The GOES-13 satellite saw Hurricane Irene on August 27, 2011 at 10:10 a.m. EDT after it made landfall at 8 a.m. in Cape Lookout, North Carolina. Irene's outer bands had already extended into New England.
Credit: NASA/NOAA GOES Project
There is an old expression that a hurricane is Mother Nature’s way of telling us that she is angry. But of course hurricanes are not just a byproduct of nature’s ire.
Hurricanes — or more broadly, tropical cyclones — generally begin as clusters of thunderstorms over tropical ocean waters, taking anywhere from several hours to several days to become organized and graduate to hurricane status.
There has to be a perfect storm, so to speak, of conditions for a hurricane to form, including:
- Water that is at least 80 degrees Fahrenheit (26.6 Celsius)
- Relatively moist air
- Very warm surface temperatures
- A continuous evaporation and condensation cycle
- Wind patterns of varying directions that collide (converging winds)
- A difference in air pressure between the surface and high altitude
Tropical cyclones form all around the world, generally about 300 miles (480 kilometers) north or south of the equator. When they form in the Atlantic or Eastern Pacific, the storms are called hurricanes. They are called typhoons in the western North Pacific and cyclones in the South Pacific and Indian Ocean. [Infographic: How, When & Where Hurricanes Form]
The Atlantic hurricane season is from June through November, when the storms take shape on the coast of Africa. The Eastern Pacific season runs from mid-May through November. Typhoons occur year-round but peak in late August. In the South Pacific, the cyclone season begins in October and ends in May.
In the Atlantic region, hurricanes form anywhere from the tropical central Atlantic to the Gulf of Mexico. Hurricanes typically follow three paths [Infographic: Storm Targets: Where the Hurricanes Hit]:
- originating off the West Coast of Africa near the Cape Verde Islands and traveling west toward the Caribbean and the East Coast of the United States
- originating in the Western Caribbean, and moving into the Gulf Coast, or along the U.S. East Coast
- originating in the Gulf of Mexico and moving into the Gulf Coast states from Texas to Florida.
There are distinct levels of progression as a storm becomes a hurricane. The first stage is a tropical disturbance, which is essentially a significant cluster of showers and thunderstorms. As it becomes a tropical depression, it is slightly more organized and the winds pick up to 25 to 38 mph (40 to 61 km/h). It is classified as a tropical storm when winds reach 39 to 73 mph (62 to 117 km/h). Once the winds reach 74 mph, it is classified as a hurricane and its intensity is measured by the Saffir-Simpson Scale.
The Saffir-Simpson hurricane scale was developed in 1971 by civil engineer Herbert Saffir and meteorologist Bob Simpson, who at the time was director of the U.S. National Hurricane Center.
- Category 1: wind 74-95 mph (119-153 km/h)
- Category 2: winds 96-110 mph (154-177 km/h)
- Category 3: 111-129 mph (178-208 km/h)
- Category 4: 130-156 mph (209-251 km/h)
- Category 5: exceeding 157 mph (252 km/h)
Some storms are called super-typhoons when wind speeds reach 150 mph (241 km/h), according to the U.S. Joint Typhoon Warning Center. "Super-typhoons are much more intense than regular typhoons; they have a higher wind speed," said Harold Brooks, a research meteorologist at the National Oceanic and Atmospheric Administration (NOAA).
The main parts of a hurricane are the rainbands, the eye and the eyewall. Air spirals in toward the center in a counter-clockwise pattern in the Northern Hemisphere (clockwise in the Southern Hemisphere) and out the top in the opposite direction. In the very center of the storm, air sinks, forming an "eye" that is mostly cloud-free and extends 20 to 40 miles (32 to 64 km) in diameter.
The eye is surrounded by the eyewall, a ring of towering thunderstorms that inflict some of the storm’s most severe punishment. Curved bands of clouds and thunderstorms trail away from the eye wall in a spiral fashion. These rainbands can produce heavy bursts of rain and wind, as well as tornadoes.
Tropical cyclones can get up to 300 miles (483 km) wide, but size is not necessarily an indication of intensity. A hurricane's destructive winds and rains can extend outward more than 150 miles (242 km).
Winds are not the only hazard from tropical cyclones. Storm surges, when water is pushed toward the shore by the force of the winds, can increase the average water level 15 feet (4.5 meters) or more. Flooding also occurs, and in fact, causes most of the deaths during a tropical cyclone. More people are killed by floods than by any other weather-related cause. [Hurricane Preparation: What to Do]
Naming the storms
Hurricane names are determined by the World Meteorological Organization in Geneva. The organization maintains six lists of alphabetical names that are used in rotation and recycled every six years. There are separate lists for Atlantic, Eastern North Pacific, Central North Pacific and other zones. Names are retired after a particularly deadly or costly storm. Most recently, the names Wilma (2005), Rita (2005) and Katrina (2005) were among the names removed from the lists. [Infographic: The Costliest Hurricanes in History]