Hurricanes and Typhoons

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 [Page 514]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 tropical 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.

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