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Hurricanes: Science and Society
Glossary - E
easterly wave
A migratory wavelike disturbance in the tropical region (over the North Atlantic and Caribbean/ general easterly trade wind circulation) that develops off the sub-Saharan, East African coast at rate of one every three days. It is a wave within the broad easterly current and moves from east to west, causing areas of cloudiness and thunderstorms. Although best described in terms of its wavelike characteristics in the wind field, it also consists of a weak trough of low pressure. The presence of a disturbance like this indicates atmospheric instability and is often associated with tropical cyclone development.
eastern north Pacific basin
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. Source NOAA-NHC.
eddies in the ocean
A closed warm or cold core circulation system in the ocean, produced as an offshoot from an ocean current.
El Nino
A warming of the ocean current along the coasts of Peru and Ecuador that is generally associated with dramatic changes in the weather patterns of the region; a major El NiƱo event generally occurs every 3 to 7 years and is associated with changes in the weather patterns worldwide. Source: NOAA NWS.
El Nino-Southern Oscillation (ENSO)
ENSO is the term used to define the oceanic El Nino/ La Nina cycle and the associated atmospheric Southern Oscillation.
A species whose continued existence is in jeopardy and is provided special protection by law. Source: USGS
ensemble mean
The average of a predicted variable or field over an ensemble of forecasts. Source: (c) 1999, American Meteorological Society. Used with permission.
A plant, fungus, or microbe sustained entirely by nutrients and water received nonparasitically from within the canopy in which it resides.
Geographically, on the surface of the earth, the imaginary great circle of latitude 0 degrees, which is equidistant from the poles, and which separates the Northern Hemisphere from the Southern Hemisphere.
The process in which a material is worn away by a stream of liquid (water) or air, often due to the presence of abrasive particles in the stream. Source: USGS.
The area where the salty ocean meets a freshwater stream/river. Source: US NPS.
evacuation zone
Any area required to evacuate during a storm event.
The process of a liquid changing into a vapor or gas. Source: NOAA-NWS ERH.
explosive deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 mb/hr for at least 12 hours or 5 mb/hr for at least six hours. Source NOAA-NHC.
external forcing
The influence on the Earth system (or one of its components) by an external agent such as solar radiation, volcanic eruptions, or the impact of extraterrestrial bodies such as meteorites.
extratropical cyclone
A cyclone of any intensity for which the primary energy source is baroclinic, that is, results from the temperature contrast between warm and cold air masses. Extratropical cyclones have cold air at their core, and derive their energy from the release of potential energy when cold and warm air masses interact. It is important to note that systems can become extratropical and still retain winds of hurricane or tropical storm force. Extratropical cyclones are also referred to as mid-latitude cyclones or wave cyclones. Source NOAA-NHC.
Extratropical Transition (ET)
The transformation of a tropical cyclone into an extratropical cyclone. More than 40% of Atlantic tropical cyclones undergo such a transformation at the end of their tropical existence. The metamorphosis typically starts as the system begins to be driven by atmospheric temperature differences rather than latent heat release. The cyclone begins losing its tropical characteristics and often develops front-like qualities. During the transformation, the rain area usually shifts to the left of the storm track while the strongest winds diminish but become elongated to the right of the storm track. Some extratropical transition events can result in a more powerful storm than the originating tropical cyclone. Source: Canadian Hurricane Centre.
The relatively calm center in a hurricane that is at least halfway surrounded by clouds comprising the eyewall. The winds are light, the skies are partly cloudy or even clear (the skies are usually free of rain), and radar depicts it as an echo-free area within the eyewall. The hurricane eye typically forms when the maximum sustained tangential wind speeds exceeds about 125 km/h (78 mph) The eye diameter, as depicted by radar, ranges typically from as small as 8 to 16 km (5 to 10 mi) upwards to about 160 km (100 mi). The average hurricane eye diameter is a little over 32 km (20 mi). When the eye is shrinking in size, the hurricane is generally intensifying. Source: NOAA Jetstream.
An organized ring or band of cumulonimbus clouds that surround the eye of a tropical cyclone. This roughly circular ring of deep convection is the area of heaviest rain, strongest surface winds, and generally the most severe turbulence in the tropical cyclone. Source NOAA-NHC.