HURRICANE BASICS 1
2.1. General. The ingredients for a hurricane include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods we associate with this phenomenon.
2.1.1 Breeding Grounds. In the eastern Pacific, hurricanes begin forming by mid-May, while in the Atlantic, Caribbean, and Gulf of Mexico, hurricane development starts in June. For the United States, the peak hurricane threat exists from mid-August to late October although the official hurricane season extends through November. Over other parts of the world, such as the western Pacific, hurricanes can occur year-round. Developing hurricanes gather heat and energy through contact with warm ocean waters. The addition of moisture by evaporation from the sea surface powers them like giant heat engines. Each year on average, ten tropical storms develop over the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Many of these remain over the ocean. Six of these storms become hurricanes each year. In an average 3-year period, roughly five hurricanes strike the United States coastline, killing approximately 50 to 100 people anywhere from Texas to Maine. Of these, two are typically major hurricanes (winds greater than 110 mph).
2.1.2. What is a Hurricane? A hurricane is a type of tropical cyclone, which is a generic term for a low-pressure system that generally forms in the tropics. The cyclone is accompanied by thunderstorms and, in the Northern Hemisphere, a counterclockwise circulation of winds near the earth's surface. Tropical cyclones are classified as follows:
188.8.131.52. Tropical Depression: An organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of 38 mph (33 knots) or less.
184.108.40.206. Tropical Storm: An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 39-73 mph (34 - 63 knots).
220.127.116.11. Hurricane: An intense tropical weather system of strong thunderstorms with a well-defined surface circulation and maximum sustained winds of 74 mph (64 knots) or higher.
2.2. Hurricane Categories. Hurricanes are categorized according to the strength of their winds using the Saffir-Simpson Hurricane Scale (Table 2.1.). A Category 1 storm has the lowest wind speeds, while a Category 5 hurricane has the strongest. These are relative terms, because lower category storms can sometimes inflict greater damage than higher category storms, depending on where they strike and the particular hazards they bring. In fact, tropical storms can also produce significant damage and loss of life, mainly due to flooding.
2.3. Hurricane Names. When the winds from these storms reach 39 mph (34 knots), the cyclone is given a name. Years ago, an international committee developed six separate lists of names for these storms. Each list alternates between male and female names. The use of these easily remembered names greatly reduces confusion when two or more tropical cyclones occur at the same time. Each list is reused every six years, although hurricane names that have resulted in substantial damage or death are retired. The names assigned for the period 2005 – 2010 are shown (Table 2.2.).
HURRICANE CATEGORIES (DISASTER POTENTIAL SCALE)
2.4. Hurricane Season. The official hurricane season for the Atlantic Basin (the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico) is from 1 June to 30 November. As seen in the graph to the right (Figure 2.1), the peak of the season is mid-August through late October. However, deadly hurricanes can occur anytime in the hurricane season.
2.5. Origin and Life Cycle.
2.5.1. The Birth of a Tropical Cyclone. Tropical cyclones form over warm waters from pre-existing disturbances. These disturbances typically emerge every three or four days from the coast of Africa as "tropical waves" that consist of areas of unsettled weather. Tropical cyclones can also form from the trailing ends of cold fronts and occasionally from upper-level lows. The process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on at least three conditions shown in Figure 2.2. Figure 2.2. Tropical Cyclone Process. • A pre-existing disturbance with thunderstorms. • Warm (at least 80° F) ocean temperatures to a depth of about 150 feet. • Light upper level winds that do not change much in direction and speed throughout the depth of the atmosphere (low wind shear).
2.5.2. Growth and Maturity : Hurricane Growth and Maturity. . In these early stages, the system appears on the satellite image as a relatively unorganized cluster of thunderstorms. If weather and ocean conditions continue to be favorable, the system can strengthen and become a tropical depression (winds less than 38 mph or 33 knots). At this point, the storm begins to take on the familiar spiral appearance due to the flow of the winds and the rotation of the earth (Figure 2.3.). If the storm continues to strengthen to tropical storm status (winds 39 - 73 mph, 34 - 63 knots), the bands of thunderstorms contribute additional heat and moisture to the storm. The storm becomes a hurricane when winds reach a minimum of 74 mph (64 knots). At this time, the cloud-free hurricane eye typically forms because rapidly sinking air at the center dries and warms the area. The center, or eye, of a hurricane is relatively calm. The most violent activity takes place in the area immediately around the eye, called the eyewall. During their life span, hurricanes can last for more than two weeks over the ocean and can travel up the entire Atlantic Coast.
2.5.3. The Storm's End. Just as many factors contribute to the birth of a hurricane, there are many reasons why a hurricane begins to decay. Wind shear can tear the hurricane apart. Moving over cooler water or drier areas can lead to weakening as well. Landfall typically shuts off the hurricane's main moisture source, and the surface circulation can be reduced by friction when it passes over land. Generally, a weakening hurricane or tropical cyclone can re-intensify if it moves into a more favorable region or interacts with mid-latitude frontal systems.