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What is a hurricane? Facts to know

How are hurricanes different from typhoons or cyclones? When is hurricane season? Read about categories, names and other hurricane facts.

The official Atlantic hurricane season begins June 1 and runs through Nov. 30, although hurricanes can form for a few weeks before and after that window. In other parts of the world, like the western Pacific, hurricanes can form year-round.

Understanding a hurricane

Sept. 23, 201704:03

What is a hurricane?

"Hurricane" is the name used for the most powerful tropical cyclones in the Atlantic Ocean and the northeastern Pacific Ocean — those bearing sustained winds of 74 mph and above. Hurricanes are generally called "typhoons" in the northwestern Pacific Ocean and "severe cyclonic storms" in the south Pacific and the Indian Ocean.

Tropical cyclones are rotating low-pressure systems carrying thunderstorms but no identifiable "front," which is the boundary that separates two air masses of different densities in most storm systems.

Tropical cyclones with sustained winds of less than 39 mph are called "tropical depressions," while those with sustained winds of 39 mph to 73 mph are called "tropical storms."

What are the hurricane categories?

A hurricane's power is measured on the Saffir–Simpson Hurricane Wind Scale, which was developed in 1971 by Herbert Saffir, U.N. civil engineer who died in 2007, and Robert Simpson, then the director of the National Hurricane Center, who died in 2014.

The scale separates hurricanes into five categories, with 1 being the weakest and 5 being the strongest. Category 3, 4 and 5 hurricanes are called "major hurricanes."

  • Category 1 (74 mph to 95 mph): Very dangerous winds with some damage to homes and trees.
  • Category 2 (96 mph to 110 mph): Extremely dangerous winds causing extensive damage to homes, trees and roads, often resulting in near-total power loss for several days to weeks.
  • Category 3 (111 mph to 129 mph): Devastating damage to homes, trees and roads. Electricity and water are often unavailable for several days to weeks.
  • Category 4 (130 mph to 156 mph): Catastrophic damage to homes, trees and power infrastructure. Most areas are uninhabitable and without power for weeks or months.
  • Category 5 (157 mph and higher): Most homes and other structures are destroyed. Most areas are uninhabitable and without power for weeks or months.

National Hurricane Center: About the Saffir-Simpson Hurricane Wind Scale (PDF)

How hurricanes are formed

Sept. 19, 201701:41

How do hurricanes form?

Tropical cyclones develop when low-pressure systems gather heat and energy as they absorb warm ocean water near the equator. Evaporation from the ocean surface fuels them like giant heat engines, creating powerful winds twisting around a relatively peaceful center, called the "eye." (The winds rotate counterclockwise north of the equator and clockwise south of the equator.)

When a tropical cyclone makes landfall or crosses over cooler waters, its source of power disappears, so the storm begins to weaken.

How are hurricanes named?

Hurricanes and other tropical cyclones are given men's and women's names by the World Meteorological Organization to make them easier to keep straight.

Different lists of names are used for different regions, reflecting local naming customs (the letters Q, U, X, Y and Z are never used for Atlantic hurricanes). When a tropical cyclone is particularly deadly or damaging, its name is retired.

As of January, 82 Atlantic hurricane names had been retired. The most in any year, five, blew through in 2005: Dennis, Katrina, Rita, Stan and Wilma.

National Hurricane Center: Tropical cyclone names through 2022

Which modern hurricanes have caused the most damage?

Reliable records have been kept only for the last half-century or so. Ranking hurricanes by the extent of the damage they cause is difficult, because monetary inflation and ever-improving building codes make comparisons imprecise.

Here's a list of the deadliest Atlantic hurricanes since 1900:

  1. Hurricane Mitch, Central America/Yucatán Peninsula/South Florida, October-November 1998 — 11,374 people killed
  2. Hurricane Fifi-Orlene, Jamaica/Central America/Mexico, September 1974 — 8,200 people killed
  3. Hurricane Flora, Caribbean/Florida, September-October 1963 — 7,193 people killed
  4. Unnamed hurricane ("Galveston" hurricane), Caribbean/Texas, August-September 1900 — 6,000 to 12,000 people killed
  5. Unnamed hurricane ("Okeechobee" hurricane), Lesser Antilles/Puerto Rico/Florida, September 1928 — 4,075 people killed

And here's a list of the deadliest tropical cyclones around the world since 1900:

  • Great Bhola Cyclone, Bangladesh, November 1970 — 300,000 to 500,000 people killed
  • Super Typhoon Nina, China, August 1975 — about 171,000 people killed
  • Cyclone 02B, Bangladesh, May 1991 — 138,866 people killed
  • Cyclone Nargis, Myanmar, May 2008 — 138,366 people killed
  • Unnamed typhoon, Swatow (now Shantou), China, July 1922 — about 100,000 people killed

SOURCES: U.S. National Hurricane Center; Hurricane Research Division, Atlantic Oceanographic & Meteorological Laboratory; U.N. Office for Disaster Risk Reduction; World Meteorological Organization; Centre for Research on the Epidemiology of Disasters, Catholic University of Louvain, Brussels;, University of Manchester, England