• A “hurricane” is the most severe category of the meteorological phenomenon known as the “tropical cyclone.”
• Tropical cyclones are low pressure systems that have thunderstorm activity and rotate counterclockwise. A tropical cyclone that has winds of 38 mph (33 kt) or less is called a tropical depression. When the tropical cyclone’s winds reach 39-73 mph (34-63 kt), it is called a tropical storm. When the winds exceed 74 mph (64 kt), the storm is considered to be a hurricane.
• The Saffir-Simpson Hurricane Scale defines hurricane strength by categories. A Category 1 storm is the weakest hurricane (winds 74-95 mph or 64-82 kt); a Category 5 hurricane is the strongest (winds greater than 155 mph or 135 kt).
• The category of the storm does not necessarily relate directly to the damage it will inflict. Lower category storms (and even tropical storms) can cause substantial damage depending on what other weather features they interact with, where they strike, and how slow they move.
Anatomy of a Hurricane
• Typical hurricanes are about 300 miles wide although they can vary considerably in size.
• The eye at a hurricane’s center is a relatively calm, clear area approximately 20-40 miles across.
• The eyewall surrounding the eye is composed of dense clouds that contain the highest winds in the storm.
• The storm’s outer rainbands (often with hurricane or tropical storm-force winds) are made up of dense bands of thunderstorms ranging from a few miles to tens of miles wide and 50 to 300 miles long.
• Hurricane-force winds can extend outward to about 25 miles in a small hurricane and to more than 150 miles for a large one. Tropical storm-force winds can stretch out as far as 300 miles from the center of a large hurricane.
• Frequently, the right side of a hurricane is the most dangerous in terms of storm surge, winds, and tornadoes.
• A hurricane’s speed and path depend on complex ocean and atmospheric interactions, including the presence or absence of other weather patterns. This complexity of the flow makes it very difficult to predict the speed and direction of a hurricane.
• Do not focus on the eye or the track–hurricanes are immense systems that can move in complex patterns that are difficult to predict. Be prepared for changes in size, intensity, speed, and direction.
What is a Hurricane?
A hurricane is a type of tropical cyclone, which is a generic term for a low pressure system that generally forms in the tropics. The cyclone is accompanied by thunderstorms and, in the Northern Hemisphere, a counterclockwise circulation of winds near the earth’s surface. Tropical cyclones are classified as follows:
• Tropical Depression An organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds* of 38 mph (33 kt**) or less
• Tropical Storm An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 39-73 mph (34-63 kt)
• Hurricane An intense tropical weather system of strong thunderstorms with a well-defined surface Circulation and maximum sustained winds of 74 mph (64 kt) or higher
* Sustained winds are defined as a 1-minute average wind measured at about 33 ft (10 meters) above the surface.
** 1 knot = 1 nautical mile per hour or 1.15 statute miles per hour. Abbreviated as “kt”.
Hurricanes are categorized according to the strength of their winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest wind speeds, while a Category 5 hurricane has the strongest. These are relative terms, because lower category storms can sometimes inflict greater damage than higher category storms, depending on where they strike and the particular hazards they bring. In fact, tropical storms can also produce significant damage and loss of life, mainly due to flooding.