tropical storms

1. Unit 2 – Tropical Revolving Systems(TRS)
•
•
Tropical Revolving Storms: Local names, seasons and
areas affected; Origin, structure, movement and
lifespan of TRS; Weather associated with TRS;
Definitions and nomenclature; Warning signs of an
approaching TRS; Characteristics of TRS; Forecasting
techniques Past and present; Action to be taken when
the presence of TRS is confirmed; Cyclone tracking
and warning bulletins under international conventions;
Practical Rules for navigation for maneuvering in the
vicinity of TRS; Avoidance of storm centers and the
dangerous quadrants; Ideal conditions for the
formation of TRS; Comparison between a TRS and a
temperate latitude depression; Avoiding TRS – 1-2-3
theory and sector theory.

2. TROPICAL REVOLVING STORMS(TRS)

4. TROPICAL CYCLONES
TROPICAL CYCLONES

5. Tropical Revolving Storm(TRS)
•
•
•
•
A tropical revolving storm is a rapidly-rotating storm
characterized by low pressure centre, strong winds
and a spiral arrangement of thunderstorms.
TRS forms over large bodies of warm water and they
develop the energy from the ocean surface.
These tropical cyclones take birth in the warm
oceanic region and move towards the continents.
TRS tends to move in counter-clockwise in Northern
hemisphere and clockwise in Southern hemisphere

6. Great Danger to Shipping
•
•
•
TRSs in the Atlantic and Pacific are generally
more violent than in the Arabian sea and the Bay
of Bengal(Wind sp reach 100 Kts).
Nevertheless, TRS are a great danger, to
shipping regardless of where they are
encountered and require a special study.
Whenever a vessel is in an area where TRSs are
likely to be encountered, careful watch should be
kept for the warning signs of an approaching
TRS and take early evasive action.

7. TROPICAL CYCLONE
DEFINED!
A warm core, non-frontal,
synoptic scale system
with cyclonically rotating
winds
A warm core, non-frontal,
synoptic scale system
with cyclonically rotating
winds Anti Clockwise in
NH & Clockwise in SH,
characterized by a rapid
decrease in pressure and
increase in winds toward
the center of the storm.
Cyclones develop over
tropical or subtropical
waters and have a
definite organized
circulation
A warm core, non-frontal,
synoptic scale system
with cyclonically rotating
winds
A warm core, non-frontal,
synoptic scale system
with cyclonically rotating
winds Anti Clockwise in
NH & Clockwise in SH,
characterized by a rapid
decrease in pressure and
increase in winds toward
the center of the storm.
Cyclones develop over
tropical or subtropical
waters and have a
definite organized
circulation.

8. Different Names – Different Areas
•
•
•
•
•
•
•
•
Tropical Revolving Storms: Tropical revolving
storms are severe low pressure systems of tropical
areas. They can be viewed as an intense vortex or
whirl in the atmosphere. They are called by different
names in different areas.
Hurricane in Atlantic Ocean
Typhoons in Pacific Ocean
Willy Willy in Australian region
Cyclonic storms in Indian region
Baguio in Philippines and
Taifu in Japan
These tropical cyclones take birth in the warm
oceanic region and move towards the continents.
Very high wind speeds are seen in association with
these low pressure systems. There are different
stages of these TRS which are given below based on
the wind speed.

9. Different Names(Local Names) in Different Areas
• Tropical revolving storms are severe low pressure
systems of tropical areas. They can be viewed as an
intense vortex or whirl in the atmosphere.
(i) Hurricane in North Atlantic (Western Side)- Jun to Nov
(ii)Typhoons or Baguios in North Pacific(Western Side)- All
the year round. Worst period From June to November
(iii)Hurricane or Cordonazo in North Pacific(Eastern Side)-
June to November
(iv)South Pacific(Western Side) Hurricane - Dec to Apr

10. Different Names in Different Areas
•
•
(v) South Indian Ocean (Eastern Side) Willy Willy in (Australian
region also) – December to April
(vi) South Indian Ocean(Western side) Cyclone - Dec - April
(vii) Cyclone in Indian region – Arabian sea – During change of
monsoon : Mid Apr
Mid June, October & November . Worst months are May,
October & November.
Bay of Bengal – May to Dec. Worst months are May, Oct,
Nov and December.
(viii) Baguio in Philippines and (ix) Taifu in Japan
These tropical cyclones take birth in warm oceanic region & move
to-
wards the continents. Very high wind speeds seen in low pressurea
area

11. Tropical revolving storms occur all around
the world, but are called different names.

12.  The map below shows where the seven basins noted for TC
development are located and typical tracks for each. It also has
the average number of tropical storms, and hurricanes, created in
each basin.
TROPICAL CYCLONE
FORMATION,
LOCATION

13. •
•
•
•
Hurricanes Around the Globe
Atlantic and eastern Pacific tropical cyclones are known as
hurricanes, while over the western Pacific they are referred to
as typhoons
Over the Indian Ocean and Australia they are known as cyclones
The eastern North Pacific has the highest frequency of tropical
cyclones for the globe with an annual average of 16
The South Atlantic produces none as the basin is too small to
initiate cyclogenesis
Tropical cyclone
genesis areas and
related storm tracks

14. Different Stages of TRS Based on wind Sp.

15. How are they measured?
Although developed in the USA, the Saffir-Simpson scale is
used to grade tropical storm wind strength in many parts of
the world.
Category
km/hr
km/hr
Wind Speed
Wind Speed
Wind Speed
km/hr
Wind Speed
Mph Surge m
Surge m
Storm
Storm
Storm
Surge m
Tropical storm 0-62 0
depression
depression
Tropical
Tropical
Tropical
depression
63-117 0-0.9
1 119-153 74-95 1.2-1.5
2 154-177 96-110 1.8-2.4
3 178-209 111-130 2.7-3.7
4 210-249 131-155 4.0-5.5
5 >250 >155 <5.5

16. HURRICANE CATEGORIES
Category 1 (Minimal) - Winds 64 to 82 knots, storm surge 4 to 5 ft
above normal. No real damage to building Structures. Low lying
coastal areas flooded, minor damage to piers. Examples Irene
1999 and Allison 1995
Category 2 (Moderate) - Winds 83 to 95 knots, storm surge 6 to 8 ft
above normal. Minor damage to structures, poorly constructed
buildings major damage. Coastal and low lying escape routes
flooded over, considerable pier damage. Examples: Bonnie 1998,
Georges (FL & LA) 1998 and Gloria 1985
Category 3 (Extensive) - Winds 96 to 112 knots, storm surge 9 to 12
ft above normal. Major damage to structures, poorly constructed
building destroyed. Serious flooding along the coast, extensive
flooding may extend inland 8 miles. Examples: Keith 2000, Fran
1996, Opal 1995, Alicia 1983 and Betsy 1965
Category 4 (Extreme) Winds 113 to 136 knots, stormsurge 13 to 18
ft above normal Extensive roofing and windowdamage,
complete destruction of mobile homes. Areas above 10
ftflooded inland up to 6 m, major erosion of beaches,
massiveevacuation of coastal areas.
Category 5 (Catastrophic)Winds above 137 knots, storm surge
greater than 18ft above normal. Complete failure of roof
structures and very severe window and door damage, some
completebuildings fail. Major damage tostructures lower than
15 ft abovesea level, massive evacuations of residential units
within 10 miles ofthe coast.

17. What drives a Tropical Storm?
Cold polar air
moves towards
the warm
equator.
Hot tropical air
moves towards
the cold poles.
The planet is not evenly heated. At
A the heating is more direct =
intense.
Whereas at B it is spread over a
larger surface area = less intense.

18. What causes wind?
• This uneven heating causes air to move
around the planet, trying to even out the
temperature.

around the
planet causes
our wind, which
generally
follows a known
pattern.
around the
planet causes
our wind, which
generally
follows a known
pattern.
Air moving
planet causes
our wind, which
generally
follows a known
pattern.
Air moving
planet causes
our wind, which
generally
follows a known
pattern.
Air moving
around the
our wind, which
generally
follows a known
pattern.
Air moving
around the
our wind, which
generally
follows a known
pattern.
Air moving
around the
planet causes
generally
follows a known
pattern.
Air moving
around the
planet causes
generally
follows a known
pattern.
Air moving
around the
planet causes
our wind, which
follows a known
pattern.
Air moving
around the
planet causes
our wind, which
follows a known
pattern.
Air moving
around the
planet causes
our wind, which
generally
pattern.
Air moving
around the
planet causes
our wind, which
generally
pattern.
Air moving
around the
planet causes
our wind, which
generally
follows a known
Air moving
around the
planet causes
our wind, which
generally
follows a known
Air moving
around the
planet causes
our wind, which
generally
follows a known
pattern.

19. Tropical storms
form between
5ºand 20º North
& South of the
equator.
They need warm
water ~ above
27ºc – hence
their location.
As the Earth rotates, this provides the ‘spin’
needed to start the tropical storm on its journey
across the Atlantic towards America.

20. Favourable/Ideal conditions for the formation of TRS
1. Sea surface temperature must be 27oC or more. Such a
high temperature is needed to produce steep lapse rate in
troposphere to maintain vertical circulation.
2. Fair amount of Coriolis force (Latitude more than 5
Deg North/South): The circulation has to be sustained
and hence the formation can take place where Coriolis
force is available. Hence no Cyclogenesis occurs close to
the equator. That is, within a belt of 5 deg latitude of both
sides of equator no cyclone forms. The maximum
cyclonic development takes place around lat 150. About
65% of TRS of the world develop between lat 10 to 200.
3. Low pressure area surrounded by areas of High
Pressure: Cyclones develop from a pre-existing trough or
diffused low-pressure areas associated with shear lines.
This makes the cyclones to have preferred period and
regions of formations in the oceans. The periods are
linked with the oscillations of Inter tropical Convergence
Zone (ITCZ) and they form on Doldrums on the sea area.

21. Conditions Contd……
4.Convection current(daytime over large islands):
Ensure that air rises continuously so that adiabatic
cooling results in condensation that liberates latent
heat, which provides energy for the TRS.)
5. Weak prevailing winds(During change of season)-
If the prevailing winds are strong, the air would not
rise vertically. It would be carried off horizontally,
thereby not allowing a TRS to form. It exists during
day time over large tropical islands, in mid ocean,
between lat 5 & 20 Deg, during change of monsoon
in Indian waters, mid Apr – mid June & from Oct to
Dec.
6. High humidity is required which is available in
maritime atmosphere. TRS form with warm moist
air below and steep lapse rate aloft. They form on
the western side of the ocean in summer or autumn.

22. Here is a check list
of what is needed
for a tropical
revolving storm
to grow.
A storm can travel anywhere from 15 to 40mph
Once the storm
has developed
it can grow 400
miles wide.

23. How do they develop?
–
–
–
–
–
–
–
Favorable environmental conditions that must be
in place before a tropical cyclone can form:
Warm ocean waters (at least 80°F / 27°C).
An atmosphere which cools fast with height
(potentially unstable).
Moist air near the mid-level of the
troposphere (16,000 ft / 4,900 m).
Generally a minimum distance of at least
300
miles (480 km) from the equator.
A pre-existing near-surface disturbance.
Little vertical wind shear between the
surface and
the upper troposphere. (Vertical wind shear is
the change in wind speed with height.)
Outflow aloft/exhaust

24. Common Areas for TRS
d.
1. North Atlantic It extends upto lat 30oN in the Atlantic
Ocean. About 8 TRS occur in a year. Their subdivisions are
a.Gulf of Mexico- from June to October (coastal areas of
Mexico and USA).
b.West Indies- Its TRS are very violent. Florida coast is
worst affected during the period June to October.
c.Carribean - The northern part is affected by TRS during
June to October while the southern part affected during
May to November.
Island of Cape de verde - TRS affect the island during
August to September (close to Africa).
2.North Pacific : This is situated close to Mexico and
Central America. 7-8 TRS affect every year during June to
November.

25. 3. China Sea [North West Pacific Ocean] : This includes
the coasts of Philippines, Japan, Vietnam and South China.
About 22 Typhoons occur every year during May to
December. This is where the frequency of TRS is maximum
in the world.
4.Australian Sea : It extends from east coast of Australia
to Society island, Christian island Hawaiian island. During
April to December, TRS affect this area.
5.North Indian Ocean : These are cyclonic storm affecting
India from Bay of Bengal (5 to 6) or Arabian sea (1 or 2)
during April to December.
6.South Indian Ocean : It Includes Madagascar, Reunion
and Mauritius. The frequency of TRS is 1 to 2 in a year from
Nov - April. In South Atlantic Ocean, the TRS do not form
because the area of their origin, which is doldrums, is
always north of the Equator (Over Northern Africa) with
respect to South Atlantic Ocean.

26. Origin, Movement of TRS
•
•
•
TRS originate in latitudes between 5 & 20 Deg and travel
between W and WNW in NH and between W and WSW in
SH, at a sp. of about 12 knots. Somewhere along their
track, they curve away from the Equator–curve to N and
then recurve to NE in NH; curve to S and then recurve to SE
in SH.
The recurving is such that the storm travels around the
oceanic high (which is situated at about 30 deg N.& 30
deg S in the middle of large oceans). After recurving, the
speed Of travel increases to about 15-20 Kts. Sometimes,
a TRS does not curve or recurve at all, but continues on its
original path, crosses the coast and dissipates quickly
thereafter due to friction and lack of moisture.
All TRSs do not follow such definite paths and speeds. In
their initial stages, occasional storms have remained
practi- cally stationary or made small loops for as long as
4 days. The life span of TRS anything from 1-19 days
Average 6 days.

27. Life Cycle of TRS
• Life period of a TRS is 4 to 7 days. The four
stages are formation stage, Immature stage,
matured stage and the decaying stage. The
winds around a low pressure may persist for
several days and then strengthen. Clouds
begin to show a formation. Afterwards the
growth stage starts. Pressure beings to fall
rapidly and near circular pattern of winds and
Isobars at the center take place. Wind speed
starts increasing enormously and a central
eye region forms. In the matured stage the
pressure fall at center and increase of

28. Life Cycle of TRS Contd….
wind speed is arrested but the system starts
expanding horizontally. A TRS can be 200 to
500 kms in horizontal extent and vertically, it
can be upto 5 to 10 kms tall with strong
expanding horizontally. A TRS can be 200 to
500 kms in horizontal extent and vertically, it
can upto 5 to 10 kms tall with strong
convective clouds. After crossing coast the
wind speed decreases and the cloud
organization gradually dissipates. The life
cycle consists of 4 stages.

29. Life Cycle of TRS

31. TROPICAL DEPRESSION
(Formative Stage)
Winds < 34 kts
Tropical wave develops a
weak cyclonic circulation
Identified by thickening
clusters of thunderstorms
on satellite
Central pressure falls
rapidly below 1002mb if
system intensifies

32. Life Cycle of TRS
•
•
•
•
3. Matured stage : At this stage, the pressure fall and increase
in wind speed are stopped. The circulation expands outwards
and an asymmetry sets in with high wind speed extending
more to the right side of the system, which constitutes an
area of Dangerous semicircle.
4.Dissipating stage : When a TRS enters the land it start
decaying due to the:
i) Moisture source being cut off the energy of the system
begins to weaken.
ii)Due to the roughness of the land, the frictional drag retards
the wind speed. The system also decays when it moves over a
cold ocean surface where the sea surface temperature is less
than 26o C. This happens when TRS moves northwards to
higher latitudes

33. TROPICAL STORM
(Immature to Mature Stage)
Winds 34 - 63 kts
Closed formation expands with
spiral
bands becoming better organized
Increasing sea state makes
navigation near the center
increasingly
difficult and dangerous

34. HURRICANE / TYPHOON / WILLY-WILLY
(Mature Stage)
Winds > 63 kts
DANGEROUSLY HIGH
SEAS navigation severely impaired
Radius of strong winds may exceed
350 NM
Gale Force Winds extend out further
in right front quadrant (typically 120
NM)

35. Structure of TRS
• Both horizontal and vertical wise, the inner most portion
of the matured cyclone is called ‘eye’. It has a diameter of
10 to 30 km. This area is calm with little cloud and low rain,
as the centrifugal force keeps the cloud mass away. The
temp. inside the eye may be about 5oC higher than the
surrounding. If outside the storm temperature is 26oC, at
the eye it can become about 30oC. Next region is the ‘eye
wall’ or ‘inner ring’ area. This is 30-50 km wide. This area
has maximum wind speed. Here the wind speed can reach
up to 100kts or even more. Terrible thunder, continuous
lightning, heavy continuous pouring rain from thick dark
Nimbostratus and Cumulonimbus clouds, violent winds
and squally weather with gales, tidal waves, storm surges

36. Structure of TRS Contd…..
•
•
Next is the ‘outer ring’ area. This is from
50-200 km. In this area, the strength of
wind speed decreases rapidly and
pressure rises fast and the intensity of
rainfall keeps on decreasing.
The eye and eye wall regions together
constitute the core of the cyclone. That is
upto 50 KM

40. Vertical Structure of 3 Layers
1. Inflow layer : It is from the ground to 3km
where there will be mass convergence of the air.
The air motion is in a spiral band and at the
center maximum spin or vorticity is seen. In the
eye region some sinking motion of air can be
seen.
2. Mid tropospheric layer : From 3 to 8 km, large
scale vertical motion of the atmosphere will be
experienced. There is no radial motion of the
wind into the system in mid tropospheric layer.
3. Outer flow layer : It is encountered above
12km, where there is mass divergence of the
atmospheric air. Beyond 300km, the diverging air
will start sinking groundwards.

42. Vertical Structure

43. Vertical Structure

44. Tropical Storm Feature
•
•
•
•
•
Circular feature of average 400/500 nm
diameter
Centre eye diameter 15/30 miles
Steep pressure gradient with high wind speeds
The eye wall which is approx.15 nm wide has an
area of dense cloud associated with heavy
rainfall and high waves.
Generate between Latitudes 5 to 10 deg North or
South of the Equator.

46. Principle Danger of TRS







Gales and strong winds
Damage installations, dwellings, communication
systems, trees., etc. Resulting in loss of life and
property.
Torrential rain
May cause river floods
Storm surges or high tidal waves
A storm surge is an abnormal rise of sea level near
the coast caused by a severe tropical cyclone
As a result, sea water inundates low lying areas of
coastal regions drowning human beings and live-
stock, eroding beaches and embankments,
destroying vegetation and reducing soil fertility

47. Here is a 3D
image of a
Tropical
revolving
storm. Notice
the lower level
winds being
drawn in and
spiralling
counter
clockwise.
The lines ‘isobars’ get closer together,
indicating faster flowing air.

48. –
–
Sinking air, warming adiabatically, causes air in the eye to be
warmer than elsewhere
Relative humidities are lower in this region due to higher
temperatures
Tropical
cyclone
structure

49. Tracks of TRS
The TRS travels at the speed of 10-30 km/hr.
In a day, it can cover a distance of about 300
KM. It moves from east to west, steered by
high level easterly wind. Sometimes the
movement is erratic, with loops and wobbling.
The TRS gets pulled polewards and therefore
general motion is northwest in the northern
hemisphere. There is a tendency to move in
areas of warm water. They weaken as they
enter in cold water or strike a coast.

50. Tracks of TRS

51. Track of TRS in Bay of Bengal/Arabian Sea
•
•
In the Bay of Bengal, 2 to 3 TRS are common
during Oct- Nov. Rarely, 1 to 2 TRS are seen in
April and May. The track of most of the TRS is
north westerly and strike the eastern coast of
India. Sometimes, off Orissa coast, TRS recurve
and cross near Burma (Myanmar) coast.
For Arabian Sea, the formation of TRS are rare.
However, the storms that form over Bay of
Bengal move across Peninsular India and
emerge out in Arabian Sea. The TRS get
rejuvenation over the Arabian Sea, move in a
Northerly course and strike the Saurashtra-kutch
coasts. The Frequency of storms in Arabian Sea
is very much less compared to Bay of Bengal

53. Evidence for Determination of TRS
•
•
•
•
•
•
•
•
•
•
Geographic conditions of storms experienced Lat 5 – 35
Deg.
Location and Season NH - Jun-Nov & SH – Dec – May
A heavy swell develops, usually from direction of storm
and may be experienced upto 1000 nm from the storms
centre.
An unsteady barometer or a cessation in the diurnal range.
Increased wind velocity or a change in the trade wind,
becoming violent.
Open ocean, high sea temperature over 27 deg.
A growth of cumulus and/or cumulonimbus will develop
with bands of showers
Thunderstorms may occur within 100 nm radius of storm.
Oppressive atmosphere, with squally and heavy rainfall in
the vicinity of the storm.
Changing appearance of the sky cirriform cloud.

54. Recurvature of TRS
When the TRS reach higher latitudes, they change
the directions. They move north and then north east
in northern hemisphere. In southern hemisphere,
they move south and then south east. This process
is called ‘recurvature’. Before the recurvature, they
remain stationary for a day or more. After
recurvature, they move very fast. The recurvature is
decided by upper level winds, which is called
steering level. When two cyclones exist near each
other on the ocean, they interact and move
anticlockwise with respect to each other. In India the
TRS after recurvature get broken up over Himalayas
and wakens. Elsewhere over the world, as in Atlantic
ocean after recurvature, the TRS acquires the
properties of an Extra tropical depression

55. Cyclone Map of India

56. Definitions of TRS
•
•
•
•
•
•
•
VORTEX: The central calm of the storm.
Right Hand Semicircle(RHSC): That half of the storm centre
that lies to the right of the observer who faces along the path
of the storm.
Left Hand Semicircle(LHSC): That half of the storm centre
that lies to the left of the observer who faces along the path of
the storm.
DANGEROUS SEMI-CIRCLE: The half of the storm, which lies to
the right of the path in the Northern Hemisphere and to the left
of the path in the Southern Hemisphere.
DANGEROUS QUADRANT: The advance quadrant of the RHSC
in NH & LHSC in SH. When the existence of a TRS om the
vicinity has been established, evasive action has to be taken to
keep the vessel out of this quadrant.
NAVIGABLE SEMI-CIRCLE: The half of the storm, which lies to
the left of the path in the Northern Hemisphere and to the right
of the path in the Southern Hemisphere.
TROUGH LINE: A line through the centre of the storm at right
angles to the path. The dividing line between falling and rising
pressure

57. •
•
•
•
•
•
•
•
PATH: The direction in which the storm is moving.
TRACK: The area, which the storm centre has
traversed.
STORM FIELD: The horizontal area covered by the
cyclone conditions of the storm.
SOURCE REGION: The region where the storm first
forms.
VERTEX or Cod: The furthest westerly point reached
by the storm centre.
EYE OF THE STORM: The storm centre.
BAR OF THE STORM: The advancing edge of the
storm field.
ANGLE OF INDRAUGHT: The angle, which the wind
makes with the isobars

58. Weather in TRS
Pressure : In the eye region the lowest pressure occurs.
Here, 15 to 20 HPA fall of pressure is common. In extreme
case, the central pressure was as low as 920 HPA. In the
outer storm area, the fall of pressure ahead of the trough,
and the rise of pressure behind it, is slow. The semi-diurnal
variation of pressure may still be visible on the trace of a
barograph. In the eye-wall, the fall of pressure ahead of the
trough, and the rise of pressure behind it, is very sharp.
The trace of the barograph is very steep, nearly vertical.
Semi-diurnal variation is not visible on it. The pressure
gradient in this region can be as high as 11 mb in 15 miles.
The pressure keeps increasing rapidly behind a TRS across
eye wall and outer storm area.
Wind : Wind speed will be very high in eye wall region of the
order of greater than 100 kmph normally. If the wind veers
continuously, the ship is in the right hand semi circle and if
the wind backs continuously, the ship is in the left hand

59. Wind
•
•
•
•
Angle of indraft: The angle of indraft, in the outer fringes
of the storm, is about 45 Deg and gradually decreases
until it is 0 Deg in eye wall.
Wind Force: The wind force will increase as the
atmospheric pressure falls and after the trough, or eye as
the case may be, has passed, wind force will gradually
decrease as the atmospheric pressure increases.
Wind direction & Force in the Eye: As soon as a vessel
passes from the eye-wall into the eye, the wind dies down
into light airs but the swell is mountainous and confused.
It must not be presumed that a vessel in the eye of a TRS
is in a comfortable and safe position. On the contrary, she
is in a most dangerous situation. After a short while, as
the vessel passes into the eye-wall behind the trough, the
sudden hurricane force wind from the opposite direction
as before, strikes the vessel and may cause it to heel over
by as much as 80 deg. Or more and would hold it like that,
leaving practically no margin for rolling further.

60. Clouds & Temperature
•
•
Clouds : All types of clouds can be seen from eye wall
circle onwards. Nimbostratus clouds will give very heavy
continuous rain. In the outer fringers of the storm, cirrus
in the form of strands or filaments generally so aligned,
that may be said to point towards the storm centre.
Then cirrostratus followed by altostratus. Around the
eye-wall, thick nimbostratus(giving continuous rain) and
small patches of cumulus, may be seen. At the eye wall,
towering anvil shaped cumulonimbus gives torrential
rain. Directly above the eye, a small circular patch of
blue sky may be seen, indicating an absence of cloud
therein and consequent cessation of precipitation.
Atmospheric Temperature. Since a TRS exists in one air-
mass only, no drastic changes of atmospheric temp. are
experienced on its passage. However, atmospheric temp.
would decrease during rain. In the eye, a slight increase
may be registered due to adiabatic heating of the
subsiding air.

61. Visibility & Storm Surge
•
•
Visibility : Visibility will be good except during rain
which occurs in the eye wall and outer storm circles.
Around the eye-wall, under the nimbostratus clouds, it
becomes poor due to rain. In the eye-wall, it is poor
due to driving rain and spray. In the eye, it is poor due
to mist or fog.
Storm Surge : When pressure becomes low, water will
rise which is known as inverted barometeric effect.
The strong winds also increase the wave. The waves
reach as high as 20 metres or so. This is called storm
surge or tidal wave or storm wave.
The storm surge will get amplified when the storm
crosses the land in a bay with shallow water or in a
river estuary especially during spring tide.
In a TRS the maximum damage is due to storm
surge. About 90% of the death toll is due to storm
surge which affects about 25 km inland.

63. Services provided for Storm
Warnings
•
•
•
•
•
•
•
Storm warnings are transmitted over various
means of transmission.
They include:
Navtex – Continuous however available only in
coastal areas – not for long ocean passages
VHF – port information
Weather FAX – all over the world
Inmarsat C – Same broadcast as Navtex but
for coverage area of Inmarsat
High Frequency (HF) broadcasts – Ocean
passages

64. Strom Warnings/Cyclone Forecast
Weather reports based on satellite pictures and
observations from other vessels may contain storm
warnings. A satellite picture cannot indicate the
atmospheric pressure at the storm centre. A vessel
which notices the warning signs of a TRS, is the first
and only one to do so and must warn others about it.
She should first send out a safety message containing
the storm warning and thence increase the frequency
of its weather reports. In a Cyclone forecast 4
important aspects are:
1. Position of the cyclone
2. Its strength or intensity, whether it is a depression
or cyclone.
3. Whether the system is intensifying or weakening.
4. Future path or course of the system

65. Major basins and warning
centers.
Basins Warning Centre(RSMC/TCWC
North Atlantic National Hurricane Center (US)
North-East Pacific National Hurricane Center (US)
North-Central Pacific Central Pacific Hurricane
Center (US)
North-West Pacific Japan Meteorological Agency
North Indian Ocean India Meteorological
Dept(India)
South-West Indian Ocean India Meteorological
Dept(India)

66. Four Aspects(hints) of Deciding Cyclone Forecast
i)Weather charts which consists of
observations on the surface as well as
upper atmospheric data observed at fixed
time like 00,03,06,09,12,18,21, UTC.
ii) Cloud observations from satellite
imageries and also from Radar observations.
iii) Ship and aircraft observations
iv) Weather buoys.

68. Cyclone Movement Prediction
•
Cyclones move in a path where there is maximum.
1. Pressure fall, 2.Pressure departure from normal,3.
Clouds & rainfall
4. Path depends on the wind directions at about 10km
in the upper atmosphere which is called steering level.
5. When the isobaric analysis is carried out the cyclone
field has elliptical shaped isobars. The cyclone has the
tendency to move along the major axis of the ellipse.
6. Future path depends on the past track also, which is
called aspersistent climatology.
7. A Cyclone track atlas is maintained by each cyclone
prone country which covers hundreds of tracks of
cyclones in the past numbers of years. The present
cyclone is compared with the past tracks depending
upon its positions and its future path is decided.
The cyclone prediction is not based on one factor or
other but it is based on the combination of the factors
contributing to its path and movement

69. Warning Signs of an approaching TRS
a) SWELL:Swells are the waves generated by a distant
TRS. They have a long wave length and appear in a
quick succession. From the eye-wall region, they
emerge radially in all directions and reach the coast
hundreds of miles away.
b) Pressure: Falls steadily. After filtering diurnal
variation of surface pressure if the pressure is less
than 5 HPA or more, it means there is a TRS is in the
vicinity.
c) Lot of clouding, heavy continuous rain with squally
gale force wind is a clear warning signal.
d) Storm warning bulletins issued by Meteorological
Department is a good guidance for mariners

70. Weather
•
•
•
•
•
•
•
Cirrus clouds in bands or filaments aligned
towards the direction of the storm centre.
Unusually clear visibility may occur.
Sometimes peculiar dark red/copper colour of
sky is seen at sunset before a TRS.
Increase of wind force as the pressure falls.
Threatening appearance of dense, heavy clouds
on the horizon.
Frequent lightning may be seen
Succession of Squallls, with or without rain.

71. Obtain the bearing of the storm centre.
(a)
(b)
‘Face the wind, and according to Buys law the storm
centre will lie 8 – 12 points on your right in the NH, Left
in the SH. If the pressure has fallen 5 mb below normal,
allow 12 points as it means that either the vessel is in
the outer fringes of a well developed TRS, or that a new
TRS is forming in the vicinity.
If the pressure has fallen 20 mb or more below normal,
allow 8 points as it means that the vessel is near the
eye of a well developed TRS.
(c) If the wind is veering continuously, the ship is in right
hand semi circle. If the wind is backing continuously, the
ship is in the left hand semi circle.
(d) If the ship is at the port at the time of crossing of a TRS
it is advisable to sail off the shore, so that the ship can be
prevented from collision with the port structure or running
aground. While out in the high seas, efforts should be
taken to keep the ship away from the storm centre

72. Dangerous Semi Circle: Right Hand Semicircle in NH
& Left Hand Semicircle in SH
Navigable Semi Circle: Left Hand Semicircle in
NH & Right Hand Semicircle in SH

73. To ascertain in which semicircle vessel lies
•
•
•
For a stationary observer, if the wind veers, vessel is in the
RHSC and if it backs, LHSC. This holds good for both
NH & SH. While determining the semicircle, the
following points should be noted.
Wind Observations, though logged every hour during
bad weather, should be compared with that 2 hours
earlier. This is to give time for significant veering or
backing and hence weed out errors that may be
caused by irregular gusts of wind.
Veering or backing, once detected, should be
continuous while the observer remains stationary, ie. a
veering wind should continue to veer and a backing
wind should continue to back. If the wind veers at first
and then backs, or if it backs at first and then veers,
the vessel must have passed from one semicircle into
another, due to change of path of the storm.

74. Semicircle vessel lies contd……
•
•
During the two-hour interval between
observations, while veering or backing of wind is
being decided, the observer must be
stationary(ie. Vessel should remain hove to). If
not, the conclusion arrived at regarding RHSC or
LHSC may be wrong and disastrous
consequences may result.
If a vessel was overtaking a TRS or it if was
approaching a stationary TRS from its rear, the
wind would veer in the LHSC and back in the
RHSC. An unwary navigator would then arrive at
a wrong conclusion regarding semi-circle and
take wrong action, which action, instead of
taking him away from the storm centre, would

76. Track
Equator
20˚N
Vertex
20˚S
Vertex
Characteristic Track
Alternative Path
Eye
Eye
Dangerous
Semicircle
Dangerous
Semicircle
Navigable
Semicircle
Navigable
Semicircle
Alternative Path
Characteristic Track

79. Action when approach of a TRS is
confirmed
1. Obtain the bearing of the storm centre.
2. Ascertain in which semi-circle the vessel
lies.
3. Take avoiding action.

80. Avoiding Action
•
•
•
Any avoiding action should aim to keep the vessel well
out of eye-/eye-wall.
If a vessel is in the Dangerous Quadrant: Proceed as
fast as practicable with the wind 1-4 points on the stbd
bow(port bow in SH)-1 point for slow vessels (less
than 12 kts) altering course as the wind veers(backs in
SH). This action should be kept up until the pressure
rises back to normal ie. Until vessel is outside the outer
storm area. If there is insufficient sea room, the vessel
should heave to with the wind on the stbd bow(port
bow in SH) until the storm passes over.
If vessel is in the path of the storm or if in the
Navigable semi-circle: Proceed as fast as practicable
with the wind about 4 points on the stbd quarter (port
quarter in SH), altering course as the wind backs(veers
in SH). This action should be kept up until the
pressure rises back to normal ie. Until vessel is outside
the outer storm area.

83. 1-2-3 Rule

84. In NH, while in Right Hand Semi Circle, that is dangerous semi
circle, proceed starboard side by about 1 – 4 points(for a fast
moving vessel) till the pressure becomes normal and wind
becomes light (outside the storm)

85. If the ship is in the circle in SH, it should proceed along with
the wind by 4 points on the Stbd side and get out of the storm
area.

86. Tropical Storm / Hurricane Key Facts
Hurricanes are located in the low-pressure belt near the equator as the sun
heats the oceans to a critical temperature of 27C.
The oceans heat up all through the summer making their warmest
temperatures in the Autumn (specific heat capacity); this is Hurricane
season in the Atlantic.
The hurricane starts as a tropical depression, gaining more and more
energy from the warm ocean as it crosses the Atlantic. The circulation of
the depression gets tighter (isobars are closer together) and the wind
speed increases into a tropical storm.
The wind speeds continue to increase, becoming a category 1 up to
category 5 for the most violent storm. As the hurricane makes landfall
damage is done by the winds and rain and an advancing storm surge,
which raise sea level by 5m+ in some cases.
The hurricane quickly looses its energy as it crosses the cooler land, as the
energy supply has been cut off. The hurricane dies.
Sometimes remnants of hurricanes are brought across back across the
Atlantic at the mid latitudes giving us very strong depressions in the UK.

87. Warm Oceans
The ‘food’ of a tropical storm
is the warm moist water
found near the equator. The
air here is under LOW
pressure, which means it
can lift easily. This lifting
encourages the air to cool
and condense, as it does
latent heat is released. This
is the name given to energy
produced when a substance
changes state ie vapour to a
liquid.

88. Cyclone Movement Prediction
1.
Cyclones move in a path where there is maximum.
Pressure fall. 2. Pressure departure from normal
3. Clouds and rainfall
4. Path depends on the wind directions at about 10km in the upper atmosphere
which is called steering level.
5.When the isobaric analysis is carried out the cyclone field has elliptical shaped
isobars. The cyclone has the tendency to move along the major axis of the
ellipse.
6.Future path depends on the past track also, which is called as persistent
climatology.
7.A Cyclone track atlas is maintained by each cyclone prone country which
covers hundreds of tracks of cyclones in the past numbers of years. The
present cyclone is compared with the past tracks depending upon its positions
and its future path is decided. The cyclone prediction is not based on one factor

89. Ideal conditions for the formation of
TRS
•
•
•
•
•
(a) High Relative Humidity(open sea) & (b) High
Temperature(Tropical areas)-Ensure that Large quantity of
water vapour is present in the air.
(c) LP Area surrounded by areas of HP & (d) Convection
current(daytime over large islands) - Ensure that air rises
continuously so that adiabatic cooling results in condensation
that liberates latent heat, which provides energy for the TRS.
(e) Fair amount of Coriolis force(latitude more than 5 Deg N/
S)- Ensures that when the winds blow, from surrounding areas
of HP to LP area inside, they get deflected sufficiently to blow
spirally inwards(Cyclonic)
(f) Weak prevailing winds(During change of season)- If the
prevailing winds are strong, the air would not rise vertically. It
would be carried off horizontally, thereby not allowing a TRS to
form.
It exists during day time over large tropical islands, in mid
ocean, between lat 5 & 20 Deg, during change of monsoon in
Indian waters, mid Apr – mid June & from Oct to Dec.