Hurricanes/Typhoons

Development and Measurement

Tropical cyclones are nonfrontal centers of low pressure that develop over tropical or subtropical waters, and are composed of intense, organized convection and surface winds that circulate cyclonically (e.g., counterclockwise in the Northern Hemisphere). Tropical cyclones typically develop from tropical disturbances or easterly waves, which are ill-defined areas of convective activity sustained for 24 hours or longer, frequently with broad, weak, cyclonic circulation. These disturbances may originate from convection moving offshore from a continent, a sustained convection near the Intertropical Convergence Zone (ITCZ), the tail end of mid-latitude frontal systems, or even from upper-level low pressure that eventually works its way down to the surface. If these disturbances become more vigorous or organized, and develop a surface center of cyclonic circulation with maximum sustained surface winds (one minute average) of less than 39 mph, then they are referred to as tropical depressions.

Tropical depressions are typically assigned consecutive numbers as they develop throughout the season. If the tropical cyclone continues to strengthen and develops sustained surface winds of at least 39 mph, then it becomes a tropical storm, at which point it is assigned a name based on designated lists developed by the World Meteorological Organization for each ocean basin. If the tropical cyclone intensifies, and sustained surface winds reach 74 mph, then it is called a hurricane in the Atlantic Ocean, northeast Pacific Ocean east of the dateline, or the south Pacific Ocean east of 160 E; a typhoon over the northwest Pacific Ocean west of the dateline; severe tropical cyclone in the southwest Pacific Ocean west of 160 E, or southeast Indian Ocean east of 90 E; a severe cyclonic storm over the north Indian Ocean; or a tropical cyclone in the southwest Indian Ocean.

Once a tropical cyclone reaches hurricane strength in the Atlantic or northeast Pacific Oceans, the United States uses the Saffir-Simpson hurricane intensity scale to rank the strength of the hurricane. The Saffir-Simpson scale begins with the weakest, a Category 1 hurricane, with sustained winds between 74 and 95 mph; and continues up to the strongest, a Category 5 hurricane, [Page 328]which has sustained surface winds of 156 mph or greater. Fortunately, Category 5 hurricanes are the least frequent. Wind gusts in a hurricane can be 10–15 percent higher than the sustained wind speed. The Saffir-Simpson scale also includes estimates of central atmospheric pressure and estimated storm surge height based on sustained wind speeds.

Saffir-Simpson Hurricane Scale

Category 3,4, and 5 hurricanes are referred to as major hurricanes. They cause the majority of damage in the United States, although they are less frequent than weaker tropical cyclones. However, weak hurricanes as well as tropical storms and depressions may still produce major damage and casualties, particularly from flooding, as was the case with Tropical Storm Allison in Texas in 2001 or Tropical Storm Alberto in 1994 in Georgia and Florida. Storm strength does not appear to be related to the size or shape of the tropical cyclone.

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