Tuesday, February 2, 2010

Notable Hurricanes: Longevity; Hurricane ACE

Among the many Atlantic hurricanes that have formed in the past 150 years or so, most are notable because of their intensity or for their impact. In this post, starting with the 12th, the top twelve hurricanes in terms of longevity will be listed. But first, a little word about ACE, or Accumulated Cyclone Energy, which combines the intensity and longevity of a cyclone into one statistic.

ACE is a formula that adds together the intensities of a cyclone every six-hour period throughout the total history of the cyclone. For each period of six hours, the maximum sustained wind speed is estimated, squared, and divided by ten thousand. This is the ACE figure for that six hour interval. The sum of all the intervals determines the ACE of a storm. However, there are a few extra rules associated with ACE. For one, the ACE is only taken when a cyclone is tropical (tropical meaning that the cyclone has hurricane-like characteristics). When a cyclone is extratropical or subtropical (midway between extratropical and tropical, but there is no given line between these categories) ACE is not taken. Also, when the tropical cyclone has winds less than 40 mph, i.e. a tropical depression, the ACE figure drops to zero. Most long-lived hurricanes have high ACE's, but this isn't always true. Since the statistic applies to longevity and intensity, an intense long-lived storm will have the highest ACE.

#12: Hurricane Ivan (2004) September 2-24 18.75 days (as a tropical cyclone)

Hurricane Ivan was a Cape Verde hurricane that became a tropical depression on September 2, 2004. The system became a tropical depression and then a hurricane quickly. Ivan also set the record for most southerly Atlantic major hurricane when it became a Category 3 on September 5 at 10.6 N latitude. It continued west-north-westward and became a Category 5 hurricane on September 9. Hurricane Ivan, continuing through the Caribbean and fluctuating between Category 4 and 5 once or twice, devastated the Cayman Islands, Cuba, Jamaica, and Grenada. Billions of dollars in damage and over 50 deaths occurred during this rampage, despite the fact that Hurricane Ivan did not make landfall in any of these places, but passed by them. On September 13, the storm entered the Gulf of Mexico and approached the coast of Alabama. The storm made landfall in the United States at Category 3 intensity on September 16 and became a remnant low on the 18th. Causing 54 deaths in the region, the low continued northeastward The low was disregarded while it slowly turned to the south, and emerged back into the Gulf from Florida. On September 22, the system became a tropical depression again, and the National Hurricane Center, recognizing the depression as the remnants of Ivan, called it Tropical Depression Ivan. It strengthened into a tropical storm shortly before making its second landfall in Louisiana and causing minor damage. The system dissipated on the 24th. Although the storm was only the 10th most long lived, it had the second highest ACE of any storm in the Atlantic: 70.38.

Ivan as a Category 5 in the Caribbean.

Track of Ivan. The circles denote the storm as tropical with blue the weakest and red the strongest. The triangles show the remnants of Ivan when it was not tropical.

#11 Hurricane Nine (1893) September 25-October 15 19.25 days (as a tropical cyclone)

Due to the lack of radar and other tracking instruments in 1893, it is not known whether the storm actually originated on the 25th of September. However, it is known that a Cape Verde hurricane formed in late September of this year, and very slowly made its way westward. The system quickly strengthened into a hurricane and then a major hurricane. Since names weren't introduced until 1950, the system was simply called Hurricane Nine. It took a turn north and then once again followed a westerly path as a Category 3 hurricane through the Bahamas. On October 13, the hurricane struck Myrtle Beach, South Carolina, and accelerated northward, dissipating on the 15th. The system caused 28 deaths and had an ACE of 63.5.

Track of Nine.

#10 Hurricane Four (1926) September 2-24 19.5 days (as a tropical cyclone)

A tropical storm formed on September 2 and moved northwest very slowly for the next two weeks. During this time, the system became a Category 4 hurricane, and its movement slowed even more. Then, it turned northeast and made a tight loop. It became extratropical shortly after and dissipated on the 24th. Although the system caused no damage to land, it had the third highest ACE of any Atlantic hurricane, at 67.59.

Track of Four.

#9 Hurricane Alberto (2000) August 3-23 19.75 days (as a tropical cyclone)

The tropical depression that was to become Hurricane Alberto formed very close to the African coast on August 3. The system moved generally westward, and strengthened into a tropical storm and then a hurricane. However, on August 9, the system experienced wind sheer, and weakened to a tropical storm, and turned northeast. After moving out of the influence of the wind sheer, the system strengthened to a hurricane and then reached its peak intensity of 125 mph and 950 millibars on August 12. Then, due to the influence of a subtropical ridge, Alberto began a large loop in the north-Central Atlantic, and once again weakened to a tropical storm on the 14th of August. Then, it began to turn south, and then west again, before becoming a hurricane for the third time on August 18. It became a Category 2 hurricane again, and reached a secondary peak intensity of 105 mph winds Then, Alberto finally turned northward once again, starting slow, and then accelerating, and finally becoming extratropical late on August 23. Although no damage resulted from Alberto, it was notable for its odd track, and it had an ACE of 36.9, which is fairly low for Alberto's longevity, but since it never went beyond Category 3, it wasn't intense for much of its lifetime.

Hurricane Alberto at its primary peak intensity as a major hurricane.

Track of Alberto.

The top ten is continued on Notable Hurricanes: Longevity; Part 2.

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