WORLD METEOROLOGICAL ORGANIZATION
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TCM-VI/Doc. 4.12
(12.X.2009)
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SIXTH TROPICAL CYCLONE RSMCs/TCWCs
TECHNICAL COORDINATION MEETING
ITEM 4.12
BRISBANE, AUSTRALIA
2 TO 5 NOVEMBER 2009
ENGLISH ONLY
Review of terminology/classification of tropical cyclones
(Submitted by the Secretariat)
Summary and Purpose of Document
This document provides the list of standardized terminology/classification of
tropical cyclones adopted during previous coordination meetings to bring about
better international coordination and understanding.
ACTION PROPOSED
The meeting is invited to review the list of terminologies and classifications for possible
revision, if needed, and proposals, if any, for addition to the list.
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LIST OF STANDARDIZED TERMINOLOGY
CENTRAL PRESSURE OF A TROPICAL CYCLONE – Surface pressure at the centre of the
tropical cyclone as measured or estimated.
CENTRE FIX OF THE TROPICAL CYCLONE – The estimated location of the centre of a tropical
cyclone.
CENTRE OF THE TROPICAL CYCLONE – The centre of the cloud eye or, if not discernible, the
wind/pressure centre.
CONFIDENCE IN THE CENTRE POSITION – Degree of confidence in the centre position of a
tropical cyclone expressed as the radius of the smallest circle within which the centre may be
located by the analysis.
“Position good” – implies a radius of less than 30 nautical miles (55 kilometers)
“Position fair” – implies a radius of 30 to 60 nautical miles (55 to 110 km) and
“Position poor” – implies a radius of greater than 60 nautical miles (110 km).
DIRECTION OF MOVEMENT OF THE TROPICAL CYCLONE – The direction towards which the
centre of the tropical cyclone is moving.
EXTRA-TROPICAL CYCLONE – Low –pressure system which develops in latitudes outside the
tropics.
EXTRATROPICAL TRANSITION – is an evolutionary process by which a symmetric warm-core
tropical cyclone transforms to an asymmetric cold core extratropical cyclone. This process includes
a change in the distribution of clouds, winds, and precipitation. Also, the primary energy source
changes from latent heat release in deep convective clouds of the tropical cyclone to baroclinic
conversion of available potential energy in the extratropical cyclone.
EYE OF THE TROPICAL CYCLONE – The relatively clear and calm area inside the circular wall of
convective clouds, the geometric centre of which is the centre of the tropical cyclone.
GALE FORCE WIND – maximum sustained surface wind speed of 34 to 47 knots
HURRICANE/TYPHOON FORCE WIND – maximum sustained surface wind speed of 64 knots or
more.
LOW PRESSURE AREA – Region of the atmosphere in which the pressures are lower than those
of the surrounding region at the same level.
*MAXIMUM SUSTAINED WIND SPEED –
depending upon the regional practices.
Averaging period of one, three or ten minutes
RECONNAISSANCE AIRCRAFT CENTRE FIX OF THE TROPICAL CYCLONE, VORTEX FIX –
The location of the centre of a tropical cyclone obtained by reconnaissance aircraft penetration.
SPEED OF MOVEMENT OF THE TROPICAL CYCLONE – Speed of movement of the centre of
the tropical cyclone.
STORM FORCE WIND – maximum sustained surface wind speed of 48 to 63 knots
STORM SURGE – The difference between the actual water level under influence of a
meteorological disturbance (storm tide) and the level which would have been attained in the
absence of the meteorological disturbance (i.e. astronomical tide).
STORM TIDE – The actual sea level as influenced by a weather disturbance. The storm tide
consists of the normal astronomical tide and the storm surge.
SUB-TROPICAL CYCLONE – A low pressure system, developing over sub-tropical waters which
initially contains few tropical characteristics. With time the sub-tropical cyclone can become
tropical.
TROPICAL CYCLONE – Generic term for a warm-core non-frontal synoptic scale cyclone
originating over tropical or sub-tropical waters with organized deep convection and closed cyclonic
surface wind circulation. The term is also used for a storm in the South-West Indian Ocean in
which the maximum sustained wind speed is estimated to be in the range of 64 to 90 knots and in
the South Pacific and South-East Indian Ocean with the maximum sustained surface wind speed
greater than 33 knots.
TROPICAL DEPRESSION WIND – maximum sustained surface wind speed less than 34 knots
TROPICAL DISTURBANCE – A non-frontal synoptic scale cyclone originating in the tropics or
sub-tropics with enhanced convection and light surface winds.
TROPICAL STORM WIND – maximum sustained surface wind speed of 34 to 63 knots
TROPICAL WAVE – A trough or cyclonic curvature maximum in the trade wind easterlies or
equatorial westerlies. The wave may reach maximum amplitude in the lower middle troposphere,
or may be the reflection of an upper-troposphere cold low or equatorial extension of a mid-latitude
trough.
ZONE OF DISTURBED WEATHER – A zone in which the pressure is low relative to the
surrounding region and there are convective cloud masses which are not organized.
WEATHER WARNING – Meteorological message issued to provide appropriate warnings of
hazardous weather conditions.