pressure gradient driving the flow. This causes the flow to rotate cyclonically about its center, producing geostrophic flow. Tropical cyclones require a minimum distance of from the equator to sustain tropical cyclogenesis. Cyclone An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. D 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. Compare indirect hit, strike. E Eastern North Pacific Basin The portion of the North Pacific Ocean east of 140W. The National Hurricane Center in Miami, Florida is responsible for tracking tropical cyclones in this region. 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. Extratropical A term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force. 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. F Fujiwhara effect The tendency of two nearby tropical cyclones to rotate cyclonically about each other. G Gale Warning A warning of 1-minute or 10-minute sustained surface winds in the range 34 kn (39 mph or 63 km/h) to 47 kn (54 mph or 87 km/h) inclusive, either predicted or occurring and not directly associated with tropical cyclones. H High Wind Warning A high wind warning is defined as 1-minute average