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Category 4, the second-highest classification on the Saffir–Simpson Hurricane Scale, is used for tropical cyclones that have winds of 130–156 mph (209–251 km/h; 113–136 kn). The division of the eastern and central Pacific basins occurs at 140° W; the eastern Pacific covers area east of 140° W, while the central Pacific extends between 140° W to 180° W. Both basins' division points are at 66° N as a northern point and the equator as the southern point. As of , 96 hurricanes have attained Category 4 status in the northeastern Pacific basins. This list does not include storms that attained Category 5 status on the scale. Numerous climatological factors influence the formation of hurricanes in the Pacific basins. The North Pacific High and Aleutian Low, usually present between January and April, cause strong wind shear and unfavorable conditions for the development of hurricanes. During its presence, El Niño results in increased numbers of powerful hurricanes through weaker wind shear, while La Niña reduces the number of such hurricanes through the opposite. Global warming may also influence the formation of tropical cyclones in the Pacific basin. During a thirty-year period with two sub-periods, the first between 1975 and 1989 and the second between 1990 and 2004, an increase of thirteen Category 4 or 5 storms was observed from the first sub-period. == Statistics and background == On the Saffir–Simpson Hurricane Scale, "Category 4" is the second-most powerful classification, with winds ranging between 130–156 mph (209–251 km/h; 113–136 kn). When these hurricanes make landfall, impacts are usually severe but are not as destructive as Category 5 hurricanes that come ashore. The term "maximum sustained wind" refers to the average wind speed measured during the period of one minute at the height of above the ground. The windspeed is measured at that height to prevent disruption from obstructions. Wind gusts in tropical cyclones are usually approximately 30% stronger than the one-minute maximum sustained winds. The northeastern Pacific hurricane basins are divided into two parts – eastern and central. The eastern Pacific basin extends from all areas of the Pacific north of the equator east of 140° W, while the central Pacific basin includes areas north of the equator between 140° W and 180° W. Both basins extend to the Arctic Circle at 66° N.〔Longshore 1998, pp. 249–250〕 When tropical cyclones cross from the Atlantic into the Pacific, the name of the previous storm is retained if the system continues to exhibit tropical characteristics; however, when hurricanes degenerate into a remnant low-pressure area, the system is designated with the next name on the rotating eastern Pacific hurricane naming list. Since 1900, 99 Category 4 hurricanes have been recorded in the eastern and central Pacific basins. Of these, thirteen have attained Category 4 status on more than one occasion, by weakening to a status on the Saffir–Simpson Hurricane Scale lower than Category 4 and later restrengthening into a Category 4. Such storms are demarcated by the dates they first attained and the final time they lost the intensity. Only two storms, Hurricane Fico in 1978 and Hurricane Norbert in 1984, reached Category 4 status three times or more.〔 Between 1970 and 1975, advisories for systems in the eastern Pacific basins were initiated by the Eastern Pacific Hurricane Center (EPHC) as part of the National Weather Service (NWS) office in San Francisco, California. At that time, the advisories released were written in cooperation with the United States Navy Fleet Weather Center in Alameda and the Air Force Hurricane Liaison Officer at the McClellan Air Force Base. Following the move of the hurricane center to Redwood City in 1976, track files were created and altered by Arthur Pike and were later re-modified following the release of a study in 1980. The National Hurricane Center (NHC) extended its authority to the EPHC in 1988, and subsequently began maintaining the tracks.〔Blake et al. 2009, p. 6〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「List of Category 4 Pacific hurricanes」の詳細全文を読む スポンサード リンク
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