Components of the index
The Hurricane Severity Index is a 50-point scale, with wind intensity and size contributing equally.
Determining size points
| Wind Radii | Size Point Range |
|---|---|
| 35 kn | 1–3 |
| 50 kn | 1–4 |
| 65 kn | 1–8 |
| 87 kn | 1–10 |
The Hurricane Severity Index (or HSI) measures the strength and destructive capability of a storm based on its size and wind intensity. [1] The HSI attempts to demonstrate that two hurricanes of similar intensity may have different destructive capability due to variances in size, and furthermore that a less intense, but very large hurricane, may in fact be more destructive than a smaller, more intense hurricane. It is very similar to the Chicago Mercantile Exchange Hurricane Index, which also factors both size and intensity of a hurricane. [2] HSI was developed by a private company program in competition with the National Weather Service's accumulated cyclone energy index.
The Hurricane Severity Index is a 50-point scale, with wind intensity and size contributing equally.
| Wind Radii | Size Point Range |
|---|---|
| 35 kn | 1–3 |
| 50 kn | 1–4 |
| 65 kn | 1–8 |
| 87 kn | 1–10 |
In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale.
Tropical cyclone warnings and watches are alerts issued by national weather forecasting bodies to coastal areas threatened by the imminent approach of a tropical cyclone of tropical storm or hurricane intensity. They are notices to the local population and civil authorities to make appropriate preparation for the cyclone, including evacuation of vulnerable areas where necessary. It is important that interests throughout the area of an alert make preparations to protect life and property, and do not disregard it on the strength of the detailed forecast track.
Typhoon Tip, known in the Philippines as Super Typhoon Warling, was an exceptionally large, extremely powerful, and long-lived tropical cyclone that traversed the Western Pacific for 20 days, shattering multiple records worldwide. The forty-third tropical depression, nineteenth tropical storm, twelfth typhoon, and third super typhoon of the 1979 Pacific typhoon season, Tip developed out of a disturbance within the monsoon trough on October 4 near Pohnpei in Micronesia. Initially, Tropical Storm Roger to the northwest hindered the development and motion of Tip, though after the storm tracked farther north, Tip was able to intensify. After passing Guam, Tip rapidly intensified and reached peak sustained winds of 305 km/h (190 mph) and a worldwide record-low sea-level pressure of 870 hPa (25.69 inHg) on October 12. At its peak intensity, Tip was the largest tropical cyclone on record, with a wind diameter of 2,220 km (1,380 mi). Tip slowly weakened as it continued west-northwestward and later turned to the northeast, in response to an approaching trough. The typhoon made landfall in southern Japan on October 19, and became an extratropical cyclone shortly thereafter. Tip's extratropical remnants continued moving east-northeastward, until they dissipated near the Aleutian Islands on October 24.
Accumulated cyclone energy (ACE) is a metric used to compare overall activity of tropical cyclones, utilizing the available records of windspeeds at six-hour intervals to synthesize storm duration and strength into a single index value. The ACE index may refer to a single storm or to groups of storms such as those within a particular month, a full season or combined seasons. It is calculated by summing the square of tropical cyclones' maximum sustained winds, as recorded every six hours, but only for windspeeds of at least tropical storm strength ; the resulting figure is divided by 10,000 to place it on a more manageable scale.
Tropical cyclones are ranked on one of five tropical cyclone intensity scales, according to their maximum sustained winds and which tropical cyclone basins they are located in. Only a few classifications are used officially by the meteorological agencies monitoring the tropical cyclones, but other scales also exist, such as accumulated cyclone energy, the Power Dissipation Index, the Integrated Kinetic Energy Index, and the Hurricane Severity Index.
An Atlantic hurricane is a type of tropical cyclone that forms in the Atlantic Ocean primarily between June and November. The terms "hurricane", "typhoon", and "cyclone" can be used interchangeably to describe this weather phenomenon. These storms are continuously rotating around a low pressure center, which causes stormy weather across a large area, which is not limited to just the eye of the storm. They are organized systems of clouds and thunderstorms that originate over tropical or subtropical waters and have closed low-level circulation, and should not be confused with tornadoes, which are just another type of cyclone. They form over low pressure systems. In the North Atlantic and the Eastern Pacific, the term "hurricane" is used, whereas "typhoon" is used in the Western Pacific near Asia. The more general term "cyclone" is used in the rest of the ocean basins, namely the South Pacific and Indian Ocean.
The eye is a region of mostly calm weather at the center of a tropical cyclone. The eye of a storm is a roughly circular area, typically 30–65 kilometers in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather and highest winds of the cyclone occur. The cyclone's lowest barometric pressure occurs in the eye and can be as much as 15 percent lower than the pressure outside the storm.
The central dense overcast, or CDO, of a tropical cyclone or strong subtropical cyclone is the large central area of thunderstorms surrounding its circulation center, caused by the formation of its eyewall. It can be round, angular, oval, or irregular in shape. This feature shows up in tropical cyclones of tropical storm or hurricane strength. How far the center is embedded within the CDO, and the temperature difference between the cloud tops within the CDO and the cyclone's eye, can help determine a tropical cyclone's intensity with the Dvorak technique. Locating the center within the CDO can be a problem with strong tropical storms and minimal hurricanes as its location can be obscured by the CDO's high cloud canopy. This center location problem can be resolved through the use of microwave satellite imagery.
Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occur are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.
A tropical cyclone is a rapidly rotating storm system with a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its location and strength, a tropical cyclone is called a hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a strong tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean. A typhoon occurs in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific, comparable storms are referred to as "tropical cyclones". In modern times, on average around 80 to 90 named tropical cyclones form each year around the world, over half of which develop hurricane-force winds of 65 kn or more.
The effects of tropical cyclones include heavy rain, strong wind, large storm surges near landfall, and tornadoes. The destruction from a tropical cyclone, such as a hurricane or tropical storm, depends mainly on its intensity, its size, and its location. Tropical cyclones remove forest canopy as well as change the landscape near coastal areas, by moving and reshaping sand dunes and causing extensive erosion along the coast. Even well inland, heavy rainfall can lead to landslides in mountainous areas. Their effects can be sensed over time by studying the concentration of the Oxygen-18 isotope within caves.
The radius of maximum wind (RMW) is the distance between the center of a cyclone and its band of strongest winds. It is a parameter in atmospheric dynamics and tropical cyclone forecasting. The highest rainfall rates occur near the RMW of tropical cyclones. The extent of a cyclone's storm surge and its maximum potential intensity can be determined using the RMW. As maximum sustained winds increase, the RMW decreases. Recently, RMW has been used in descriptions of tornadoes. When designing buildings to prevent against failure from atmospheric pressure change, RMW can be used in the calculations.
The Chicago Mercantile Exchange Hurricane Index (CMEHI) is an index which describes the potential for damage from an Atlantic hurricane in the United States. The CMEHI is used as the basis for trading hurricane futures and options on the Chicago Mercantile Exchange (CME). It is very similar to the Hurricane Severity Index, which also factors both size and intensity of a hurricane.
The following outline is provided as an overview of and topical guide to tropical cyclones:
Hurricane Debbie was an intense and long-lived hurricane that formed during August 1969. The fifth tropical cyclone, fourth named storm, third hurricane and second major hurricane of the 1969 Atlantic hurricane season, Debbie formed on August 14 in the southern Atlantic Ocean and took a general northwesterly path until turning northward into the central Atlantic. The storm was characterized by numerous fluctuations in intensity, but it still reached winds corresponding to Category 3 status on the Saffir–Simpson scale. The hurricane bypassed the island of Bermuda to the southeast on August 22, before ultimately brushing southeastern Newfoundland with strong winds. It dissipated over the cold waters east of Greenland. Although Debbie had little effect on land, it was extensively researched and was subject to a weather modification experiment by Project Stormfury, in which it was seeded with silver iodide.
During 2007, tropical cyclones formed within seven different tropical cyclone basins, located within various parts of the Atlantic, Pacific and Indian Oceans. During the year, a total of 129 systems formed with 79 of these developing further and were named by the responsible warning centre. The strongest tropical cyclone of the year was Cyclone George, which was estimated to have a minimum barometric pressure of 902 hPa (26.64 inHg). The deadliest tropical cyclone of the year was Cyclone Sidr in the North Indian Ocean which killed 15,000 people in Bangladesh. The costliest tropical cyclone of the year was Cyclone Gonu in the North Indian Ocean, which caused more than $4.41 billion in damage after striking Oman, United Arab Emirates, Iran and Pakistan. The most active basin in the year was the Western Pacific, which documented 24 named systems. The North Atlantic had an above-average season with 15 named storms. The Eastern Pacific hurricane season experienced a below-average number of tropical storm intensity systems, numbering 11. Activity across the Southern Hemisphere's three basins – South-West Indian, Australian, and South Pacific – was fairly significant, with the regions recording 25 named storms altogether, with the most intense storm of the year coming from the Australian basin. Throughout 2007, twenty one major tropical cyclones formed, including five Category 5 tropical cyclones in the year. The accumulated cyclone energy (ACE) index for the 2007, as calculated by Colorado State University was 568.1 units.
The following is a glossary of tropical cyclone terms.
Throughout 2013, 139 tropical cyclones formed in seven different areas called basins. Of these, 67 have been named by various weather agencies when they attained maximum sustained winds of 35 knots. The strongest and deadliest tropical cyclone of the year was Typhoon Haiyan, which was estimated to have a minimum barometric pressure of 895 hPa (26.43 inHg) and caused at least 6,300 deaths in the Philippines. The costliest tropical cyclone of the year was Hurricane Manuel, which was responsible for at least $4.2 billion worth of damages in Mexico. 21 major tropical cyclones formed in 2013, including five Category 5 tropical cyclones. The accumulated cyclone energy (ACE) index for the 2013, as calculated by Colorado State University was 618.5 units.
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