long and awaited death.
Conditions for Tropical Development
Whenever a tropical system develops over the warm waters of the Atlantic, Gulf of Mexico, or Caribbean Sea, conditions over these areas must be perfect for storm development. If one of the conditions is not perfect, the development of the system could be hindered or even halted leading up to the storm to possibly prematurally begin to die off. The basics for tropical development include: 80 degree F water or greater, high pressure aloft, and no shear. It is possible for storm development to take place when these basic conditions are not meet. Development will be slow at this point and somethimes development could be stopped because of this.

There are a total of 6 major pieces that must be in place in order to have tropical development to occur. These pieces must be in place in order to have the system to begin development. They are: warm ocean waters, atmosphere that cools fast enough with height, moist layers in the mid-latitudes, distance from the equator, pre-existing near-surface low, low values of verticle shear.

The first part of this equation, warm ocean waters, means that the surface water temperature have to be warmer then 80 degrees F to an unknown depth. These waters have to be this warm so far down cause it is the heat of the water that fuels the giant heat ovens inside of the tropical system.

Piece 2, atmosphere that cools fast enough with height, means that the atmosphere has to stay cooled at various heights so that thunderstorm development can continue on. If the thunderstorm structure begins to break down and fall apart, the tropical system begins to break down and fall apart. The activity of the thunderstorms allows the heat stored in the warm ocean temperatures to allow for tropical development.

Moist layers in the mid-latitudes of the troposphere are necassary because dry layers are not conductive to thunderstorm development. Without the continuing development of thunderstorms, the tropical system would begin to weaken and possibly die off.

The forth piece is having development occur away from the equator. If development takes place next to the equator, this would prohibit tropical development because of the Coriolos effect the earth's motion has on the system. If there was no pull on the system, the low pressure system the storm developed would not be able to be maintained, allowing the storm to begin to break down and die off.

The fifth element, pre-exisiting surface-low, means that a tropical system cannot develop by itself. It needs some sort of circulation in place that has already established a sizable spin and low level inflow. Tropical systems cannot be born from nothing so they need help to first get there wheels turning.

The final element, low level vertical shear means, that in order for develop to even get going and once it has gotten going, winds need to be spinning in a counter-clockwise motion from sea-level all the way up to the top of the system. If winds begin to blow from one certain direction into the storm, the tops and structures of the thunderstorms would begin to be pulled away from the center of circulation. This shear could be only a temperory thing, but it can have an ever lasting affect on the storm.
Forecasting Hurricanes