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Hurricane Glossary

  1. Air Pressure- Simply put, the weight of the air. The more air, the higher the pressure, the less air, the lower the pressure.
  2. Anticyclone- Also known as 'ridge' or 'high pressure', a clockwise rotating area of air that tends to denote good weather. It is the opposite of cyclone.
  3. Barometric Pressure- (Atmospheric pressure) The amount of pressure exterted by the atmosphere at a specific point. We measure the weight of the air with a BAROMETER.
  4. Cyclone- Counter-clockwise rotating area of air that usually denotes unsettled weather. A hurricane is a (warm-core) cyclone.
  5. Hurricane- A hurricane is a tropical storm with winds that have reached a constant speed of 74 miles per hour or more. Hurricane winds blow in a large spiral around a relative calm center known as the "eye." The "eye" is generally 20 to 30 miles wide, and the storm may extend outward 400 miles. As a hurricane approaches, the skies will begin to darken and winds will grow in strength. As a hurricane nears land, it can bring torrential rains, high winds, and storm surges. A single hurricane can last for more than 2 weeks over open waters and can run a path across the entire length of the eastern seaboard. August and September are peak months during the hurricane season that lasts from June 1 through November 30. The center, or eye, of a hurricane is relatively calm. The most violent activity takes place in the area immediately around the eye, called the eyewall. At the top of the eyewall (about 50,000 feet), most of the air is propelled outward, increasing the air's upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.
  6. Hurricane Season- The portion of the year having a relatively high incidence of hurricanes. The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30. The hurricane season in the Eastern Pacific basin runs from May 15 to November 30. The hurricane season in the Central Pacific basin runs from June 1 to November 30.
  7. Hurricane Warning- A warning that sustained winds of 64 knots (74 mph or 119 kph) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.
  8. Hurricane Watch- An announcement of specific coastal areas that a hurricane or an incipient hurricane condition poses a possible threat, generally within 36 hours.
  9. Knot- A unit of speed most often used by marine interests in which one nautical mile per hour is achieved. One knot equals approximately 1.15 miles per hour.
  10. Rapid Deepening- A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 mb/hr or 42 mb for 24 hours. Hurricane Opal in 1995 experienced rapid deeping while out over the very warm waters of the Gulf of Mexico. Fortunately, Opal weakened just prior to landfall on the Gulf Coast.
  11. 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. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.
  12. Storm Tide- The actual level of sea water resulting from the astronomic tide combined with the storm surge.
  13. Tornadoes- Hurricanes also produce tornadoes, which add to the hurricane's destructive power. Typically, the more intense a hurricane is, the greater the tornado threat. When a hurricane brings its winds inland, the fast-moving air hits terrain and structures, causing a frictional convergence which enhances lifting. Frictional convergence may be at least a contributing factor to tornado formation in hurricanes. The greatest concentration of tornadoes occurs in the right front quadrant of the hurricane. A number of theories exist about their origin, but in the case of Hurricane Andrew, severe damage was inflicted by small spin-up vortices that developed in regions of strong wind-shear found in the hurricane's the eye wall. The strong damaging winds of the hurricane frequently cover the smaller tornado paths, making the separation of their damaging effects very difficult.
  14. Tropical Cyclone-The generic term given to all tropical depressions, tropical storms and hurricanes. A tropical cyclone has a warm core and must have a warm ocean and humid environment in order to survive. Also, a tropical cyclone will have its most intense winds concentrated close to the center.
  15. Tropical Disturbance- A distinct tropical weather system of apparently organized convection originating in the tropics or subtropics and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field (in other words, a tropical disturbance does not always have a center of lowest air pressure- where the wind spirals towards).
  16. Tropical Storm- A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 34 knots (39 mph or 63 kph) to 63 knots (73 mph or 118 kph).
  17. Tropical Storm Warning- A warning for tropical storm conditions including sustained winds within the range of 34 to 63 knots (39 to 73 mph or 63 to 118 kph) that are expected in a specified coastal area within 24 hours or less.
  18. Tropical Storm Watch- An announcement that a tropical storm poses or tropical storm conditions pose a threat to coastal areas generally within 36 hours. A tropical storm watch should normally not be issued if the system is forecast to attain hurricane strength.
  19. Tropical Wave- An area of converging air (and relatively low air pressure) that is embedded within the deep easterlies. May lead to tropical cyclone development.
  20. Tropical Depression- A closed-low pressure area with organized convection, heavy rain, and winds up to 38 mph. The first stage of hurricane development.
  21. Trough- An elongated area of low pressure (where the air is rising) usually between two areas of high pressure. Troughs tend to deflect tropical systems away from the US coast.
  22. Upper Level Low- An area of cold air aloft (20,000 to 30,000 feet) that is rotating counter-clockwise. These sometimes steer hurricanes and less frequently, transform into a warm core low and work their way down to the surface to become a tropical cyclone.
  23. Upwelling- The "bringing up of water" from deeper water towards the surface. A hurricane can "upwell" colder water to the surface as it moves along and churns up the ocean. This can actually weaken a hurricane if it becomes stationary and thus upwells colder water underneath itself.