Radius of outermost closed isobar

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Size descriptions of tropical cyclones
Less than 2 degrees latitude Very small/midget
2 to 3 degrees of latitude Small
3 to 6 degrees of latitude Medium/Average
6 to 8 degrees of latitude Large
Over 8 degrees of latitude Very large [1]

The radius of outermost closed isobar (ROCI) is one of the quantities used to determine the size of a tropical cyclone. It is determined by measuring the radii from the center of the storm to its outermost closed isobar in four quadrants, which is then averaged to come up with a scalar value. It generally delimits the outermost extent of a tropical cyclone's wind circulation. [2]

Tropical cyclone Is a rotating storm system

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".

A scalar or scalar quantity in physics is a physical quantity that can be described by a single element of a number field such as a real number, often accompanied by units of measurement. A scalar is usually said to be a physical quantity that only has magnitude and no other characteristics. This is in contrast to vectors, tensors, etc. which are described by several numbers that characterize their magnitude, direction, and so on.


Use of this measure has objectively determined that tropical cyclones in the northwest Pacific Ocean are the largest on earth on average, with North Atlantic tropical cyclones roughly half their size. [3] Active databases of ROCI are maintained by the National Hurricane Center for systems tracked in the eastern north Pacific and north Atlantic basins.

An active database is a database that includes an event-driven architecture which can respond to conditions both inside and outside the database. Possible uses include security monitoring, alerting, statistics gathering and authorization.

National Hurricane Center division of the United States National Weather Service

The National Hurricane Center (NHC) is the division of the United States' National Weather Service responsible for tracking and predicting tropical weather systems between the Prime Meridian and the 140th meridian west poleward to the 30th parallel north in the northeast Pacific Ocean and the 31st parallel north in the northern Atlantic Ocean. The agency, which is co-located with the Miami branch of the National Weather Service, is situated on the campus of Florida International University in University Park, Florida.


Hurricane Sandy at peak intensity. Sandy Oct 25 2012 0400Z.JPG
Hurricane Sandy at peak intensity.

Active databases of ROCI are maintained by the National Hurricane Center for systems tracked in the eastern north Pacific and north Atlantic basins, within a database known as the Extended Best Track Database. The values are determined in real-time every six hours. The eastern north Pacific database runs from 2001 to present, while the north Atlantic database runs from 1988 to present. [4] Other than these official databases, a global once-daily dataset was compiled for a 1984 research paper, which covered global tropical cyclones between 1957 and 1977. [3] Previously, a database was created to determine ROCI values for the western north Pacific Ocean in 1972, using data from 1945 to 1968. [5] Another database with additional ROCI information is currently being modified at the Hydrometeorological Prediction Center for use in matching ongoing tropical cyclones to past systems for the purposes of finding rainfall analogs to an ongoing event, [6] which has fairly continuous data running back to 1959 for the north Atlantic ocean. [7]


Tropical cyclones tend to be smaller during the mid-summer, and largest in October in the Northern Hemisphere. As tropical cyclones initially develop, the size of their ROCI initially contracts. [8] Once tropical cyclones reach maturity, their isobaric patterns increase in size. During the mature stage, the broadening pressure pattern leads to some reduction in their maximum sustained wind, but the extent of their tropical storm and hurricane-force winds is the most extensive within the storm's life cycle. [9] An increase in size is also noted as a tropical cyclone gains latitude. [8] As tropical cyclones weaken, their ROCI values diminish. In general, the size of a tropical cyclone shows little relation to its intensity. Use of this measure has objectively determined that tropical cyclones in the northwest Pacific Ocean are the largest on earth on average, with North Atlantic tropical cyclones roughly half their size. [3]

Northern Hemisphere half of Earth that is north of the equator

The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's North Pole.

The maximum sustained wind associated with a tropical cyclone is a common indicator of the intensity of the storm. Within a mature tropical cyclone, it is found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are determined via their sampling and averaging the sampled results over a period of time. Wind measuring has been standardized globally to reflect the winds at 10 metres (33 ft) above the Earth's surface, and the maximum sustained wind represents the highest average wind over either a one-minute (US) or ten-minute time span, anywhere within the tropical cyclone. Surface winds are highly variable due to friction between the atmosphere and the Earth's surface, as well as near hills and mountains over land.

See also

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  1. Joint Typhoon Warning Center. Q: What is the average size of a tropical cyclone? United States Navy. Retrieved on 2007-07-04.
  2. FRÉDÉRIC MOUTON and OLA NORDBECK. Cyclone Database Manager. Archived 2011-07-18 at the Wayback Machine . Retrieved on 2008-07-14.
  3. 1 2 3 Robert T. Merrill. A Comparison of Large and Small Tropical Cyclones. Retrieved on 2008-07-14.
  4. Mark DeMaria (2009). Atlantic Extended Best Track Database. Colorado State University. Retrieved on 2009-06-14.
  5. K. S. Liu and Johnny C. L. Chan. Size of Tropical Cyclones as Inferred from ERS-1 and ERS-2 Data. Retrieved on 2008-07-14.
  6. David M. Roth and Kyle S. Griffin (2009-06-07). Cliqr Rainfall Analog. Hydrometeorological Prediction Center. Retrieved on 2009-06-14.
  7. David M. Roth (2013). "CLIQR Database". Hydrometeorological Prediction Center . Retrieved 2013-01-04.
  8. 1 2 Edward Morgan Brooks (1945). An Analysis of an Unusual Rainfall Distribution in a Hurricane. Archived 2015-04-03 at the Wayback Machine . Massachusetts Institute of Technology. Retrieved on 2008-12-31.
  9. Gordon E. Dunn and Banner I. Miller (1960). Atlantic Hurricanes. Louisiana State University Press. p. 32. ASIN   B0006BM85S.