Appendix – Tropical Storm Classification

Meteorological and Hurricane Data

Tropical Storm Classification

Tropical systems go through several stages before developing into a full-fledged hurricane. The following defines the four stages of the development cycle, based on information from the Atlantic Oceanographic and Meteorological Laboratory (AOML). The AOML is the hurricane research division of the National Oceanic and Atmospheric Administration (NOAA). Windspeeds mentioned here are for those measured or estimated as the top speed sustained for one minute at 10 meters above the surface. Peak gusts would be on the order of 10-25 percent higher.

Tropical Disturbance

An individual tropical weather system of apparently organized convection (upward transfer of heat and moisture into the atmosphere from the surface). Generally 100 to 300 nautical miles in diameter, originating in the tropics or subtropics. Having a non-frontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable disturbance of the wind field. Surface upheavals associated with disturbances in the wind field and progressing through the tropics from east to west are also known as easterly waves.

Tropical Depression

A tropical cyclone (a gathering of thunderstorms that rotate around a central vortex) in which the maximum sustained windspeed (using the U.S. 1 minute average standard) is 38 m.p.h. (33 knots) or less. Depressions have a closed circulation.

Tropical Storm

A tropical cyclone in which the maximum sustained surface windspeed (using the U.S. 1 minute average standard) ranges from 39to 73 m.p.h. (34 to 63 knots). The convection in tropical storms is usually more concentrated near the center, with outer rainfall organizing into distinct bands.


When winds in a tropical cyclone equal or exceed 74 m.p.h. (64 knots), it is called a hurricane (in the Atlantic and eastern and central Pacific Oceans). Hurricanes are further designated by categories on the Saffir-Simpson scale. Hurricanes in categories 3, 4 and 5 are known as Major or Intense Hurricanes.

The Saffir-Simpson Scale of Hurricane Intensity

In 1969, The World Meteorological Organization was preparing a report on structural damage to dwellings hit by windstorms. They recognized the need for a standardized scale to which all with weather-related interests could refer, concerning the strength of hurricanes. The task of formulating such a scale fell to Dr. Bob Simpson, director of the National Hurricane Center at the time, and his colleague Herbert Saffir, a consulting engineer.

The two men took into account that the destructive power of a hurricane depends on the way a number of factors combine:

  • storm surge
  • wind
  • forward motion
  • the configuration of the land it strikes
  • and other aspects of the weather event.

They came up with a five-category scale that assigns each hurricane (any tropical storm with sustained winds over 74 m.p.h.) a number, according to its intensity.

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Mr. Saffir contributed applicable standards of potential for causing property damage, while Dr. Simpson added information regarding potential for storm surge heights that accompany hurricanes in each category. This information supplies a fairly reliable estimate of the potential property damage and flooding that may be expected along coastal areas faced with the imminent arrival of a hurricane.

The resulting Saffir-Simpson Scale is now used by NOAA’s hurricane forecasters to make storm comparisons easier, and to clarify to government emergency managers the predicted hazards of approaching hurricanes.


CATEGORY 1 – No real damage to building structures.
Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal road flooding and minor pier damage.

CATEGORY 2 – Some roofing material, door, and window damage to buildings. Considerable damage to vegetation, mobile homes, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of center. Small craft in unprotected anchorages break moorings.

CATEGORY 3 – Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures, with larger structures damaged by floating debris. Terrain continuously lower than 5 feet ASL* may be flooded inland 8 miles or more.

CATEGORY 4 – More extensive curtainwall failures, with some complete roof structure failure on small residences. Major erosion of beach. Major damage to lower floors of structures near the shore. Terrain continuously lower than 10 feet ASL may be flooded, requiring massive evacuation of residential areas inland as far as 6 miles.

CATEGORY 5 – Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Major damage to lower floors of all structures located less than 15 feet ASL, and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5 to 10 miles of the shoreline may be required.

*ASL = Above Sea Level

Historic Examples in Each Category

  1. Florence, 1988, LA; Charley, 1988, NC
  2. Kate, 1985, FL; Bob, 1991, NY
  3. Alicia, 1983, TX
  4. Andrew, 1992, FL; Hugo, 1989, NC
  5. Camille, 1969, MS; Labor Day Hurricane, 1935, FL Keys

25 Costliest Atlantic Basin Hurricanes 

Hurricane Diane, which caused the flood this book is about, used to be #16 on the list of costliest US landfalling hurricanes since 1900; but after 2005, all those numbers went out the window. Now she doesn’t even make this chart! This list ranks tropical cyclones within the Atlantic basin that have accrued at least USD$1 billion in damage, not adjusted for inflation.

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