STORMS – Tropical Revolving Storms


Table of Contents

What is a tropical revolving storm?

A tropical revolving storm is a powerful meteorological phenomenon characterized by a low-
pressure center, strong gale-force winds circulating around it, and a spiraling pattern influenced by
the Coriolis effect. The direction of rotation depends on the hemisphere, with anticlockwise
circulation in the Northern Hemisphere and clockwise circulation in the Southern Hemisphere. These
storms are common in tropical regions and can have significant impacts on the weather and climate
of affected areas.

What is the Coriolis Effect?

The Coriolis effect is a result of the Earth’s rotation and influences the direction of moving objects,
including air masses.
In the NH, the Coriolis effect causes moving air to be deflected to the right, leading to anticlockwise
circulation around low-pressure centers.
In the SH, the Coriolis effect causes deflection to the left, resulting in clockwise circulation.

What are the various names used for a Tropical Revolving Storm (TRS)?

TRS, known by different names in various regions:
Hurricane- on the western side of the North Atlantic and South Pacific.
Typhoon – on the western side of the North Pacific.
Cordonazo – on the eastern side of the North Pacific.
Willy-willy- on the eastern side of the South Indian Ocean.
Cyclones- in the Bay of Bengal and the Arabian Sea.

What leads to the formation of Tropical Revolving Storms (TRS)?

Tropical Revolving Storms (TRS) originate when warm, moist air, typically with temperatures
exceeding 27 degrees Celsius, ascends from the sea surface.
As this air rises, it encounters cooler air, leading to condensation and the formation of clouds and
rain. The process of condensation releases significant amounts of energy, giving rise to strong winds.
The rotation of the Earth contributes to the development of these winds, causing them to circulate
in a spiraling pattern. As the Earth rotates, the winds are forcefully drawn upwards into a “vortex”,
which can span up to 1,000 kilometers wide. Wind speeds within this vortex can reach as high as 200
kilometers per hour (about 125 miles per hour).

However, when these storms make landfall, their energy source shifts. The storms are initially fueled
by the presence of moist air over warm ocean surfaces. Once they reach land, the absence of the
warm, moist air leads to a shift. Dry air is drawn into the system, causing the storm to lose energy.
This transition can weaken the storm and impact its intensity as it moves over land.

What is a vortex?

In the case of tropical revolving storms (TRS), a vortex is created around the storm’s central low-
pressure area. The Coriolis effect, resulting from the Earth’s rotation, influences the direction of the
circulating winds, causing them to spiral around the center of the storm.
This spiraling motion contributes to the characteristic shape of a tropical cyclone and is often
referred to as the storm’s vortex.

From where do Tropical Cyclones (TRS) originate?

The origin and track of Tropical Revolving Storms (TRS) are influenced by specific latitudes and
prevailing wind patterns. TRS typically originate in latitudes between 5° and 15°, where the
conditions for their formation are most favorable. In the Northern Hemisphere (NH), these storms
generally travel between the west (W) and west-northwest (WNW) directions. In the Southern
Hemisphere (SH), their track extends between the west (W) and west-southwest (WSW) directions.
The speed of their movement is between 8 to 10 knots.

What are Track, Path, Vertex & Semi circles (RHSC & LHSC) in a TRS?

The Track refers to the path or route that a Tropical Revolving Storm (TRS) has already traversed.

The Path indicates the projected course where there is a potential for the Tropical Revolving Storm
(TRS) to pass in the near future.

The Vertex is the westernmost point during the recurving of the TRS.

The Semi circles- When a storm is bisected along its path, two segments emerge: the Right-hand
semicircle (RHSC) on the storm’s right side and the Left-hand semicircle (LHSC) on the left side.

The Right-hand semicircle (RHSC) represents the storm’s segment situated to the observer’s right
along the storm’s route. In this zone, for a stationary observer, the wind consistently veers.

The Left-hand semicircle (LHSC) represents the storm’s segment positioned to the left of an observer
facing the storm’s route. In this area, for a stationary observer, the wind consistently backs.

What is the typical path of a Tropical Cyclone (TRS)?

As TRS progress along their track, a notable pattern emerges. Somewhere along their path, they
undergo a significant change in direction. In the Northern Hemisphere, TRS tend to curve away from
the equator, initially curving to the north (N) and then recurving to the northeast (NE).In the
Southern Hemisphere, A similar pattern is observed, with the storms initially curving to the south (S)
and then recurving to the southeast (SE).
TRSs follow a path that involves recurving around the oceanic high near 30°N and 30°S. After
recurving, their speed increases to 15-20 knots.

Some TRSs, however, don’t recurve and instead cross the coast, dissipating quickly due to friction
and moisture absence. Notably, TRS paths and speeds vary, with occasional storms staying
stationary or making small loops for up to 4 to 5 days in their initial stages.
This curving and recurving behavior is a result of the complex interaction between the storm system
and the environmental conditions it encounters.
Factors such as the Coriolis effect, steering winds, and the distribution of temperature and pressure
play crucial roles in determining the trajectory of TRS along their track. The re-curvature often marks
a transition phase in the life cycle of these storms, influencing their intensity and potential impact on
surrounding regions.

What are various stages of a Tropical cyclone?

There are 4 stages of Tropical Cyclone:
Formation Stage
Tropical cyclones form in the Inter Tropical Convergence Zone, hinging on six conducive
environmental factors. They derive energy from latent heat generated by substantial thunderstorm
activity and moist air condensation. This intricate process, further fueled by Earth’s gravity and
rotation, transforms the cyclone formation into a gigantic vertical heat engine. Observable on
satellite images, the early stage presents as an unusually active yet disorganized convection zone
with thunderstorms. The presence of curved cumulus clouds indicates the central location of the
developing cyclone. Inland movement generally results in minimal direct damage, but the cyclones
can unleash heavy rainfall and potential flooding, underscoring the need for understanding these
phenomena to prepare and mitigate their impact on vulnerable regions.

Nascent stage
During this phase, the convection area consolidates and intensifies, accompanied by simultaneous
strengthening. Surface pressure markedly decreases, plunging below normal levels, fostering the
development of gale-force winds due to an intensified pressure gradient. The circulation center
becomes well-defined, potentially initiating the formation of an eye. Satellite and radar observations
reveal a distinctive spiral banding pattern. The nascent Stage of a tropical cyclone poses a threat,
inducing destructive wind and storm surge effects along coastlines. However, the resulting damage
typically confines itself to a localized area due to the cyclone’s early developmental status.

Developed Stage     

Given sustained favorable conditions in the ocean and atmosphere, a cyclone can progress to the
severe cyclone stage,  marked by heightened peril. Around half of cyclones reach this critical phase.
Here, cyclonic circulation intensifies significantly, and gales extend their reach. Satellite imagery
depicts highly organized cloud fields, symmetrically arranged, often featuring a well-defined, central
eye. This stage, characterized by maximum intensity, typically persists for about a day, contingent
upon the cyclone’s continuous exposure to an exceptionally favorable environment. The severe
cyclone stage represents a peak threat, emphasizing the potential for significant impacts and
necessitating close monitoring for timely warnings and preparations.

Decay Stage
In this stage, the Tropical Cyclone undergoes a critical transformation as its warm core disintegrates,
resulting in an elevation of central pressure and a weakening of maximum surface winds. The decay
process accelerates swiftly if the system enters an inhospitable atmospheric or geographic
environment. During this stage, heavy or moderate rainfall can persist, marking the lingering
influence of the deteriorating cyclone and the potential for continued weather impacts despite the
diminishing intensity of core elements.

What is the typical Life cycle of a tropical cyclone?

The entire life cycle of a tropical cyclone typically spans around 9 days, though it can be as brief as 2
or 3 days or extend beyond 20 days. The duration of a cyclone’s life cycle is influenced by various
factors such as environmental conditions, sea surface temperatures, and interactions with land
masses, leading to variability in their development, maturity, and eventual dissipation.

What are the signs/indications of approaching TRS?

The initial sign of a Tropical Cyclone (TRS) is the onset of heavy and extensive swells originating from
its center, detectable up to a thousand miles away. This swell often serves as the primary indicator
of an approaching storm.
Critical information regarding the storm’s location, atmospheric pressure, and likely trajectory can
be obtained from official alerts, meteorological updates featuring satellite imagery, and firsthand
accounts from fellow vessels.

A significant decrease in barometric pressure, dropping more than 5 millibars below the usual level,
serves as a corroborating indicator of an impending Tropical Cyclone, enhancing the confirmation
provided by other elements.
The wind direction experiences a noticeable change, aligning with the rotational pattern of the
cyclonic spin.
The air becomes infused with thick, ominous clouds.
Frequent occurrences of lightning become a prominent characteristic.
Squalls, increasing both in frequency and intensity, become more apparent.

The ocean encounters the beginning of intense and hazardous waves.
Distinct bands or filaments of cirrus clouds take shape, aligning in the direction of the storm center.
At sunset, the clouds adopt a dark red or coppery hue.

What actions are to be taken when the approach of a TRS is confirmed?

Determine the direction of the storm center.

Stand facing the wind, and the storm center will likely be positioned approximately 8 to 12 points to
the right in the Northern Hemisphere (or to the left in the Southern Hemisphere), following Buys
Ballot’s Law. The swell direction serves as a rough indicator of the storm center, as does the
orientation of the densest section of a large cloud bank. Additionally, a barometric pressure drops of
5 millibars below the normal level suggests the ship is likely in the outer region of a well-developed
storm. If the barometric pressure falls by 20 millibars or more below the normal level, there is a
possibility that the ship is near the eye of a thoroughly developed Tropical Cyclone.

Determine the semi-circle in which the vessel is located.

Bring the ship to a stationary position or heave to (moving very slowly in one direction) at the
current location. Monitor and record wind directions every 2 hours. If the wind direction shifts
clockwise (veering), the vessel is in the Right-hand semicircle (RHSC). If the wind direction changes
counterclockwise (backing), the vessel is in the Left-hand semicircle (LHSC) in both hemispheres.
Once identified, the veering or backing of the wind should be consistent while the observer stays
stationary— meaning a veering wind should persist in veering, and a backing wind should
consistently back. If there is an initial veer followed by a back, or an initial back followed by a veer, it
indicates that the vessel has transitioned from one semicircle to another, likely due to a change in
the storm’s path .

Take evasive measures during Tropical Cyclone conditions.

If feasible, attempt to head to the open sea, so that there is ample sea room, adequate water depth
& safe distance from coastline. Otherwise, follow these procedures:
1. Take head count & ensure all persons are on board.
2. Keep the engine on standby.
3. Double the moorings.
4. Place sufficient fenders between the ship and the jetty.
5. Rig lifelines both fore and aft.
6. Securely batten down all hatches, hatchways & weathertight doors on open decks.
7. Lower and secure all derricks/cranes.
8. Top up all tanks & ensure there is no slack in tanks.

9. Keep all life-saving appliances (LSA) ready for immediate use.

If feasible, attempt to head to the open sea, so that there is ample sea room, adequate water depth
& safe distance from coastline or relocate to a secure anchorage with adequate shelter. Otherwise,
follow these procedures:
1. Take head count & ensure all persons are on board.
2. Keep the engine on standby.
3. If relocating to sheltered anchorage, then drop both anchors with several cables in the water.
4. Rig lifelines both fore and aft.
5. Securely batten down all hatches, hatchways & weathertight doors on open decks.
6. Lower and secure all derricks/cranes.
7. Top up all tanks & ensure there are no slack in tanks.
8. Keep all life-saving appliances (LSA) ready for use in case needed.
9. Ensure all bridge equipment and navigational lights are in standby mode.

1. If the vessel is in the dangerous quadrant:
Proceed as quickly as practical with the wind at 1 to 4 points on the starboard bow (port bow in SH)
– choose 1 point for slow vessels (less than 12 knots) and 4 points for fast vessels (more than 12
knots), adjusting the course as the wind veers (backs in SH). Maintain this action until the pressure
returns to normal, signifying that the vessel is beyond the outer storm area. If there is limited sea
room, the vessel should heave to with the wind on the starboard bow (port bow in SH) until the
storm passes over.
2. If the vessel is in the path of the storm OR if in the navigable semi-circle:
Proceed as quickly as practical with the wind approximately 4 points on the starboard quarter (port
quarter in SH), adjusting the course as the wind backs (veers in SH). Maintain this maneuver until the
pressure returns to normal, signifying the vessel is beyond the outer storm area.

Check out the article  Heavy Weather Preparations & Checklist

Frequently Asked Questions(FAQs)

1.What is TRS 1-2-3 rule?
The TRS 1-2-3 rule, also known as the “Danger Area” rule, is a guideline used to help vessels avoid the dangerous winds(greater than 34kts) associated with a tropical cyclone (hurricanes or typhoons). It provides a simple way to estimate the potential path of a storm and the areas likely to be affected by its hazardous winds.

The 1-2-3 Rule is means of avoiding winds associated with a tropical cyclone by taking into account the forecast track error of the National Weather Service over a 10 year period which is approximately 100 nm in 24 hours, 200 nm for 48 hours and 300 nm in 72 hours.

The rule states:

24 Hours: Move 100 nautical miles away from the forecasted position of the storm.
48 Hours: Move 200 nautical miles away from the forecasted position of the storm.
72 Hours: Move 300 nautical miles away from the forecasted position of the storm.

It provides a simple and conservative approach to ensure safety during these severe weather events.

2. Where does tropical revolving storm get its energy from?

Tropical revolving storms, also known as tropical cyclones, hurricanes, or typhoons, derive their energy primarily from the warm ocean waters and the release of latent heat from condensation within the storm system.


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