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5. Terms used in international exchanges in the
South-West Indian Ocean |
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A |
B |
C |
D |
E |
F |
G |
H |
I |
J | K |
L |
M |
N |
P | O |
Q | R |
S |
T |
U |
V |
W |
X | Y |
Z
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| TERM |
MEANING |
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Average/mean wind speed
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Speed of the wind averaged over the previous 10 minutes
(in the following classification, the wind speed, as it is described
in the definitions, relates either to observed winds at least
in half of the circulation near the centre, or to observed winds
within the system in nearly all of the depression circulation).
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Centre fix of the tropical cyclone
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Estimated location of the centre of a tropical disturbance.
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Centre of the tropical disturbance |
Geometric centre of the cloud eye or, if not discernible,
the wind/pressure centre.
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Confidence in the centre position |
Degree of confidence in the centre position of a
tropical disturbance expressed as the radius of
the smallest circle within which the centre
may be located by the analysis.
- Position good implies a radius
of 30 nautical miles (55 kilometres) or less.
- Position fair, a radius of
30 to 60 nautical miles (55 to 110 km) and
- Position poor, a radius of
greater than 60 nautical miles (110 km).
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Cyclone advisory |
Priority message for exchanging information between national meteorological services or meteorological offices concerning tropical disturbances or incipient tropical disturbances : observational data of special significance (e.g., radar eye report), analyses (e.g. of satellite imagery) ; forecasts (e.g. of movement), warnings issued nationally.
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| Cyclone season |
Period of the year during which most of the tropical disturbances occur. In the South- West Indian Ocean, this period is between 1 November and 30 April (for Mauritius and Seychelles: this period is between 1 November and 15 May).
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Direction of movement
of the tropical disturbance
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Direction towards which the tropical
disturbance centre is moving.
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Extra-tropical disturbance |
Synoptic scale low pressure area outside of the tropics or former tropical disturbance having lost its tropical characteristics.
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Eye of the tropical disturbance |
Relatively clear and calm area inside the circular wall of
the convective clouds, the geometric centre of which is
the centre of the tropical disturbance.
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Gale warning |
Warning of mean wind speeds in the range of 34 to 47 knots
(63 to 88 km/h, force 8 or 9 in the Beaufort scale).
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Gust |
Instantaneous peak value of surface wind speed.
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Hurricane warning |
Warning of mean wind speeds of 64 knots or higher
(118 km/h or higher, force 12 in the Beaufort scale).
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Intense tropical cyclone
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Tropical disturbance in which the maximum of the average wind
speed is estimated to be in the range 90 to 115 knots
(166 to 212 km/h).
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Low pressure area
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Region of the atmosphere in which the pressures are lower than
those of the surrounding region at the same level and where the
cloud masses do not appear to be organized.
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Moderate tropical storm |
Tropical disturbance in which the maximum of the average wind
speed is estimated to be in the range 34 to 47 knots
(63 to 88 km/h, force 8 or 9 in the Beaufort scale).
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Near gale warning |
Warning of mean wind speeds in the range of 28 to 33 knots
(51 to 62 km/h, force 7 in the Beaufort scale).
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Severe tropical storm |
Tropical disturbance in which the maximum of the average wind
speed is estimated to be in the range 48 to 63 knots
(89 to 117 km/h, force 10 or 11 in the Beaufort scale).
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Speed of movement of the tropical disturbance |
Speed of movement of the centre of the tropical disturbance.
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Storm surge |
Difference between the actual water level as influenced by a meteorological disturbance (i.e., the storm tide) and the level which would have been attained in the absence of the meteorological disturbance (i.e., astronomical tide). Storm surge results from the shoreward movement of water combined with the comparatively minor effects of low barometric pressure.
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Storm tide |
Actual sea level as influenced by a weather disturbance.
The storm tide consists of the normal astronomical tide and the storm surge.
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Storm warning |
Warning of mean wind speeds in the range of 48 to 63 knots
(89 to 117 km/h, force 10 or 11 in the Beaufort scale).
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Sub-tropical disturbance |
Synoptic scale low pressure area having during its life,
characteristic which could belong to both tropical and
extra-tropical depressions. In the South West Indian Ocean,
the genesis of such system is regularly observed over the
South of Mozambique Channel.
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Sustained wind speed
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Surface wind speed averaged over the previous 1 minute
(this value is sometimes used as the mean wind speed,
particularly in Region IV).
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Tropical cyclone |
Tropical disturbance in which the maximum of the average wind
speed is estimated to be in the range 64 to 89 knots
(118 to 165 km/h, force 12 in the Beaufort scale).
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Tropical depression |
Tropical disturbance in which the maximum of the average wind
speed is estimated to be in the range 28 to 33 knots
(51 to 62 km/h, force 7 in the Beaufort scale).
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Tropical disturbance |
Generic term for a non-frontal synoptic scale low pressure area,
originating over tropical or sub-tropical waters with organized
convection and definite cyclonic surface wind circulation
(wind estimated to be not exceeding 27 knots (50 km/h,
force 6 in the Beaufort scale)).
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Tropical wave |
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 reflexion of an upper-troposphere cold
low or equatorial extension of a mid-latitude trough.
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Very intense tropical cyclone |
Tropical disturbance in which the maximum of the average wind
speed is estimated to exceed 115 knots (212 km/h).
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Weather warning |
Meteorological message issued to provide appropriate warnings
of hazardous weather conditions.
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Zone of disturbed weather |
non-frontal synoptic scale low
pressure area originating in the tropics or sub-tropics
with enhanced convection and light surface winds.
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