Met Office College – Course Notes
Tropical Meteorology – Tropical cyclones
Contents
1
Introduction
2
Classification
3
Formation Requirements
4
Life-cycle
5
Structure and Characteristics
6
Surface Pressure
7
Wind
8
Cloud and Precipitation
9
Sea and Swell
10 Movement
11 Occurrence
12 Forecasting by Centres
13 The Ship-borne Forecaster
14 Avoidance Rules
15 Tropical Cyclone Warnings
16 Acknowledgements
17 Further reading
 Crown Copyright. Permission to quote from this document must be obtained from The Principal, Met
Office College, FitzRoy Road, Exeter, Devon, EX1 3PB
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1 Introduction
The tropical cyclone is the most fearsome and powerful of all
meteorological phenomena. The destructive potential is vast but,
fortunately, they are well documented and ships heeding the
frequent warnings issued by appropriate centres should rarely be
troubled. However, because the initial disturbances can develop
rapidly, the forecaster must remain alert to the danger signs.
In the past tropical cyclones have been known by various general
names such as ‘tropical revolving storms’, ‘severe cyclonic
storms’ and tropical storms’ the latter being in particular common
usage today. However, because tropical storm is also a term used
for a discrete stage in its life-cycle (see para 4 below), the
generic name tropical cyclone will be used exclusively in this
handout.
2 Classification
Tropical cyclones are categorised by their intensity measured in
terms of the highest sustained winds near their centres. The
phrase ‘highest sustained winds’ equates to the mean maximum over
a 10 minute period in the case of most Warning Centres, but
American Centres use a one minute period. As a general rule of
thumb, reducing the American wind by 10% will give a reasonable
approximation of the 10 minute highest sustained wind.
Although some warning centres use their own terminology,
nomenclature as laid down by the World Meteorological Organisation
(WMO) is followed by most i.e.
a. TROPICAL DEPRESSION (TD): Highest sustained winds of at least
22 knots but less than 34 knots (i.e. Beaufort force 6 or 7).
b. TROPICAL STORM (TS): Highest sustained winds from 34 to 63
knots. However, this category may be subdivided further:
MODERATE TROPICAL STORM: Highest sustained winds from 34 to 47
knots ie. Beaufort force 8 or 9. Centres using this category
may drop the term ‘MODERATE’.
SEVERE TROPICAL STORM (STS): Highest sustained winds from 48 to
63 knots i.e. Beaufort force 10 or 11.
The Americans do not use this subdivision in the Pacific. ie.
their Tropical Storm embraces Beaufort forces 8 to 11.
c. HURRICANE (H) or TYPHOON (T) or CYCLONE (C): Highest sustained
winds of 64 knots or more i.e. Beaufort force 12 or more.
HURRICANE is used in the Atlantic and in the Pacific east of
longitude 180. TYPHOON is used in the Pacific west of 180 and
in the China Seas. CYCLONE is used in the Indian Ocean and by
Australian warning authorities.
The Americans (and Australians) further categorise tropical
cyclones with names once they have achieved tropical storm status.
Prior to December 1979 only female names were used but
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subsequently the Americans, no doubt bowing to sexist accusations,
have used male and female names altering in alphabetical order.
The WMO have also introduced a numbering system which is widely
adopted eg. STS WINONA (8513) means that Severe Tropical Storm
Winona was the 13th tropical cyclone to affect the region in 1985.
The minimum criteria for a hurricane is 64 knots but considerably
greater strengths can be achieved with highest sustained winds of
more than 200 knots not unknown and more than 150 knots fairly
common in the more intense typhoons of the western Pacific. It
should also be noted that gusts can be considerably greater than
the highest sustained winds, up to 1.5 times greater over the sea
and even as high as 2 times over land. There seems to be no
universal terminology for severe hurricanes although the Americans
may use Major Hurricane for tropical cyclones with sustained winds
of l00knots or more and, in the western Pacific, a Super Typhoon
signifies sustained winds in excess of 130 knots.
3 Formation Requirements
Unlike depressions at higher latitudes, tropical cyclones are warm
core features which depend on latent heat of condensation to fuel
and perpetuate their existence. They thus require the vast
quantities of water vapour only found over warm tropical oceans
and are unable to develop overland. The requirements for their
formation may be summarised as follows:
a. A PRE-EXISTING LOW PRESSURE AREA Since the minimum criteria for
a tropical depression is 22 knots it follows that a low born of
some tropical disturbance must already be present. The
disturbance may be found within the monsoon trough or
associated with tropical waves or TUTT vortices extending to
the surface. Such disturbances are commonplace in summer but
fortunately only about 10% are likely to develop into tropical
cyclones.
b. SEA SURFACE TEMPERATURE OF AT LEAST 26C This provides
sufficient water vapour to produce the warm core during the
condensation process. The warmer the sea, the more conducive it
is to tropical cyclogenesis which helps explain why summer is
the Hurricane Season (along with the disturbances also
prevalent at this time of year). Although empirical, the 26C
value has proved so critical that even developed tropical
cyclones will weaken if they move over waters cooler than this.
c. A MINIMUM VALUE OF LATITUDE OF 5 DEGREES Tropical cyclones
require enough coriolis force to produce a cyclonic spiralling
circulation. This means they cannot form equatorward of 5
degrees and are rare equatorward of 8 degrees particularly in
the Atlantic. So, even though a cyclonic disturbance with
enhanced convective activity can form close to the equator, it
will not develop into a tropical cyclone unless it moves
poleward to the required amount of latitude.
d. UPPER LEVEL DIVERGENCE Without outflow aloft the low level
inflow would create a build up of air resulting ultimately in
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rising pressure. Upper level divergence acts rather like an
extractor fan, removing the rising air from the top of the
tropical cyclone ‘chimney’. A Tropical Depression or Storm
passing under a ridge of high pressure at the 200 hPa level
will often develop to Hurricane/Typhoon status.
e. LITTLE OR NO VERTICAL WIND SHEAR Significant shear prohibits
the formation of the warm core thus preventing development. The
critical value is thought to be about 20 knots between 950 and
200 hPa. If there are westerly winds below the 300 hPa level,
wind shear would probably prevent tropical cyclone formation
i.e. deep easterly component winds extending to at least
30000ft are necessary for development and such conditions only
occur in the summer months.
4 Life-cycle
Assuming the requirements for formation are present, the lifecycle of a tropical cyclone from the incipient disturbance to the
dissipating phase may be described in 4 stages:
a. FORMATIVE This represents the transition from disturbance to
Tropical Depression (TD). Surface pressure begins falling,
though remaining above 1000 hPa and winds increase near the
centre. The strongest winds are normally found on the poleward
side between the centre and the sub-tropical high, but they
will still be less than gale force during this stage.
b. INTENSIFICATION This is the period of major deepening to
Tropical Storm (TS) or Hurricane strength with pressures
rapidly falling below 1000 hPa, possibly to as low as 900 hPa
or even less. This stage may take several days or occur
explosively within 24 hours depending on the conditions
prevailing at the time. The tropical cyclone becomes clearly
evident in its pressure and wind fields both horizontally and
vertically and it is during this stage that the eye develops.
The maximum speed annulus actually shrinks during
intensification and with large systems the ring of highest
winds may contract from a diameter of hundreds of miles to 60
nm or less. However, the winds within this ring continue to
increase and the region of strong winds beyond also expands
slowly during the intensification stage. Convective clouds
increase in amount and vertical extent and form a band-like
structure. Showers and squalls increase and rain becomes
widespread near the centre.
c. MATURE This stage is the period when the tropical cyclone
remains at or near its maximum intensity although this may vary
greatly from storm to storm. Some mature as small intense
systems while others develop into the most powerful of typhoons
or hurricanes. The mature stage can last from a few hours to a
week or more during which time the central pressure remains
roughly constant, although it may fluctuate in the persistent
storms if they interact with upper troughs extending into the
tropics from higher latitudes.
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d. DECAYING The decaying stage represents the period when the
system either fills and loses its identity, or it assumes
extra-tropical characteristics. Tropical cyclones moving inland
will fill rapidly as their energy supply is cut off from below,
particularly if the terrain is rough. Moving over seas colder
than 26 deg C will also promote decay as will a low level feed
of cold air such as incursion of the winter monsoon from Asia
into the system’s circulation (it should be noted, however,
that cold bursts of polar air can have the reverse effect,
creating intense cyclogenesis in storms which have become
extra-tropical). When tropical cyclones recurve into higher
latitudes they generally assume extra-tropical characteristics
i.e. polar air is drawn into the system causing the ‘eye’ and
tight band of maximum winds to become less defined.
5 Structure and Characteristics
Figure 1 shows a schematic cross-section of a mature tropical
cyclone. The right hand side shows the convection increasing
towards the centre becoming most marked at what is termed the ‘eye
wall’. The left hand side depicts the common occurrence where
convection is also concentrated in a weather band some distance
from the wall with an interruption of activity between the two,
often called the ‘annular zone’. The ‘inflow layer’ comprises the
bottom 3km of the storm, most of the inflow occurring below one
km. Outflow occurs at the top of the storm with subsidence at the
outer limit and in the annular zone. The ‘outflow layer’ extends
from about 8km to the top of the storm, the maximum outflow in
mature storms occurring near 12km. Some descent also takes place
within the eye, helping to maintain the raised temperature in the
mid-tropospheric levels and causing it to be relatively cloud
free.
Figure 1. Schematic cross section of vertical motion, clouds, and
pressure for the idealised hurricane/typhoon.
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6 Surface Pressure
Mature storms have very low central pressures dropping below 900
hPa at times although 920 to 930 hPa are more typical and some
quite intense hurricanes may have central pressures around 950
hPa. A barograph trace of a tropical cyclone passing close to the
observation point is shown at Figure 2. Note the diurnal variation
prior to the main pressure fall which takes place over a period
less than 12 hours. Pressure falls prior to this were not
particularly significant, thus demonstrating the limited value of
pressure tendency as a long term forecasting aid. Once pressure
falls are noticed it is likely that the ship or station is already
well within the tropical cyclone circulation.
Figure 2. Barograph trace during the passage of a severe cyclone
in the western South Indian Ocean, 1959.
7 Wind
The low level wind-field surrounding tropical cyclones has 3
distinct regions:
a. The outer region extending from the periphery to the ring of
maximum winds. The radius of gale force winds tends to be
greater on the poleward side in the stronger pressure gradient
between the tropical cyclone and the subtropical high. Also as
the storm itself moves poleward, the radius of gales increases
from typically 100 to 200 nm at latitudes 20 or less to double
these values at latitude 30 to 35.
b. The maximum wind belt surrounds the eye peaking within the
inner margin of the eye-wall cloud. The width of the band is of
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the order of 5 to 10 nm but the radius of hurricane force winds
may lie between 20 and 80 nm.
c. The eye is the region of lightest winds decreasing rapidly from
the wall cloud to the centre. The radius of the eye can vary
from 10 nm or less, to 40 nm in the very large tropical
cyclones.
Vector addition of the tropical cyclone’s own speed of movement to
the winds means that they are normally strongest in the right hand
semicircle (NH), relative to the direction of movement, with
localised maxima in the right rear quadrant. Figure 3 illustrates
this asymmetry of the wind field and is based on mean data derived
from reconnaissance flights into 13 mature Pacific tropical
cyclones.
Figure 3. Total wind speed (knots) near 1000 feet. Arrow indicates
direction of storm movement.
8 Cloud and Precipitation
The basic structure has already been seen in Figure 1. Because of
the subsidence at the periphery the precursor of the close
approach of the tropical cyclone is unusually warm and fine
weather. The first cloud signs would be thin cirrus gradually
thickening and lowering until the bands of cumulus cloud become
evident. These too increase in vertical extent as the system draws
nearer. The eye wall represents the densest cloud with tops
extending to 15km or more and is accompanied by very heavy
precipitation and rapid pressure falls. The eye is often cloud
free apart from some patchy low stratus and perhaps some upper
cloud. The intense precipitation in the wall and outer bands of
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cloud (sometimes called feeder bands) greatly reduce visibility
and lower cloud base although the hurricane force winds reduce the
visibility even more from blowing spray. The banded structure
shows well on radar. To observe such a structure, radar
wavelengths need to be of the order of 100 metres. Lesser
wavelengths would be severely attenuated and the old fashioned 3cm
weather radar would probably not penetrate the first band with
sufficient energy to produce detectable returns from subsequent
bands.
At sea, wind and sea states are the prime hazards, but inland and
at coastal stations the main killers are landslips and flooding
due to excessive precipitation and abnormal rises in sea-level.
The accumulated rainfall from the passage of a tropical cyclone
can be staggering. As much as 4000 mm (100 inches) has fallen from
a single typhoon in the Philippines and amounts of more than 250
mm (10 inches) over a 12 hour period at a single station are not
uncommon. Intense rain bands may supply over 100 mm (4 inches) in
one hour. To put these figures in perspective London’s average
annual rainfall is about 600 mm (24 inches).
Figure 4. Representation of swell emanating from a tropical
cyclone.
9 Sea and Swell
Despite limited fetches in the maximum wind zones, the very high
speeds still manage to produce phenomenal wave heights in excess
of 15m, particularly in the right hand semicircle (NH) where winds
are strongest. Constructive interference may even produce the odd
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wave in excess of 30m. Obviously such seas can inflict severe
damage and are capable of capsizing ships even of frigate or
destroyer size. In fact, it is thought that typhoons inflicted
more damage to the US Pacific Fleet during WW2 than the entire
Japanese Navy. One particular typhoon, encountered by the US 3rd
Fleet, caused 3 destroyers to be capsized and lost, 146 aircraft
blown overboard or damaged beyond repair, 13 other vessels
requiring major repairs and 9 others minor repairs.
At low latitudes tropical cyclones move at speeds of the order of
10 knots or less. It follows, therefore, that waves will move out
of the storm area as swell ahead of the system. The waves
propagate radially outwards as shown in Figure 4 with the largest
ones, generated in the right rear quadrant, moving out almost
parallel with the storm track. Thus, swell is often a forerunner
of the tropical cyclone. For coastal stations there is the
additional hazard of storm surges, sometimes called hurricane
tides. As the tropical cyclone approaches the land, the winds in
the right hand semicircle (NH) will pile up water along the
coastline to the right of the storm track. If this occurs at the
top of the normal astronomical tide, abnormal rises in the sea
level will occur, particularly if the water is piling up in
narrowing inlets. The resulting flooding in low lying areas can be
catastrophic. In one particular typhoon in Hong Kong in 1937,
11000 people were killed, largely by drowning in such a surge
situation.
10 Movement
As a broad generalisation, tropical cyclones form on the
equatorward side of the sub-tropical highs and then move in the
anticyclonic flow. They thus move westward in the early stages,
normally at about 10 knots. Depending on the configuration of the
high the tropical cyclone may continue its westward movement,
usually with a poleward component, or recurve round its western
periphery. Figure 5 shows generalised mean tracks around the world
but it must be emphasised that individual tracks can be highly
erratic.
Figure 5. Generalised mean tracks of tropical storms for the
various development areas.
The strength of the steering currents depends on the intensity of
the sub-tropical high. When these currents are very weak the
tropical cyclones become slow moving or almost stationary. In this
event there is a tendency, due to the tropical cyclones own
internal forces, for a poleward drift.
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The presence of another tropical cyclone within about 800 nm can
create interaction between the two where they dumb-bell around
each other cyclonically. This is known as the ‘Fujiwhara Effect’
in the Pacific where the higher occurrence of tropical cyclones
makes such interaction a not unusual happening. The interaction is
complex and its extent is dependent on the strength of the
steering currents so it is not really feasible for the ship borne
forecaster to make accurate allowances for it.
As previously stated, tropical cyclones generally move at around
10 knots in the early stages but during intensification they slow
down particularly if the deepening is rapid. Recurving storms also
slow down during the actual recurvature process but then speed up
again as they engage the higher latitude westerlies and have been
known to move eastward at speeds up to 40 knots as they become
extra-tropical.
11 Occurrence
Figure 6 shows the average number of tropical cyclones per year
which reach tropical storm intensity or greater plus the
percentage of the global total. As can be seen none develop within
5 deg of the equator and certain regions, notably the S Atlantic
and SE Pacific do not experience any. Vertical wind shear
considerations alone can explain these gaps, but also the trade
wind trough does not migrate far enough from the equator to
produce embryonic disturbances with the required degree of
latitude for tropical cyclogenesis. Additionally, sea temperatures
are too cool to sustain storms in the SE Pacific and make them
rare in the central N Pacific.
Figure 6. Average annual number (and global percentage) of
tropical storms in each development area.
The greatest number occur in the NW Pacific which experiences
about 30% of the global total. Of these about two thirds are
likely to reach typhoon intensity in comparison with ~ in the NE
Pacific while in the N Atlantic about 60% reach hurricane
strength. Almost 75% of the world’s tropical cyclones occur in the
northern hemisphere.
Another interesting feature is the latitude of occurrence. In the
NW Pacific early and late season storms tend to form within 5 to
15 deg N compared to 10 to 25 deg N mid-season corresponding to
the northward migration of the monsoon trough. In the N Atlantic,
formation occurs at higher latitudes, even as high as 25 to 35 deg
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N, which reflects the fact that tropical cyclones in this region
are more likely to form from tropical waves or TUTT disturbances.
Figure 7. Average frequency of tropical cyclones (excluding
tropical depressions). Abscissae start with January in the
northern hemisphere and July south of the equator.
The seasonal variation for the oceans of the world is depicted at
Figure 7. In general tropical cyclones are most frequent in the
summer and autumn although there are some interesting departures.
Conditions are such that they can occur in any month in the NW
Pacific though they are considerably rarer in winter. Some regions
also display a double peak frequency structure. This is primarily
due to the transition of the monsoon trough across those seas
during the appropriate months, moving poleward in early summer and
equatorward later. In the Arabian Sea and Bay of Bengal vertical
wind shear also inhibits tropical cyclone formation in the midsummer months although 1 or 2 depressions can form in the extreme
northern part of the Bay of Bengal.
12 Forecasting by Centres
Forecasting the movement of tropical cyclones has improved
dramatically over the last 2 or 3 decades largely as a result of
improved input data ie. accurate positioning of the centre using
satellite imagery. However, storms are still renowned for their
erratic movement and so forecasting remains problematical and
better left to warning centres ashore which have access to large
quantities of data, not to mention the experience of their
forecasters. Such centres normally use several objective
forecasting techniques which would probably include the following:
a. Persistence: Simple extrapolation of recent past movement.
b. Climatology: The mean tracks taken by tropical cyclones in that
position at that time of year.
c. Blended Persistence and Climatology: A combination of the
methods in a and b. This may be either a simple average of the
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two, or more weight may be placed on persistence in the short
term and climatology in the long term.
d. Statistical Techniques: These use various regression equations
to facilitate selection of statistical predictors, from gridpoint fields, of such parameters as surface pressure, contour
heights at various levels, winds, temperatures, thickness
values etc.
e. Dynamic Techniques: These normally use the concept of a point
vortex being steered in the overall current after the removal
(by vector subtraction) of the influence of the tropical
cyclone circulation. This is usually done in 6 hour time steps
out to, perhaps, 72 hours for all standard levels. Such methods
may also have built-in feedback i.e. the previous 12 hour
forecast error is computed by examining the actual and old
forecast position, and then used to apply a bias correction for
subsequent forecast positions.
f. Analog Techniques: These scan historical data for similar
cyclones (within certain accepted limits) to the current one.
Forecast positions are then computed first assuming no
recurvature and then taking a recurving situation.
The various results of all these objective techniques are then
assessed by the forecaster and a track is selected whereupon a
tropical cyclone warning is issued to all readers of the
broadcast. Frequency of warnings may be 6 or even 3 hourly and
they contain information on storm intensity and location, wind
fields and forecast positions out to at least 48 hours.
Initial detection and location is normally by satellite imagery.
Further monitoring and analysis may be carried out by radar
networks and aircraft reconnaissance. The USAF Weather
Reconnaissance Squadrons use C 130s (Hercules) to enter tropical
cyclones at the 700 hPa level keeping the wind on the port quarter
to facilitate a spiralling penetration of the eye where dropsondes are released. Specialised techniques of analysis of
satellite pictures, particularly enhanced JR imagery, also enable
estimation of central pressure, wind strength and likely future
development.
13 The Ship-borne Forecaster
Upon receipt of a warning the METOC or other responsible officer
should immediately plot the position and forecast tracks,
regularly updating it, thereafter, on receipt of subsequent
warnings. He must, of course, keep the Command informed at all
times and be ready to advise on the best way of avoiding the
storm. Occasionally there may be warnings from more than one
centre and it is not unusual for these to be conflicting thus
requiring judgement on which is likely to be most reliable. It is
best to plot the information from different centres separately and
probably prudent to consider the most threatening tracks. If the
forecasts differ greatly then the steering currents are probably
weak and none is likely to be too reliable. As a general rule,
more confidence can be placed on forecast positions when the storm
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has definite movement, say of 8 knots or more, than when it is
slow moving. Naturally the short term forecasts will be more
reliable than the longer term ones.
With the continuous satellite coverage and a highly efficient
warning network it is most unlikely that ships would ever be
caught out by a tropical cyclone in modern times. Thus,
observation techniques are not as crucial as they once were,
although it would be prudent to remain alert to the warning signs
of an approaching storm:
a. A surface pressure 5 hPa or more below normal or a 24 hour
change of 4 hPa or more - but bear in mind comments on surface
pressure (Section 6).
b. An unusual wind direction after accounting for land/sea breezes
or other topographical effects.
c. Wind speeds 25% or more above normal, particularly if the flow
curves cyclonically.
d. Long period swells especially from an unusual direction
(Moderate NE Monsoon swell is common in the S China Sea and any
departure from this should be viewed with suspicion).
e. A solid overcast of Cirrostratus thickening and lowering
particularly if observed at several adjacent reporting
stations.
14 Avoidance Rules
If appropriate heed is paid to warnings, then ships are unlikely
to be caught in the circulation of a tropical cyclone. However,
there are avoidance rules for ships in such a situation, in the
Mariners Handbook and the Admiralty Manual of Navigation. The
rules relate to the sectors of a tropical cyclone relative to its
track as shown in Figure 8.
Figure 8. Tropical Cyclone Quadrant/Semicircle Nomenclature
(Northern Hemisphere).
The dangerous semicircle is the right hand one in the northern
hemisphere (left hand in the SH) and is so named because it has
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the strongest winds and highest seas. The ‘safer’ half is called
the navigable semicircle. The dangerous quadrant is the front
right (NH) and is to be avoided if at all possible. Here any ship
unable to make steerage way would be drawn into the path of the
storm and if there is going to be any deviation from the forecast
track, then the storm is more likely to curve towards the
dangerous quadrant.
Basically the rules are:
a. Ensure there is plenty of sea room to avoid being blown
aground. This is particularly important and will require early
decision making if the ship is in coastal waters that have no
hurricane or typhoon havens.
b. If possible, get on the safe side of the storm to avoid danger
from recurvature.
c. If caught in the dangerous semicircle, place the wind on the
starboard bow and keep it there.
d. If caught in the navigable semicircle place and maintain the
wind on the starboard quarter.
15 Tropical Cyclone Warnings
Examples of operational messages are shown at Annex A.
16 Acknowledgements
These notes form part of the material included in a Tropical
meteorology course developed at the Royal Navy School of
Meteorology and Oceanography (RNSOMO). The College is grateful for
permission to reproduce and use the material.
17 Further reading
A Synoptic Meteorology of the Tropical Oceans, Director of Naval
Oceanography and Meteorology Hydrographic Department, Ministry of
Defence, prepared by Captain T A Marshall BSc RN (retd.).
Atkinson. Forecaster’s Guide to Tropical Meteorology, Technical
Report 240.
Herbert Riehl 1954. Tropical Meteorology. McGraw-Hill
Herbert Riehl 1979. Climate and Weather in the Tropics. Academic
Press.
Maurice A. Garbell. Tropical and Equatorial Meteorology. Pitman
Publishing Corporation.
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Annex A
EXAMPLES OF TROPICAL CYCLONE WARNINGS
MIAMI WARNING
HURRICANE CLAUDETTE WARNING NO.23
1. HURRICANE CENTRE LOCATED NEAR 36.5N4 35.0W8 OR 380 NM ESE OF
LAJES, AZORES AT 152200Z.
POSITION ACCURATE WITHIN 40 MILES BASED ON SATELLITE.
PRESENT MOVEMENT TOWARDS THE EAST OR 085 DEGS AT 2OKT.
A.
MAXIMUM SUSTAINED WINDS 65 KT WITH GUSTS TO 80 KTS.
B.
RADIUS OF 64 KT WINDS 50NE 755E 75SW 5ONW.
C.
RADIUS OF 50 KT WINDS 75NE 100SE 100SW 75NW.
D.
RADIUS OF 34 KT WINDS 100NE 1755E 175SW 100NW.
E.
REPEAT CENTER LOCATED AT 36.5N4 35.0W8 AT 152200Z.
2.
FORECAST VALID 160600Z. 37.ONO 32.0W5.
A.
MAXIMUM SUSTAINED WINDS 55 KTS WITH GUSTS TO 70 KTS.
B.
RADIUS OF 50 KT WINDS 75NE 100SE 100SW 75NW.
C.
RADIUS OF 34 KT WINDS 100NE 150SE 150SW 100NW.
3.
FORECAST VALID 161800Z. 39.0N2 26.0W8.
A.
MAXIMUM SUSTAINED WINDS 45 KTS NEAR CENTER
B.
RADIUS OF 334 KT WINDS 100NE 150SE 150SW 100NW.
4.
OUTLOOK VALID 171800Z. EXTRATROPICAL.
5.
REQUEST 3 HOURLY SHIP REPORTS WITHIN 300NM OF 36.5N4 35.0W8.
6.
NEXT WARNING 160400Z.
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HONG KONG WARNING
FROM ROHK
TO HMS ARK ROYAL
TROPICAL CYCLONE WARNING FOR SEVERE TROPICAL STORM TIP ISSUED AT
130120Z JUL
AT 130000Z SEVERE TROPICAL STORM TIP (8314) HAD BEEN DOWNGRADED TO
A TROPICAL STORM WITH CENTRAL PRESSURE 994 MBS AND WAS CENTRED
WITHIN 30 N MILES OF ONE NINE POINT TWO (19.2) DEGREES NORTH, ONE
ONE ONE POINT FOUR (111.4) DEGREES EAST.
TROPICAL STORM TIP IS FORECAST TO MOVE NORTHWEST AT ABOUT 10 KNOTS
AND GRADUALLY WEAKEN.
MAXIMUM SUSTAINED WINDS NEAR THE CENTRE ARE ESTIMATED AT 40 KNOTS.
RADIUS OF 34 KNOTS OR GREATER IS 100 N MILES.
24 HOUR FORECAST POSITION FOR 140000Z 22 NORTH 109 EAST
48 HOUR FORECAST POSITION FOR 150000Z DISSIPATED OVERLAND
BT
FROM ROHK
TO HMS ARK ROYAL
TROPICAL CYCLONE WARNING FOR TROPICAL STORM VERA ISSUED 130125Z.
AT 130000Z TROPICAL STORM VERA (8315) HAD INTENSIFIED TO A SEVERE
TROPICAL STORM WITH CENTRAL PRESSURE 986 MBS AND WAS CENTRED
WITHIN 60 N MILES OF ONE ONE POINT EIGHT (11.8) DEGREES NORTH, ONE
TWO EIGHT POINT FOUR (128.4) DEGREES EAST.
SEVERE TROPICAL STORM VERA IS FORECAST TO MOVE WEST AT ABOUT 10
KNOTS AND INTENSIFY FURTHER
MAXIMUM SUSTAINED WINDS NEAR THE CENTRE ARE ESTIMATED AT 50 KNOTS
RADIUS OF 47 KNOTS OR GREATER 1560 N MILES
RADIUS OF 34 KNOTS OR GREATER IS 180 N MILES
24 HOUR FORECAST POSITION FOR 140000Z 12 NORTH 125 EAST
48 HOUR FORECAST POSITION FOR 150000Z 13 NORTH 120 EAST
BT
Page 16 of 16
Last saved date: 12 February 2016
File: ms-train-college-work-d:\106739167.doc