Hurricane Awareness: Hurricane Basics

Hurricane Ivan - 2004 (Image courtesy NOAA)

July 20, 2009

The name "Hurricane" is derived from the Caribbean god of evil, "hurican". The unpredictable behavior, high seas and devastating winds have been our nemesis for centuries.

The National Weather Service has declared the week of July 20th to be Hurricane Awareness Week in New England. The NWS Forecast Offices in Gray and Caribou will be issuing information about hurricanes and safety each day during this week, and have provided us with a wealth of information to share at Maine Prepares. Visit the Maine Prepares site every day for articles and tips on hurricane preparedness.

What is a hurricane?

A hurricane is a type of tropical cyclone, 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 miles per hour (33 knots) 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.

The tropical cyclones that affect eastern North America generally form in either the tropical Atlantic Ocean, the Caribbean Sea, or in the Gulf of Mexico. The three main conditions which favor tropical cyclone development are:

warm ocean waters,
atmospheric moisture, and
relatively light winds aloft.

In addition, an atmospheric disturbance, called a tropical wave, is needed to initiate the development the counter-clockwise wind circulation. If the favorable conditions persist for a sufficient amount of time, the tropical disturbance can strengthen to a tropical depression, tropical storm, or hurricane. When the winds in a tropical cyclone reach tropical storm strength (39 mph, 34 kt), the storm is "named".

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.

The well-developed hurricane consists of an eye, an eyewall, and spiral bands of showers and thunderstorms. In the eye, winds are relative calm and there is a gentle sinking motion in the atmosphere which leads to mostly clear skies. Surrounding the eye is the eyewall which contains the most violent winds, the most intense showers and thunderstorms in the hurricane, and can contain tornadoes. The winds in the eyewall also have the greatest potential for causing a deadly storm surge. Outside the eyewall, spiral bands of showers and thunderstorms rotate around the storm. These bands of showers and thunderstorms can also be very intense, can move into an area very rapidly, and are the most likely area in the hurricane for tornadoes to form.

Hurricanes and tropical storms bring with them four main threats: high winds, coastal storm surge, inland fresh water flooding, and tornadoes.

While hurricane season lasts from June through November, the peak of the season is from mid-August through October. Each year, an average of ten tropical storms develop over the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Many of these storms remain over the ocean, and an average of six become hurricanes each year. During an average 3-year period, roughly five hurricanes strike the US coastline, killing approximately 45 people (an average of 15.5 per year) anywhere from Texas to Maine. Of these, two are typically "major" or "intense" hurricanes (winds greater than 110 mph).

Notes:

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".

For additional information about hurricanes and hurricane safety, visit the National Hurricane Center's web site at: http://www.nhc.noaa.gov/

Also visit National Weather Service Caribou at http://www.weather.gov/caribou and Gray at http://www.weather.gov/gray

Contact:

Beth Barton or Lynette Miller
207-624-4400

Home and Family | Schools | Business | Community | Service Organizations