HURRICANES AND TYPHOONS are tropical cyclones with maximum sustained winds of at least 74 mi. per hour (64 knots). Tropical cyclones that originate over the tropical Atlantic basin or the eastern/central North Pacific basin are called hurricanes, while those that originate in the tropical western North Pacific are called typhoons. Before the tropical cyclone reaches hurricane or typhoon status, there are stages of development the system will go through from birth to decay. Initially, when a poorly-organized mass of thunderstorms produces a weak circulation, the system is known as a tropical disturbance or tropical wave. When the winds increase to between 20 and 34 knots, the system is upgraded to a tropical depression, and when the winds increase to between 35 and
64 knots, the system is then called a tropical storm. At this point, the tropical cyclone is given a name.
For each tropical cyclone basin (Atlantic, eastern North Pacific, central North Pacific, western North Pacific, Australian region, Fiji Region, Papua New Guinea Region, Philippine Region, Northern Indian, Southwest Indian), there is a list of names. In the Atlantic, there is a six-year list of names. The list of names originated in 1953, and was first compiled and maintained by the National Hurricane Center. Currently, the lists are compiled and maintained by the World Meteorological Organization. Until 1979 (1978 in the eastern North Pacific), the list of names featured only female names. With the introduction of male names in 1979, male and female names are now alternated with each letter of the alphabet. The Atlantic basin has names for each letter of the alphabet from A to W, with the exceptions of Q and U.
In the event that each letter is used for a given year (as was the case in 2005), the Greek alphabet is then used (alpha, beta, gamma, and so on) for subsequent storms. If a particular storm is deadly or costly, the name is retired and replaced by a name beginning with the same letter. For example, there were five named storms in 2005 that were retired and replaced by the World Meteorological Organization (Dennis, Katrina, Rita, Stan, and Wilma were replaced with Don, Katia, Rina, Sean, and Whitney). The eastern Pacific basin has a similar six-year list of names, except that the list includes names for X, Y and Z (Q and U are still excluded). The western Pacific has five lists of names that are compiled and used sequentially by 14 contributing nations.
The Atlantic hurricane season begins on June 1st. and runs through November 30, with the peak frequency occurring during the months of August, September, and October. These months correspond to the time period where ocean temperatures are the highest. The eastern Pacific hurricane season is slightly longer, running from May 15. through November 30, with the peak frequency occurring during July, August, and September. Typhoons in the western Pacific can occur year round due to the vast supply of warm water, but typically occur from May to November.
Hurricanes and typhoons need certain conditions in order to intensify and grow. First and foremost, they need warm water to form. The threshold temperature that is generally considered necessary for tropi-
cal cyclone formation is 79 degrees F (26 degrees C). The warm water from the ocean provides the energy that is transferred from the surface to the lowest layers of the troposphere. This causes the atmosphere to be unstable and allows for rising air. As the air rises, it cools, condenses, and releases latent heat, which causes the upper levels of the atmosphere to warm. Thus, an upper level high pressure is produced, which helps evacuate air to enhance the development of the surface low. The deeper the surface low becomes, the stronger the pressure gradient becomes, which creates stronger winds.
In addition to warm water, hurricanes and typhoons must form at non-equatorial locations, because they need Coriolis force (apparent force that causes an apparent deflection to the right of an object in the Northern Hemisphere because of the rotation of the Earth) to help the rotation, and in areas that help trigger surface convergence. One such area is the Intertropical Convergence Zone (ITCZ), which is an area where low pressure devel ops, however, few Atlantic systems develop there because of the proximity to the equator. The majority of Atlantic systems develop on the eastern side of tropical waves, which is an area that promotes convergence and rain showers. Another condition that must be satisfied for tropical cyclone development is the absence of vertical wind shear. Vertical wind shear is the change in wind speed or direction with height. When a hurricane or typhoon encounters areas with high vertical wind shear, the storm has a tendency to tilt, which inhibits the efficiency of the heat distribution in the atmosphere. Therefore, the upper level cannot form over the surface low that would cause the low to deepen. As a result, vertical wind shear causes the surface low to weaken.
The Saffir-Simpson scale is used to measure the intensity of hurricanes in the Atlantic and eastern Pacific. It uses estimated and measured maximum sustained winds in the eye wall to classify the intensity on a scale from one (weakest) to five (strongest). Only three category five hurricanes have struck the United States: Florida Keys (1935), Camille (1969), and Andrew (1992).
The impacts of global warming upon hurricane and typhoon formation are not fully understood. On one hand, an increase in global temperatures would increase ocean temperatures, which provide more energy for the tropical cyclone. On the other hand, it is not exactly known how warming would change vertical wind shear and the frequency of teleconnection patterns such as El Niño, which impact vertical wind shear. El Niño generally strengthens trade winds, particularly over the Atlantic basin, which inhibits Atlantic hurricane formation (as was the case in 2006). However, during El Niño years, there tends to be a slightly more active eastern Pacific season.
sEE ALso: Cyclones; El Niño and La Niña; Intertropical Convergance Zone; Weather.
BIBLIogRAPHY. C.D. Ahrens, Meteorology Today (Brooks/ Cole, 2007); Bob Sheets and Jack Williams, Hurricane Watch (Vintage Books, 2001); Robert Simpson, Hurricane! Coping with Disaster (American Geophysical Union, 2003).
Kevin Law Marshall University
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