The radius of outermost closed isobar (ROCI) is one of the quantities used to determine the size of a

tropical cyclone 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 and squalls. Depend ...

. 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 Scalar may refer to: *Scalar (mathematics), an element of a field, which is used to define a vector space, usually the field of real numbers * Scalar (physics), a physical quantity that can be described by a single element of a number field such ...

value. It generally delimits the outermost extent of a tropical cyclone's wind circulation. 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 cyclone An Atlantic hurricane, also known as tropical storm or simply hurricane, is a tropical cyclone that forms in the Atlantic Ocean, primarily between the months of June and November. A hurricane differs from a cyclone or typhoon only on the basis of ...

s roughly half their size.Robert T. Merrill
A Comparison of Large and Small Tropical Cyclones.
Retrieved on 2008-07-14.

Active database An active database is a database that includes an event-driven architecture (often in the form of ECA rules) which can respond to conditions both inside and outside the database. Possible uses include security monitoring, alerting, statistics gat ...

s of ROCI are maintained by the

National Hurricane Center The National Hurricane Center (NHC) is the division of the United States' NOAA/National Weather Service responsible for tracking and predicting tropical weather systems between the Prime Meridian and the 140th meridian west poleward to the 3 ...

for systems tracked in the eastern north Pacific and north Atlantic basins.

Database

Active databases of ROCI are maintained by the

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. 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. Previously, a database was created to determine ROCI values for the western north Pacific Ocean in 1972, using data from 1945 to 1968. Another database with additional ROCI information is currently being modified at the

Hydrometeorological Prediction Center The Weather Prediction Center (WPC), located in College Park, Maryland, is one of nine service centers under the umbrella of the National Centers for Environmental Prediction (NCEP), a part of the National Weather Service (NWS), which in turn is p ...

for use in matching ongoing tropical cyclones to past systems for the purposes of finding rainfall analogs to an ongoing event, which has fairly continuous data running back to 1959 for the north

Atlantic ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

Variation

Tropical cyclones tend to be smaller during the mid-summer, and largest in October in the

Northern Hemisphere 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 Nort ...

. As tropical cyclones initially develop, the size of their ROCI initially contracts. 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 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. Unl ...

, but the extent of their tropical storm and hurricane-force winds is the most extensive within the storm's life cycle. An increase in size is also noted as a tropical cyclone gains latitude.Edward Morgan Brooks (1945)
An Analysis of an Unusual Rainfall Distribution in a Hurricane.

Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the ...

. Retrieved on 2008-12-31. 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 An Atlantic hurricane, also known as tropical storm or simply hurricane, is a tropical cyclone that forms in the Atlantic Ocean, primarily between the months of June and November. A hurricane differs from a cyclone or typhoon only on the basis of ...

roughly half their size.

References

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Tropical cyclone meteorology The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

Radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

Database

Variation

See also

References