Explaining Tropical Terms as Hurricane Irene Approaches Canton

[Editor's note: Terms are courtesy of the National Weather Service.] 
Advisory:Official information issued by tropical cyclone warning centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken.

 

Cyclone:An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

 

Direct Hit:A close approach of a tropical cyclone to a particular location. For locations on the left-hand side of a tropical cyclone's track (looking in the direction of motion), a direct hit occurs when the cyclone passes to within a distance equal to the cyclone's radius of maximum wind. For locations on the right-hand side of the track, a direct hit occurs when the cyclone passes to within a distance equal to twice the radius of maximum wind.

 

Eye:The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud.

 

Eyewall/Wall Cloud:An organized band or ring of cumulonimbus clouds that surround the eye, or light-wind center of a tropical cyclone. Eyewall and wall cloud are used synonymously.

 

Hurricane / Typhoon:A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 74 mph or more. The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific tropical cyclones north of the Equator west of the International Dateline.

 

Hurricane Local Statement:A public release prepared by local National Weather Service offices in or near a threatened area giving specific details for its county/parish warning area on (1) weather conditions, (2) evacuation decisions made by local officials, and (3) other precautions necessary to protect life and property.

 

Hurricane Warning:An announcement that hurricane conditions (sustained winds of 74 mph or higher) are expectedsomewhere within the specified area. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds.

 

Hurricane Watch:An announcement that hurricane conditions (sustained winds of 74 mph or higher) are possiblewithin the specified area. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical-storm-force winds.

 

Indirect Hit:Generally refers to locations that do not experience a direct hit from a tropical cyclone, but do experience hurricane force winds (either sustained or gusts) or tides of at least 4 feet above normal.

 

Landfall:The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone's strongest winds to be experienced over land even if landfall does not occur.

 

Major Hurricane:A hurricane that is classified as Category 3 or higher.

 

Radius of Maximum Winds:The distance from the center of a tropical cyclone to the location of the cyclone's maximum winds. In well-developed hurricanes, the radius of maximum winds is generally found at the inner edge of the eyewall.

 

Saffir-Simpson Hurricane Wind Scale:The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the indicated time. The scale provides examples of the type of damage and impacts in the United States associated with winds of the indicated intensity. The following table shows the scale broken down by winds: 

 

Category Wind Speed (mph) Damage 1 74 - 95 Very dangerous winds will produce some damage 2 96 - 110 Extremely dangerous winds will cause extensive damage 3 111 - 130 Devastating damage will occur 4 131 - 155 Catastrophic damage will occur 5 > 155 Catastrophic damage will occur

 

A detailed description of the Saffir-Simpson Hurricane Wind Scale, which was revised in early 2010, is available here. 

 

Storm Surge:An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. 

 

Tropical Depression:A tropical cyclone in which the maximum sustained surface wind speed is 38 mph or less.

 

Tropical Storm:A tropical cyclone in which the maximum sustained surface wind speed ranges from 39 mph to 73 mph.

 

Tropical Storm Warning:An announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are expectedsomewhere within the specified area within 36 hours.

 

Tropical Storm Watch:An announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are possiblewithin the specified area within 48 hours.