A Tropical Revolving Storm (TRS) is a large-scale weather system characterized by a low-pressure center, powerful winds, heavy rain, and thunderstorms. TRS includes cyclones, hurricanes, and typhoons, and the structure, formation, development, and decay stages are crucial in understanding how to navigate around them safely.
Structure of a TRS:
A TRS is divided into several key components:
- The Eye:
- The center of the storm, a calm region with very low pressure. It is typically 20-50 km in diameter.
- The eye is surrounded by the eyewall, where the strongest winds and heaviest rainfall occur.
- The Eyewall:
- A ring of towering thunderstorms encircling the eye.
- This region has the most intense winds and rainfall due to the strong convection.
- Rainbands:
- Spiral bands of clouds and thunderstorms that extend outward from the eyewall.
- These bands can stretch for hundreds of kilometers and bring heavy rainfall and strong winds.
- Outflow:
- At the upper levels of the storm, air flows outward from the center, creating cirrus clouds that spread over a large area.
Formation of a TRS:
The formation of a TRS typically occurs in tropical waters where conditions are favorable for its development.
Steps in Formation:
- Warm Ocean Water (at least 26.5°C):
- Warm water provides the energy for the TRS to develop. As the water heats the air above it, moisture evaporates, rises, and cools, leading to condensation.
- Coriolis Effect:
- The Coriolis force (caused by the Earth’s rotation) initiates the spinning motion of the storm.
- This force is negligible at the equator and becomes stronger further from it, which is why TRS usually forms between 5° and 20° latitude.
- Low-Pressure Area:
- A disturbance or low-pressure system forms as warm, moist air rises and cools, leading to condensation and cloud formation.
- As the air rises, it creates a vacuum effect, drawing in surrounding air, which also rises, fueling the storm.
- Development of a Cyclonic Circulation:
- The system starts rotating due to the Coriolis effect. As it intensifies, the low-pressure area deepens, and the wind speed increases.
- At this point, the storm is classified as a tropical depression.
- Maturation:
- With continued heat and moisture input from the ocean, the storm can strengthen into a tropical storm or cyclone.
- Wind speeds exceed 64 knots (74 mph), and the storm is classified as a TRS (hurricane, cyclone, or typhoon).
Development and Intensification:
After formation, the storm develops through several stages as it gathers energy from the warm ocean and intensifies.
- Tropical Depression:
- Wind speeds: Less than 34 knots (39 mph).
- Organized thunderstorms with a low-pressure center.
- Tropical Storm:
- Wind speeds: 34-63 knots (39-73 mph).
- The storm becomes more organized, and the spiral structure with rainbands begins to form.
- Tropical Cyclone (TRS):
- Wind speeds: Over 64 knots (74 mph).
- The eye and eyewall become well-defined, and the system has a strong cyclonic rotation.
- Mature Stage:
- The storm reaches its maximum intensity with very strong winds, heavy rainfall, and a well-formed eye.
- At this stage, the storm is capable of causing significant damage if it makes landfall.
Decay of a TRS:
A TRS weakens or decays when it encounters factors that reduce its energy supply or disrupt its structure.
- Landfall:
- When a TRS moves over land, it loses access to the warm ocean water that provides it with energy.
- Increased friction with the land surface also disrupts the circulation, weakening the storm.
- Moving Over Cooler Water:
- As the storm moves into cooler waters, the reduced heat input from the ocean weakens the storm.
- Increased Wind Shear:
- Wind shear refers to changes in wind speed and direction with height.
- Strong wind shear can disrupt the storm’s vertical structure, causing it to weaken.
- Dry Air Intrusion:
- Dry air can enter the storm system, reducing the amount of moisture available for condensation, leading to a decay in the storm’s intensity.
Characteristics of a TRS
- Structure:
- Eye Wall: This area surrounds the eye and contains the most severe weather, with sustained high winds, torrential rain, and violent thunderstorms.
- Spiral Rain Bands: Beyond the eye wall, the TRS has spiral bands of clouds that extend outward, bringing heavy rain, thunderstorms, and gusty winds.
- Eye: The center of the storm, known as the eye, is a calm region with low pressure and clear skies. The eye is surrounded by the eye wall, where the strongest winds and heaviest rainfall occur.
- Winds:
- Cyclonic Winds: In the Northern Hemisphere, winds rotate counterclockwise around the storm’s center, while in the Southern Hemisphere, winds rotate clockwise.
- Wind Speed: TRS winds can reach speeds of 74 mph (119 km/h) or more, categorizing the storm as a tropical cyclone or hurricane.
- Pressure:
- Low Pressure: The TRS is a low-pressure system, with the lowest pressure at its center (the eye). The pressure gradient between the center and the outer edges of the storm is steep, resulting in strong winds.
- Precipitation:
- Heavy Rainfall: TRS systems are known for producing large amounts of rain, which can lead to flooding both at sea and on land. The spiral rain bands bring frequent squalls and thunderstorms.
- Movement:
- Forward Motion: TRS systems generally move westward in the tropics due to trade winds, then often turn poleward (northward in the Northern Hemisphere or southward in the Southern Hemisphere) as they enter higher latitudes, steered by upper-level westerly winds.
- Sea Conditions:
- High Waves: The strong winds generate extremely rough seas with high waves, which pose serious dangers to ships. Wave heights in a TRS can exceed 10 meters or more in severe cases.
- Storm Surge: In coastal areas, TRS often leads to storm surges, where the sea level rises dramatically due to the low pressure and wind force, leading to coastal flooding.
Typical Signs of Approaching a Tropical Revolving Storm (TRS):
- Fall in Atmospheric Pressure: A steady and continuous fall in barometric pressure is an early sign of a TRS. The fall is gradual initially and becomes more rapid as the storm approaches.
- Cloud Patterns: The appearance of high cirrus clouds followed by the development of denser cloud formations. Cirrostratus and cumulonimbus clouds are common as the storm intensifies.
- Sea Swell: Long-period swells coming from the direction of the storm’s center, usually preceding the TRS by up to a few days.
- Changes in Wind: Winds tend to increase and shift direction. Initially, the wind direction may become erratic, and then it may start to blow steadily from one quadrant, which shifts as the storm moves.
- Sudden Temperature Drop: A decrease in temperature, often accompanied by a noticeable increase in humidity, is another sign of a TRS.
- Heavy Rainfall: Intermittent showers or squalls followed by more continuous heavy rainfall as the storm nears.
- Halo Around the Sun or Moon: The appearance of a halo around the sun or moon is sometimes observed in the early stages of an approaching TRS.
- Radio Static Interference: Increased radio interference on longwave and shortwave bands due to the ionospheric disturbances caused by the storm’s electrical activity.
Warning Signs of an Approaching Tropical Revolving Storm (TRS) on a Ship:
Early detection of a TRS is crucial for taking evasive actions. The following warning signs help mariners recognize the approach of a TRS:
- Falling Barometric Pressure:
- A continuous and steady drop in barometric pressure is one of the earliest signs of an approaching TRS. The pressure drop becomes more rapid as the storm gets closer.
- Cloud Formation:
- High cirrus clouds, often appearing as wisps, followed by thickening clouds and cumulonimbus clouds are indicative of an approaching TRS.
- The sky might look “murky” or hazy as the storm nears.
- Change in Wind Direction and Speed:
- Winds begin to increase in speed and change direction. This shift, known as veering in the Northern Hemisphere or backing in the Southern Hemisphere, indicates that the storm is approaching.
- Winds may become erratic at first and then settle into a more consistent pattern.
- Swell Patterns:
- A noticeable, long-period swell is often the first sign that a TRS is nearby. These swells can precede the storm by several days, originating from the direction of the storm’s center.
- Darkening Sky and Rapid Deterioration of Visibility:
- As the storm approaches, the sky darkens, and visibility decreases due to heavy rainfall or thick clouds.
- Rainbands and squalls associated with the TRS bring rapid changes in weather conditions.
- Sudden Temperature and Humidity Changes:
- A sudden increase in humidity and a noticeable drop in air temperature can occur as the storm draws closer.
- Radio Interference:
- Increased static on radio communications, especially on long-wave and short-wave frequencies, is often observed due to the electrical activity in the storm.
- Halo Around the Sun or Moon:
- A halo around the sun or moon caused by the refraction of light through high cirrus clouds may be visible prior to the arrival of the TRS.
Reporting Requirements as per SOLAS:
The International Convention for the Safety of Life at Sea (SOLAS) sets out specific requirements for ships to report hazardous weather conditions, including the presence of a TRS. The key regulation concerning the reporting of TRS and other dangerous weather phenomena is found in SOLAS Chapter V – Safety of Navigation.
1. Reporting Dangerous Weather Phenomena (SOLAS Chapter V, Regulation 31):
- When a vessel encounters a TRS or any dangerous weather phenomenon, the ship’s master is obligated to send a danger message to nearby ships and relevant shore authorities.
- The danger message must include the following information:
- Type of dangerous phenomena (TRS or other hazardous weather condition).
- Time and position of the observation.
- Wind force and direction.
- Barometric pressure (including the rate of fall in pressure).
- Sea swell and other meteorological conditions (such as visibility, sea state).
- The message should be sent via the ship’s communications system, and it must reach other vessels and shore authorities as soon as possible to alert others in the storm’s vicinity.
2. Weather Reporting and Forecasting (SOLAS Chapter V, Regulation 5):
- Ships are encouraged to transmit weather reports when they encounter unusual or dangerous meteorological conditions, including TRS.
- These reports help improve the accuracy of meteorological forecasts and provide crucial data for other ships in the region. Ships can send reports directly to meteorological offices or through automatic weather-reporting systems.
3. Avoiding TRS Reporting:
If a ship is unable to avoid the TRS, the master should continuously update the ship’s position and intended track to maritime authorities to:
- Aid in the coordination of potential search and rescue efforts.
- Provide authorities and nearby ships with information about the storm’s development.
Buys Ballot’s Law:
Buys Ballot’s Law is a meteorological principle that helps mariners determine the general location of a low-pressure system (such as a tropical revolving storm or TRS) based on wind direction.
- In the Northern Hemisphere: If you stand with your back to the wind, the center of low pressure will be on your left-hand side (slightly behind).
- In the Southern Hemisphere: If you stand with your back to the wind, the center of low pressure will be on your right-hand side (slightly behind).
This principle is useful for determining the direction of a storm’s center and helps mariners plan a course to avoid it.
Veering and Backing of Wind:
These terms refer to changes in wind direction relative to a fixed location, and they are essential in understanding the behavior of wind as a TRS approaches or moves away.
Veering:
- Definition: A wind is said to “veer” when its direction changes in a clockwise manner.
- In the Southern Hemisphere: As a TRS approaches, the wind direction tends to veer. For example, if the wind initially blows from the north, it may shift to northeast and then to east as the storm gets closer.
- Relevance to TRS: Veering is an indicator that a ship is on the left-hand side (dangerous semicircle) of the TRS in the Southern Hemisphere.
Backing:
- Definition: A wind is said to “back” when its direction changes in a counterclockwise manner.
- In the Southern Hemisphere: If a ship is located on the right-hand side (navigable semicircle) of the TRS, the wind will back. For example, it may shift from southeast to east and then to northeast as the TRS moves away.
Conditions Favorable for the Formation of TRS (Both Hemispheres):
1. Warm Ocean Waters
- Sea Surface Temperature: The water temperature must be at least 26.5°C (80°F) to a depth of 50 meters or more. Warm water provides the energy for the storm by fueling convection (the rising of warm, moist air) that forms clouds and thunderstorms.
- Why: Warm water allows for the evaporation of large quantities of moisture, which fuels the storm’s convection processes and provides latent heat needed to drive the TRS.
2. Sufficient Coriolis Effect
- Coriolis Force: The Coriolis effect, caused by the Earth’s rotation, is crucial for initiating the storm’s rotational motion. A TRS cannot form near the equator because the Coriolis force is too weak.
- Formation Latitude: Typically, a TRS forms between 5° and 20° latitude in both the Northern and Southern Hemispheres.
- Why: The Coriolis effect helps initiate and maintain the cyclonic rotation (counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere).
3. Low Vertical Wind Shear
- Wind Shear: Wind shear is the change in wind speed or direction with height. For a TRS to form and intensify, the vertical wind shear must be low (less than 20 knots).
- Why: High wind shear disrupts the vertical alignment of the storm’s convection and can prevent the storm from organizing and intensifying.
4. Pre-existing Disturbance
- Tropical Disturbance: A pre-existing weather disturbance, such as a tropical wave, must be present to provide the initial low-pressure system for the storm to develop around.
- Why: A tropical disturbance provides the necessary instability in the atmosphere for the TRS to begin developing. This initial disturbance often starts as a low-pressure area with thunderstorms.
5. High Relative Humidity
- Moist Mid-Troposphere: A high level of humidity in the mid-troposphere (around 5,000 to 7,000 meters altitude) is essential to sustain deep convection and cloud formation.
- Why: Moist air allows for more efficient convective processes, fueling the storm’s development and intensification.
6. Weak Trade Wind Inversion
- Trade Wind Inversion: An inversion layer prevents cloud formation by capping rising air. A weak inversion or the absence of an inversion promotes convection and helps the storm develop.
- Why: When there is no strong inversion layer, warm air can rise freely, forming thunderstorms that eventually organize into a TRS.
Movement of TRS:
The movement of a Tropical Revolving Storm is influenced by several factors, including prevailing winds, the Coriolis effect, and ocean currents. TRS generally follows a westward track before eventually turning poleward (northward in the Northern Hemisphere, southward in the Southern Hemisphere).
1. Prevailing Winds
- Trade Winds: Initially, TRS moves westward under the influence of the easterly trade winds near the equator.
- In both hemispheres, the storm moves westward and slightly poleward due to the general wind patterns in tropical latitudes.
- Why: Trade winds push the storm westward until it moves into regions influenced by mid-latitude westerlies.
2. The Coriolis Effect
- Northern Hemisphere: The TRS rotates counterclockwise due to the Coriolis effect, and the system typically turns northwestward as it strengthens and moves away from the equator.
- Southern Hemisphere: The TRS rotates clockwise, and the system typically turns southwestward as it moves poleward.
- Why: The Coriolis effect steers the storm toward higher latitudes, curving the storm’s path to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
3. Subtropical Ridge
- Influence of High-Pressure Systems: The subtropical ridge is a high-pressure system that influences the westward and poleward movement of a TRS. The ridge is usually located to the north of the storm in the Northern Hemisphere and to the south in the Southern Hemisphere.
- Why: The ridge acts as a steering mechanism, guiding the storm’s path. If the ridge is strong, it forces the storm to stay on a westward course longer. If the ridge weakens, the storm is more likely to turn poleward.
4. Westerlies (Mid-latitude Westerly Winds)
- Poleward Turn: As the storm reaches higher latitudes, it often encounters the westerlies, which are dominant in mid-latitudes. The westerlies cause the storm to turn towards the poles and, eventually, the east.
- Why: The interaction between the westerlies and the storm’s circulation results in the storm curving poleward and sometimes recurving toward the northeast in the Northern Hemisphere or southeast in the Southern Hemisphere.
Alternate Path of a TRS:
The movement of a TRS can be affected by various atmospheric and oceanographic factors, leading to deviations from the typical path. While TRS generally follows a westward and poleward track, there are situations where an alternate path may occur.
1. Recurvature
- Description: The most common alternate path is a recurvature of the storm, where the TRS shifts from its westward track and turns toward higher latitudes (northward or southward depending on the hemisphere).
- Cause: Recurvature occurs when the TRS is influenced by the presence of mid-latitude westerlies and high-pressure systems. Once the storm moves into regions with stronger upper-level westerly winds, it can be forced to turn toward the poles (north in the Northern Hemisphere, south in the Southern Hemisphere).
- Implication for Mariners:
- Northward Recurvature: In the Northern Hemisphere, a TRS moving westward toward land may suddenly turn northward, potentially heading toward the eastern coasts of North America or Asia.
- Southward Recurvature: In the Southern Hemisphere, the storm may turn southward, affecting regions along southern Indian Ocean or Australian waters.
2. Blocking High
- Description: A blocking high-pressure system can cause the TRS to stall or change direction unexpectedly.
- Cause: When a strong high-pressure ridge forms to the north (or south in the Southern Hemisphere) of the TRS, the storm’s progress is blocked, forcing it to veer off course or slow down significantly. This can lead to erratic motion or even looping behavior.
- Implication for Mariners:
- A blocked TRS may take an unpredictable path, making it challenging for mariners to predict its movements and adjust routes accordingly. The stalling of the TRS also increases the likelihood of extended bad weather conditions in the affected region.
3. Interaction with Other Weather Systems
- Description: A TRS may interact with other low-pressure systems or troughs, causing it to deviate from its usual path.
- Cause: When two low-pressure systems come into proximity, they can influence each other’s movement through a process known as the Fujiwhara effect. This may result in the TRS moving in an unpredictable manner.
- Implication for Mariners:
- Mariners need to pay close attention to weather forecasts and monitoring systems, as the interaction between systems can lead to unexpected turns or accelerations in the TRS’s path.
4. Extratropical Transition
- Description: A TRS may undergo an extratropical transition, changing its characteristics and taking on a more mid-latitude cyclone form.
- Cause: As the TRS moves into colder waters and interacts with the jet stream or other frontal systems, it can lose its tropical characteristics and become an extratropical cyclone.
- Implication for Mariners:
- The storm may become broader in size but with less intense winds. However, the interaction with cold fronts and other systems can still produce strong winds and rough seas, requiring careful route planning.
5. Slow or Accelerated Movement
- Description: While most TRS follow predictable tracks, they may sometimes slow down or speed up due to changes in steering winds or pressure systems.
- Cause: The speed of a TRS can be influenced by the strength of steering winds (trade winds or westerlies) and the presence of high-pressure systems that either block or accelerate the storm’s movement.
- Implication for Mariners:
- Slower-moving TRS can cause prolonged periods of heavy rain and rough seas, while faster-moving storms may give less time to react for avoidance.
Movement of a Tropical Revolving Storm (TRS) in Northern Hemisphere:
The sketch above illustrates the movement of a Tropical Revolving Storm (TRS) in the Northern Hemisphere
In the Northern Hemisphere, a tropical revolving storm (TRS) typically moves in a characteristic path, driven by several factors, including the Earth’s rotation, prevailing winds, and pressure systems. The general movement can be broken down into two phases:
1. Formation and Initial Movement:
- Tropical revolving storms typically form in the low-latitude regions, between 5° and 15° North, where warm ocean waters (at least 26°C) and high humidity fuel the development of the storm.
- Initially, the TRS is steered westward by the trade winds in the tropical regions. These winds blow from east to west, causing the storm to move westward, often towards the Caribbean, Central America, or Southeast Asia.
2. Coriolis Effect and Curvature:
- Due to the Coriolis Effect, caused by the Earth’s rotation, the storm system begins to rotate counterclockwise (cyclonic) in the Northern Hemisphere.
- As the storm strengthens and moves further north, it begins to curve. The Coriolis Effect increases with latitude, causing the storm to gradually turn to the right (northward).
3. Typical Track:
- Westward Drift (Phase 1): The storm moves westward, influenced by the easterly trade winds, and typically maintains a westward trajectory as it develops in the tropical regions.
- Northwestward Turn (Phase 2): As the storm intensifies and drifts north of 20° latitude, it often encounters a subtropical high-pressure ridge. The interaction with this system causes the storm to start curving northwestward.
- Northeastward Recurvature (Phase 3): Once the TRS reaches higher latitudes, around 25° to 30° North, it begins to interact with the mid-latitude westerlies (winds from the west) and turns more sharply northeastward. This is commonly referred to as the “recurvature phase.” The storm may then move towards the North Atlantic, eastern coastlines, or the northern Pacific.
4. Decay and Dissipation:
- As the TRS moves further north or makes landfall, it loses its primary energy source—the warm ocean waters. Cooler sea temperatures, friction with land, or interaction with upper-level winds lead to the storm’s weakening and eventual dissipation.
The overall path of the TRS in the Northern Hemisphere typically follows a parabolic shape: first moving west, then curving northwest, and eventually turning northeast before dissipating.
Curving and re-curving of a Tropical Revolving Storm (TRS):
Curving and re-curving of a Tropical Revolving Storm (TRS) are determined by the interaction between the storm and the surrounding atmospheric conditions, particularly wind patterns and pressure systems. Below are the main factors that contribute to the curving and re-curving of a TRS:
- Trade Winds and Initial Curving
- Trade Winds: In the tropical regions, the prevailing easterly trade winds (blowing from east to west) push the TRS in a westward direction. Initially, a TRS follows this westward path as it forms and moves through the tropical latitudes.
- Coriolis Effect: The Coriolis force caused by Earth’s rotation contributes to the curving of the TRS. In the Northern Hemisphere, the Coriolis effect causes the storm to curve northward (to the right of its initial path), while in the Southern Hemisphere, it causes the storm to curve southward (to the left).
- Influence of Mid-Latitude Westerlies (Re-curving)
- Re-curving occurs when the TRS moves out of the tropics and enters the mid-latitudes. Here, the prevailing winds shift from easterlies to westerlies (winds that blow from west to east).
- Upper-Level Westerlies: As the TRS reaches higher latitudes, it interacts with the mid-latitude westerlies in the upper atmosphere. The westerlies push the storm from a westward direction to an eastward direction, causing the storm to re-curve. The storm’s path bends or recurves to the northeast in the Northern Hemisphere and to the southeast in the Southern Hemisphere.
- Interaction with High-Pressure Systems
- Subtropical High-Pressure Ridge: A subtropical high-pressure ridge usually exists to the north of the TRS in the Northern Hemisphere (and to the south in the Southern Hemisphere). This high-pressure system helps guide the TRS along its initial westward path. However, as the storm moves poleward, the ridge weakens or shifts, allowing the TRS to re-curve.
- Weakening of the Ridge: If the subtropical high weakens, the TRS can more easily shift into the mid-latitudes, where the westerlies take over, causing re-curvature. This typically happens when the storm encounters a break in the high-pressure system or a trough of low pressure.
- Steering by Troughs
- Troughs: A trough of low pressure in the mid-latitudes can help steer the TRS and cause re-curvature. When the storm encounters a trough, it is drawn toward the low-pressure area, accelerating the re-curving process.
Increasing in speed of TRS after re-curving:
After a TRS re-curves, its forward speed generally increases, particularly as it moves toward higher latitudes. Several factors contribute to this acceleration:
1. Transition from Trade Winds to Westerlies
- Initial Movement in the Tropics: In the tropics, the TRS moves relatively slowly because the trade winds that guide its movement are not particularly strong.
- Westerlies: Once the TRS re-curves and enters the mid-latitudes, it is steered by the stronger westerly winds that dominate this region. These upper-level westerlies are faster than the trade winds, causing the storm to move more quickly as it is carried eastward by these winds.
2. Increase in Latitude and Jet Stream Influence
- Stronger Winds at Higher Latitudes: As the TRS moves poleward into the mid-latitudes, it enters regions with stronger atmospheric dynamics, including faster-moving winds in the upper atmosphere. These winds help accelerate the storm’s forward motion.
- Jet Stream Influence: The TRS may come under the influence of the polar jet stream, a fast-moving ribbon of air at high altitudes. When the storm interacts with the jet stream, its speed increases dramatically due to the strong eastward winds of the jet stream.
3. Extratropical Transition
- Change in Dynamics: After re-curving, many TRS systems begin an extratropical transition, where they lose their tropical characteristics and become more like mid-latitude low-pressure systems. This transition often results in an increase in speed as the storm is influenced by the faster-moving mid-latitude weather systems and the jet stream.
- Larger Storm System: As the storm transitions and interacts with other frontal systems, it often grows in size and becomes more organized, enabling it to move more rapidly as a larger extratropical storm.
TRS in different parts of the world:
Tropical Revolving Storms (TRS) are referred to by different names depending on the region of the world where they occur:
- Hurricanes:
- Location: North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and Eastern Pacific Ocean.
- Example: Hurricane Katrina (2005).
- Typhoons:
- Location: Northwest Pacific Ocean (East Asia, Japan, the Philippines, and China).
- Example: Typhoon Haiyan (2013).
- Cyclones:
- Location: Indian Ocean, South Pacific Ocean, and around Australia.
- Southwest Indian Ocean: Madagascar, Mauritius, and surrounding areas (e.g., Cyclone Idai, 2019).
- Bay of Bengal and Arabian Sea: Affecting India, Bangladesh, Myanmar, and Pakistan (e.g., Cyclone Amphan, 2020).
- Australia/South Pacific: Affecting Australia and nearby Pacific islands (e.g., Cyclone Yasi, 2011).
- Willy-Willies:
- Location: North-western Australia.
- Example: Term used specifically for smaller storms in the region.
Areas Where Tropical Storms Frequently Occur, their Yearly Frequency and Local Names
Tropical Revolving Storms (TRS), known by different names around the world, occur in various tropical and subtropical regions. These storms are generally associated with warm ocean waters, and their occurrence is influenced by atmospheric conditions, such as low wind shear and strong Coriolis forces.
1. North Atlantic Ocean
- Region: North Atlantic, including the Caribbean Sea, Gulf of Mexico, and the eastern coast of the United States.
- Local Name: Hurricanes
- Yearly Frequency: 10–12 storms per year.
- Storm Season: June to November, with a peak from August to October.
2. Northeast Pacific Ocean
- Region: Eastern Pacific, along the western coast of Mexico and Central America.
- Local Name: Hurricanes
- Yearly Frequency: 15–16 storms per year.
- Storm Season: May to November, peaking in late summer.
3. Northwest Pacific Ocean
- Region: Western Pacific Ocean, including the Philippines, Japan, China, and Taiwan.
- Local Name: Typhoons
- Yearly Frequency: 25–30 storms per year (the most active region for tropical cyclones).
- Storm Season: Year-round, with peaks from July to October.
4. North Indian Ocean
- Region: Bay of Bengal and Arabian Sea.
- Local Name: Cyclones
- Yearly Frequency: 4–6 storms per year.
- Storm Season: May to June and October to November.
5. Southwest Indian Ocean
- Region: Off the eastern coast of Africa, Madagascar, and the islands of the southwest Indian Ocean (Mauritius, Seychelles).
- Local Name: Cyclones
- Yearly Frequency: 10–12 storms per year.
- Storm Season: November to April.
6. Southeast Indian Ocean
- Region: Off the northwestern coast of Australia.
- Local Name: Cyclones
- Yearly Frequency: 7–10 storms per year.
- Storm Season: November to April.
7. Southwest Pacific Ocean
- Region: Near Australia, Fiji, New Caledonia, and the Solomon Islands.
- Local Name: Cyclones
- Yearly Frequency: 7–10 storms per year.
- Storm Season: November to April.
Regions Free of Tropical Storms and Reasons
1. The South Atlantic Ocean
- Reason: Tropical storms rarely form in the South Atlantic due to high wind shear, cold sea surface temperatures, and a weak Coriolis effect in this region. The only recorded cyclone in this region was Cyclone Catarina in 2004.
2. The Southeast Pacific Ocean
- Reason: This region, particularly along the west coast of South America, experiences cooler ocean waters due to the Humboldt Current and high wind shear that prevents the formation of tropical storms.
3. Near the Equator
- Reason: Tropical storms cannot form within about 5° of the equator because the Coriolis effect (the force responsible for the storm’s rotation) is too weak. The Coriolis force is essential for the development of the cyclonic motion of a TRS.
Differences in TRS Formation and Movement in Northern and Southern Hemispheres:
1. Rotation Direction:
- Northern Hemisphere: TRS rotates counterclockwise.
- Southern Hemisphere: TRS rotates clockwise.
2. Poleward Movement:
- Northern Hemisphere: As the storm moves poleward, it typically curves northwestward and then may recurve toward the northeast if it interacts with westerlies.
- Southern Hemisphere: As the storm moves poleward, it curves southwestward and may eventually turn southeastward when encountering the westerlies.
3. Latitude of Formation:
- TRS typically forms between 5° and 20° latitude in both hemispheres due to the necessity of the Coriolis force to initiate rotation.
Steps to Avoid the Storm Center:
- Turn to Starboard (Wind on Starboard Bow):
- Alter your course so that the wind is on the starboard bow (45° to the right of the bow). This maneuver helps you to move away from the storm center while keeping the ship stable.
- Why: In the Northern Hemisphere, with a counterclockwise rotating storm, steering into the wind at this angle will take you away from the center.
- Increase Speed:
- Maintain the highest safe speed possible to escape the dangerous quadrant quickly. The faster you move, the sooner you can create distance between your ship and the storm’s center.
- Monitor Wind and Barometric Pressure:
- Continuously track the changes in wind direction and speed. As you alter course, observe whether the wind is veering (clockwise shift). If the wind is veering, you are successfully moving out of the dangerous semicircle.
- A steady fall in barometric pressure indicates the storm is approaching, so it’s vital to keep monitoring.
- Avoid Crossing the Storm’s Path:
- Do not attempt to cross the storm’s projected track. Instead, maintain your course to keep moving away from the center of the storm.
Actions to Avoid the Eye of the TRS in the Southern Hemisphere:
In the Southern Hemisphere, the rotation of the wind around a TRS is clockwise, and the TRS generally moves in a westerly direction with a southward or southwestward turn.
Identifying the Dangerous and Navigable Semicircles:
- Dangerous Semicircle:
- This is the left-hand side of the TRS (relative to its direction of movement). Winds veer (change clockwise), and the ship will face stronger winds and more dangerous conditions.
- Navigable Semicircle:
- This is the right-hand side of the TRS (relative to its direction of movement). Winds back (change counterclockwise), and conditions are somewhat less dangerous, although still hazardous.
Steps to Avoid the TRS Eye:
- Assessing the Position:
- Using Buys Ballot’s Law: If you’re in the Southern Hemisphere and face the wind with your back to it, the storm center will be to your right (and slightly behind).
- If the wind is veering, you are likely in the dangerous semicircle. If the wind is backing, you are in the navigable semicircle.
- Course of Action:
- If in the Dangerous Semicircle (left-hand side):
- Alter course so that you move with the wind coming from the starboard bow (approx. 45 degrees). This will take you away from the TRS center.Increase speed to distance yourself as much as possible from the storm.
- If in the Navigable Semicircle (right-hand side):
- Alter course so that you move with the wind coming from the starboard quarter (approx. 135 degrees). This will take you away from the TRS center.
- Use high speed to ensure a safe distance from the storm.
- If Directly in the Path of the Storm:
- If you find yourself directly in the path of the TRS, your primary goal is to avoid the storm’s center or eye.
- You should alter course to place yourself in the navigable semicircle. To do this, steer a course at an angle that keeps the wind on your starboard quarter. Maintain maximum safe speed to outrun the storm’s core.
- Monitor Weather Data:
- Continuously monitor barometric pressure, wind speed, and direction to gauge the proximity of the TRS. A rapid drop in pressure means the TRS is getting closer.
TRS: Evasive Actions in the Northern Hemisphere:
1. In the Dangerous Semicircle (left side of the storm):
- Situation: In the Northern Hemisphere, the TRS rotates counterclockwise, and the dangerous semicircle is on the left side of the storm’s path.
- Actions:
- Alter course so that the wind is on the starboard bow (approximately 45°).
- Steer away from the center of the storm, aiming to move out of the dangerous semicircle.
- Maintain maximum safe speed to create distance from the storm as quickly as possible.
2. In the Navigable Semicircle (right side of the storm):
- Situation: In the Northern Hemisphere, the navigable semicircle is on the right-hand side of the storm’s path.
- Actions:
- Alter course so that the wind is on the starboard quarter (approximately 135°).
- Use this position to move away from the storm center while taking advantage of the lighter wind conditions.
- Maintain a steady course and safe speed to outrun the storm.
3. If the Vessel is Ahead of the Storm:
- Situation: If you are in the direct path of the storm.
- Actions: Steer so that the wind is on your stern and head at maximum safe speed to move away from the TRS’s projected path.
Vessel Action to take if inside the dangerous quadrant in Northern Hemisphere:
If your vessel is inside the dangerous quadrant of a Tropical Revolving Storm (TRS) in the Northern Hemisphere, you must take immediate and decisive action to steer clear of the storm’s center. In the Northern Hemisphere, TRS rotates counterclockwise, and the dangerous quadrant is located in the right front of the storm’s path, where the storm’s rotational wind speed adds to its forward movement, making this quadrant particularly hazardous.
Steps to Take if Inside the Dangerous Quadrant in the Northern Hemisphere:
- Determine Your Position:
- Using Buys Ballot’s Law: In the Northern Hemisphere, if you stand with your back to the wind, the low-pressure center (the storm’s eye) will be on your left-hand side and slightly behind.
- If the wind is veering (shifting in a clockwise direction), you are in the dangerous quadrant.
- Alter Course to the Starboard Bow:
- The key goal is to steer away from the storm’s center and move out of the dangerous quadrant.
- Turn the ship so that the wind is on the starboard bow (approximately 45° to the right of the bow).
- This course will take you diagonally away from the storm’s center, helping you exit the dangerous quadrant as quickly as possible.
- Avoid heading directly into the wind, as this can put you on a dangerous course that risks running closer to the storm center.
- Increase Speed to Maximize Distance:
- Increase the ship’s speed to the maximum safe speed based on the ship’s conditions and weather.
- The faster you move, the sooner you can escape the dangerous quadrant and reach safer waters.
- Continuously Monitor the Weather:
- Keep monitoring barometric pressure and wind direction:
- A continuously falling pressure indicates you are still near the storm’s core, and immediate action is still required.Wind direction shifts: As the storm moves, make course adjustments based on the wind’s veering or backing behavior.
- Weather reports: Use up-to-date satellite or meteorological reports to stay informed about the storm’s trajectory and changes in intensity.
- Avoid Crossing the Storm’s Path:
- If possible, do not attempt to cross the TRS’s projected path, especially not from the dangerous quadrant. Doing so risks steering directly into the storm’s eye or the more dangerous side of the storm.
- General Guidelines While in the Dangerous Quadrant:
- Avoid head-on confrontation: Steer a course away from the storm rather than attempting to face it head-on.
- Keep the storm on your starboard side: Maintain a course that ensures the wind remains on the starboard bow to gradually take you out of the dangerous quadrant.
Vessel Action to take if inside the dangerous quadrant in Southern Hemisphere:
1. In the Dangerous Semicircle (right side of the storm):
- Situation: In the Southern Hemisphere, the TRS rotates clockwise, and the dangerous semicircle is on the right-hand side of the storm’s path.
- Actions:
- Alter course so that the wind is on the starboard bow (approximately 45°).
- Steer away from the storm center, aiming to exit the dangerous semicircle.
- Maintain high speed to increase the distance from the storm.
2. In the Navigable Semicircle (left side of the storm):
- Situation: In the Southern Hemisphere, the navigable semicircle is on the left-hand side of the storm’s path.
- Actions:
- Alter course so that the wind is on the starboard quarter (approximately 135°).
- Continue to move away from the storm while taking advantage of weaker winds in this semicircle.
- Maintain a safe speed and course to maximize the distance from the storm.
3. If the Vessel is Ahead of the Storm:
- Situation: If the vessel is directly in the path of the TRS.
- Actions:
- Alter course so that the wind is on your stern and steer away from the storm at maximum speed to avoid being caught in the eye of the storm.
Describe the action to be taken if a vessel in the southern hemisphere is in the path of a storm, experiencing winds on the Beaufort scale of 5, assuming navigable waters all around:
When a vessel in the Southern Hemisphere is in the path of a storm and experiencing winds at Beaufort scale 5 (17-21 knots, moderate conditions), specific actions need to be taken to avoid the storm center and ensure the safety of the vessel, assuming navigable waters all around.
Actions to Avoid the Storm in the Southern Hemisphere
1. Determine Your Position Relative to the Storm
- Buys Ballot’s Law: In the Southern Hemisphere, when you stand with your back to the wind, the center of the low-pressure system (storm) will be on your right and slightly behind you.
- Determine if You Are in the Dangerous or Navigable Semicircle:
- Dangerous Semicircle: The left-hand side of the storm when facing the direction of its movement (wind blows from your right to left). This is where winds are stronger and more hazardous.
- Navigable Semicircle: The right-hand side of the storm (wind blows from your left to right). This area is comparatively safer but still requires caution.
2. Alter Course Away from the Storm Center
- If in the Dangerous Semicircle:
- Action: Alter course to starboard (right) to place the storm on your port side (left side). Head with the wind on your starboard quarter (behind and to the right). This will help you move away from the storm’s center while keeping it on your port side.
- If in the Navigable Semicircle:
- Action: Alter course to port (left) so the storm remains on your starboard (right). Head into the wind at a slight angle, with the wind on your starboard bow (right front). This helps you steer a course that avoids the storm’s stronger side.
3. Reduce Speed
- Reduce Speed: Reducing speed helps reduce the impact of waves and improves the handling of the vessel, especially when encountering heavy seas. Maintaining control of the ship is essential when navigating in rough weather conditions.
4. Secure the Vessel
- Secure Cargo and Deck Equipment: Ensure that all cargo, deck equipment, and loose objects are properly secured to avoid damage or accidents due to rolling or pitching.
- Check Engine and Steering: Ensure the ship’s machinery, especially engines and steering systems, are fully operational to maintain maneuverability in rough seas.
5. Monitor Weather Reports
- Constantly Monitor Weather Updates: Stay updated with meteorological forecasts, storm warnings, and navigational advisories via radio, weather facsimile, or satellite communications to track the storm’s movement and adjust course as necessary.
6. Maintain a Safe Distance from the Storm
- Steer Clear of the Storm’s Center: Your goal is to navigate away from the storm’s path, keeping a safe distance from the low-pressure center where the strongest winds and waves are located.
7. Communication
- Inform Authorities: Notify the vessel’s company or relevant maritime authorities about the storm encounter and actions taken to avoid the storm.
Practical rules for avoiding the TRS eye (Southern Hemisphere):
In the Southern Hemisphere, the Tropical Revolving Storm (TRS) rotates clockwise, and taking practical actions to avoid the TRS’s eye is critical for safety. The following practical rules will help mariners in evading the TRS eye based on their position relative to the storm:
1. Identify Your Position Relative to the Storm
- Buys Ballot’s Law: In the Southern Hemisphere, if you stand with your back to the wind, the low-pressure area (or the TRS center) will be on your right-hand side and slightly behind.
- This law helps you determine whether you are in the dangerous semicircle (right side of the storm’s track) or the navigable semicircle (left side of the storm’s track).
2. Evasive Actions Based on the Semicircle You Are In
A. If in the Dangerous Semicircle (Right-hand side of the storm): The dangerous semicircle is the side where the TRS’s forward motion adds to the wind speed, making conditions more dangerous.
- Actions:
- Alter course so that the wind comes from your starboard bow (approximately 45° to the right of the bow).
- Steer in a direction that takes you away from the storm center. This course should lead you in a direction opposite to the storm’s movement.
- Increase speed to maximize distance from the TRS. The faster you move, the better your chances of escaping the storm’s most dangerous conditions.
- Continuously monitor changes in wind direction and pressure to adjust your course accordingly as the TRS moves.
B. If in the Navigable Semicircle (Left-hand side of the storm): The navigable semicircle is less dangerous since the forward motion of the TRS reduces the wind speed in this area.
- Actions:
- Alter course so that the wind comes from your starboard quarter (approximately 135° to the right of the stern).
- Move in a direction that keeps the wind on your starboard quarter and takes you away from the storm center.
- Increase speed to safely distance yourself from the storm.
- Continue to monitor the weather closely for any changes in the storm’s movement and adjust your course as necessary.
3. If Directly Ahead of the Storm’s Path (On the storm’s track)
If you find yourself in the direct path of the TRS, you are in the most dangerous position and should take immediate action to avoid being caught in the storm’s eye.
- Actions:
- Alter course so that the wind comes from directly astern (wind from behind).
- Steer away from the storm at maximum safe speed. This course should take you out of the storm’s track and into the navigable semicircle or completely away from the storm.
- Monitor pressure and wind changes to track the storm’s progress, and make course adjustments to stay out of the storm’s path.
4. General Guidelines for Avoiding the TRS Eye
- Monitor Weather Reports: Make use of weather information (satellite, meteorological reports, and navigational warnings) to stay informed about the TRS’s position and movement. Early detection can give you time to make evasive actions.
- Barometric Pressure Monitoring: Keep a close eye on the barometer. A sudden and steady fall in pressure indicates that the storm is getting closer.
- Maintain Maximum Safe Speed: Once you identify your position and take evasive action, speed is crucial. Moving quickly away from the storm’s center is the best way to avoid the TRS’s most dangerous areas.
- Track the Wind Changes: As you steer away from the storm, continuously monitor wind direction and speed changes. Adjust your course accordingly based on whether the wind is veering (clockwise shift) or backing (counterclockwise shift). In the Southern Hemisphere, veering winds indicate you are in the dangerous semicircle.
- Avoid Head-on Encounters: If possible, take actions well in advance of the storm’s approach to avoid being directly ahead of the TRS’s path.
5. Avoiding the TRS Eye – A Step-by-Step Example in the Southern Hemisphere
- Initial Situation: You are in the Southern Hemisphere, facing a TRS. Winds are veering (changing clockwise).
- Action: You conclude that you are in the dangerous semicircle. Immediately alter your course so that the wind is on your starboard bow (45°).
- Steering: Head in a direction that takes you away from the storm center.
- Speed: Increase speed to quickly move away from the storm.
- Monitoring: Continuously check barometric pressure and wind direction for changes that may require further course adjustments.
Reasons for Naming the “Dangerous Quadrant” in a Tropical Revolving Storm (TRS)
The “Dangerous Quadrant” of a Tropical Revolving Storm (TRS) is the part of the storm where the wind and sea conditions are most hazardous for ships and coastal regions. This quadrant is named “dangerous” due to the combination of factors that make it especially perilous for navigation and safety. Below are the key reasons why this quadrant is considered the most dangerous:
1. Strongest Wind Speeds
- The dangerous quadrant of a TRS is typically found on the right-hand side of the storm’s path (in the Northern Hemisphere) or the left-hand side (in the Southern Hemisphere) when facing the direction of the storm’s movement.
- Reason: In this quadrant, the forward movement of the storm adds to the rotational wind speed, producing the strongest winds. The winds here are significantly stronger than on the opposite side of the storm due to the combination of the storm’s rotational speed and its forward velocity.
2. Highest Sea States
- The dangerous quadrant experiences the roughest seas due to the combination of strong winds and storm surge.
- Reason: The intense winds in this quadrant create higher waves and steeper swells, which are hazardous for ships, especially smaller vessels. Additionally, the interaction between wind and ocean surface increases wave height, making navigation extremely difficult.
3. Strongest Storm Surge
- Storm surge is the abnormal rise of sea level caused by the storm’s low pressure and high winds pushing water toward the coast.
- Reason: The dangerous quadrant generates the most powerful storm surge, as the strong winds in this region push more water toward the coastline. This makes coastal areas particularly vulnerable to flooding and damage from the surge.
4. Heavy Precipitation and Squalls
- The dangerous quadrant is often accompanied by heavy rain, squalls, and thunderstorms that can significantly reduce visibility and create hazardous navigation conditions.
- Reason: The strong upward movement of warm, moist air in this quadrant leads to the formation of thick cloud bands and intense precipitation, which can overwhelm a ship’s drainage systems and lead to flooding on deck.
5. Higher Risk of Tornadoes
- Reason: The right front quadrant of a TRS (dangerous quadrant in the Northern Hemisphere) is also more likely to produce tornadoes due to the increased instability in this area. Tornadoes can develop from the intense thunderstorms embedded within the storm, further increasing the risks.
6. Forward Speed Amplifies Wind Speed
- Reason: In the dangerous quadrant, the storm’s forward speed adds to the rotational wind speed. For example, if the TRS has a rotational wind speed of 80 knots and a forward movement of 20 knots, the wind speed in the dangerous quadrant would be 100 knots. This creates more violent wind conditions than other quadrants of the storm.
Action to avoid a TRS in the Northern Hemisphere near the point of re-curvature:
Actions to Take Near the Point of Re-curvature:
1. Identify Your Position Relative to the Storm
- Use Buys Ballot’s Law: Stand with your back to the wind, and in the Northern Hemisphere, the center of the low-pressure system (TRS eye) will be to your left-hand side and slightly behind. This will help you identify whether you are in the dangerous semicircle (right side of the storm) or the navigable semicircle (left side of the storm).
- Dangerous Semicircle (right-hand side of the storm): Winds and seas are stronger here due to the additive effect of the storm’s rotational speed and its forward movement.
- Navigable Semicircle (left-hand side of the storm): Conditions are relatively less severe in this region, as the storm’s forward movement partially cancels out the rotational wind speed.
2. If You Are in the Dangerous Semicircle (Right-hand side)
- Actions:
- Turn to starboard: Alter your course so that the wind is on your starboard bow (45° to the right of the bow). This maneuver helps move the ship out of the dangerous semicircle and steers you away from the storm’s center.
- Increase speed: Maximize your ship’s safe speed to quickly distance yourself from the TRS. The faster you can move out of the storm’s path, the better.
- Monitor wind and pressure: Continuously track the veering of the wind. If the wind continues to veer, you are still in the dangerous semicircle, and further adjustments are needed. A steady drop in barometric pressure indicates the storm is still nearby.
- Avoid crossing the storm’s path: Do not attempt to move across the storm’s path to the navigable semicircle. This could put you at greater risk, especially near the point of re-curvature where storm intensity can rapidly increase.
3. If You Are in the Navigable Semicircle (Left-hand side)
- Actions:
- Turn to starboard: Adjust your course so that the wind is on your starboard quarter (135° to the right of the stern). This course allows you to move away from the storm while staying in the relatively safer navigable semicircle.
- Increase speed: Use the ship’s maximum safe speed to further distance yourself from the storm. The goal is to exit the storm’s influence before the re-curvature intensifies its effects.
- Monitor wind and pressure: Ensure the wind is backing (shifting counterclockwise), indicating that you are moving away from the storm. If pressure starts to rise and wind backs, this confirms you are successfully escaping the storm’s influence.
4. If You Are Directly Ahead of the Storm Near Re-curvature
- Actions:
- Turn to place the wind directly astern: If you are ahead of the storm, it’s critical to move as quickly as possible to avoid being caught in its path. Steer a course so that the wind is on your stern.
- Increase speed: Maximize your speed to move directly away from the storm as it recurs. The goal is to avoid being caught near the eye or the most intense part of the storm as it turns northeastward.
- Monitor storm reports: Keep track of updated weather reports and forecasts. Near the point of re-curvature, the storm’s speed and path can change rapidly, so stay informed and adjust your course accordingly.
5. General Guidelines for Avoiding a TRS Near the Point of Re-curvature
- Early Detection: Monitor weather forecasts, barometric pressure, and wind shifts carefully to detect changes in the storm’s path and intensity. The point of re-curvature is where significant changes occur, and timely detection gives you the advantage of planning evasive actions early.
- Avoid Head-on Confrontations: If you detect that you are near the projected point of re-curvature, steer away early to avoid being caught in the storm as it makes its poleward and eastward turn.
- Use Maximum Safe Speed: Speed is critical in avoiding the storm’s center. Use the ship’s full capabilities to exit the dangerous quadrant and steer away from the storm.
Impact of the Intertropical Convergence Zone (ITCZ):
The Intertropical Convergence Zone (ITCZ) plays a critical role in global weather patterns and is particularly influential in the formation of Tropical Revolving Storms (TRS) and other tropical weather phenomena. The ITCZ is a region near the equator where the northeast trade winds of the Northern Hemisphere converge with the southeast trade winds of the Southern Hemisphere. This convergence leads to significant vertical uplift of warm, moist air, causing frequent thunderstorms, heavy rainfall, and areas of low pressure.
- Weather Systems and Convection:
- The ITCZ is characterized by intense convection, as warm, moist air from the surface rises into the atmosphere. This rising air cools and condenses, forming clouds and thunderstorms. The ITCZ is responsible for much of the world’s tropical rainfall.
- In the ITCZ, tropical waves and disturbances can develop, which may lead to the formation of TRS under favorable conditions.
- Seasonal Shifts:
- The ITCZ moves north and south of the equator throughout the year, following the seasonal movement of the sun. During summer, it shifts toward the hemisphere experiencing warmer temperatures, and during winter, it shifts back toward the equator. This shifting affects rainfall patterns, especially in tropical regions, leading to wet and dry seasons.
- For example, in the Indian subcontinent, the movement of the ITCZ toward the north during the summer months triggers the Southwest Monsoon, while its retreat leads to the dry season.
- Formation of TRS:
- The ITCZ is a breeding ground for disturbances that can develop into Tropical Revolving Storms (TRS). The warm, moist air in this region provides the necessary fuel for cyclonic activity.
- However, TRS do not typically form directly on the ITCZ, because Coriolis force is minimal near the equator. TRS formation is most likely to occur at latitudes between 5° and 20° north or south of the equator, where the Coriolis effect is strong enough to create cyclonic rotation.
- Influence on Global Wind Patterns:
- The ITCZ is part of the larger atmospheric circulation system, connecting the Hadley Cells of each hemisphere.
- The ITCZ’s position affects the strength and direction of the trade winds, and any shifts in the ITCZ can cause variations in global wind patterns.
Why the ITCZ Cannot Be Termed an Equatorial Front:
While the ITCZ shares some characteristics with meteorological fronts, it cannot be accurately described as an equatorial front for several reasons:
- No Significant Temperature Gradient:
- A meteorological front, such as a cold or warm front, occurs where there is a significant temperature difference between two air masses (e.g., cold polar air and warm tropical air). In the ITCZ, however, the air masses converging from both hemispheres are both tropical and relatively homogeneous in temperature. This lack of a marked temperature contrast is one of the main reasons the ITCZ is not considered a front.
- Convergence of Similar Air Masses:
- In a typical front, different air masses with distinct properties (temperature, humidity) collide. However, in the ITCZ, the converging winds (trade winds) from the Northern and Southern Hemispheres are both warm and moist tropical air masses. This convergence does not involve the mixing of dramatically different air masses, as would be the case with a traditional front.
- Location and Characteristics:
- The ITCZ is generally located near the equator but can shift north or south depending on the season. It is a band of low pressure with intense convection rather than a distinct boundary between two air masses, which is characteristic of a front.
- A meteorological front is typically characterized by a distinct boundary line where one air mass overtakes another, but the ITCZ is more of a diffuse area of convergence rather than a well-defined boundary.
- Vertical Motion and Convection:
- Unlike fronts, which are often associated with horizontal movement of air masses (cold air pushing under warm air or warm air sliding over cold air), the ITCZ is primarily associated with vertical motion due to the convergence and uplift of moist tropical air.
- The intense convective activity (rising air that forms thunderstorms) in the ITCZ differs from the sloping air mass movements found in a traditional front.
Decay Factors Affecting a TRS:
A. Moving Over Land (Landfall):
- Description: When a TRS moves over land, it loses contact with the warm ocean waters that provide the energy needed to sustain it.
- Effects:
- Loss of Moisture: Over land, there is no longer an abundant supply of warm, moist air. This reduces the latent heat that fuels the storm, leading to rapid weakening.
- Increased Friction: The rougher terrain over land increases friction, disrupting the storm’s circulation and further contributing to its decay.
- Dry Air: Landmasses typically have drier air, which inhibits the development of the deep convection needed to maintain a TRS.
- Example: Hurricanes that make landfall in coastal areas often weaken significantly after crossing into the interior.
B. Moving Over Cooler Water:
- Description: A TRS relies on warm sea surface temperatures (at least 26.5°C or 80°F) for energy. As it moves into regions with cooler water, it loses the heat source necessary to maintain its strength.
- Effects:
- Reduction in Energy: Cooler waters limit the evaporation and convection processes that drive the storm. Without these processes, the storm’s intensity decreases.
- Example: Hurricanes moving northward from the tropics into temperate zones often weaken as they pass over cooler waters of the North Atlantic or Pacific.
C. Increased Wind Shear:
- Description: Wind shear refers to changes in wind speed or direction at different altitudes. While TRS thrive in environments with low vertical wind shear, increased wind shear can disrupt the vertical structure of the storm.
- Effects:
- Disruption of the Storm’s Structure: Strong wind shear tilts the storm’s core, preventing it from aligning its convection and reducing the storm’s ability to intensify.
- Decoupling of the Storm: When upper-level winds move faster than lower-level winds, the TRS’s upper portion can be “blown off,” preventing the storm from organizing.
- Example: Many tropical storms that encounter strong wind shear are unable to maintain their structure and quickly weaken.
D. Dry Air Intrusion:
- Description: Tropical storms require moist air to sustain convection. Dry air, which can come from surrounding high-pressure systems or from moving over land, inhibits this process.
- Effects:
- Suppression of Convection: When dry air gets entrained into the storm’s circulation, it reduces the storm’s ability to produce thunderstorms and clouds, leading to weakening.
- Example: Dry air in the mid-latitudes or over deserts (like the Sahara Air Layer in the Atlantic) can disrupt tropical storms, weakening them quickly.
E. Interaction with Mid-Latitude Systems (Extratropical Transition):
- Description: As a TRS moves away from the tropics, it may interact with mid-latitude systems, such as cold fronts or high-pressure ridges.
- Effects:
- Extratropical Transition: The storm can transition into an extratropical cyclone, losing its tropical characteristics (such as warm core and symmetric structure). This transition weakens the storm but can also expand its wind field, making it broader but less intense.
- Example: Hurricanes that move into the North Atlantic often become extratropical cyclones, losing their tropical characteristics but still causing heavy rain and strong winds.
F. Absence of Upper-Level Divergence:
- Description: TRS thrive when there is strong upper-level divergence (air flowing outward at the top of the storm), which allows rising air and convection at the surface. When upper-level divergence weakens or becomes absent, the storm struggles to sustain itself.
- Effects:
- Suppression of Development: Without upper-level divergence, air is unable to rise efficiently, reducing the convection that drives the storm.
- Example: Storms moving into regions with weak upper-level support typically weaken.
Conditions Affecting the Movement of a TRS:
A. Steering by Trade Winds:
- Description: In the tropics, TRS are primarily steered by the easterly trade winds, which push them westward and slightly poleward.
- Effects:
- Westward Movement: TRS in both hemispheres generally follow a westward track, moving toward landmasses, islands, or open ocean. Their path depends on the strength and position of the trade winds.
- Example: Hurricanes in the Atlantic typically move westward toward the Caribbean or Gulf of Mexico due to the easterly trade winds.
B. Coriolis Effect:
- Description: The Coriolis effect, caused by the Earth’s rotation, deflects the storm’s path. In the Northern Hemisphere, TRS are deflected to the right, while in the Southern Hemisphere, they are deflected to the left.
- Effects:
- Curvature of Path: As a TRS gains latitude, the Coriolis effect increases, causing the storm to turn poleward. This leads to the classic recurvature seen in many TRS tracks, where the storm turns northwestward and then northeastward (Northern Hemisphere).
- Example: Hurricanes in the Atlantic often start moving west and then curve north and northeast as they encounter stronger Coriolis forces.
C. Influence of Subtropical Ridges:
- Description: The subtropical ridge, a high-pressure system that typically sits to the north of TRS in the Northern Hemisphere and to the south in the Southern Hemisphere, plays a critical role in guiding the storm.
- Effects:
- Steering of TRS: When the ridge is strong, the TRS is steered westward. When the ridge weakens or moves, the storm is more likely to turn poleward and recurve.
- Example: The Bermuda High often guides hurricanes in the Atlantic westward. A break or weakening in the ridge can cause storms to turn northward.
D. Interaction with Westerlies:
- Description: As a TRS moves poleward, it often encounters the mid-latitude westerlies, a belt of winds that flow from west to east.
- Effects:
- Recurvature and Poleward Turn: The westerlies cause the TRS to recurve and move eastward, often turning the storm toward higher latitudes and away from the tropics. This process is common in the later stages of a TRS’s life cycle.
- Example: Many Atlantic hurricanes turn northeastward into the Atlantic Ocean after interacting with the westerlies.
E. Interaction with Landforms:
- Description: Large landmasses and mountain ranges can affect the track of a TRS.
- Effects:
- Deflection or Weakening: Interaction with landforms, such as mountain ranges, can deflect the storm or cause it to weaken as it loses energy from warm water.
- Example: Hurricanes in the Caribbean or Gulf of Mexico may be deflected or weakened as they pass over mountainous islands like Cuba or Puerto Rico.
How to Use an Aneroid Barometer to Detect a TRS?
An aneroid barometer is a key instrument used aboard ships to detect changes in atmospheric pressure, which can indicate the presence and approach of a Tropical Revolving Storm (TRS). By carefully observing pressure trends using the barometer, mariners can detect early signs of an approaching TRS and take evasive action if necessary.
1. Understanding the Basic Function of an Aneroid Barometer:
- An aneroid barometer measures atmospheric pressure without using liquid (like mercury barometers). It uses a sealed, flexible metal chamber that expands and contracts with changes in atmospheric pressure. These movements are mechanically transferred to a dial that shows the pressure reading in millibars (or inches of mercury).
2. Monitoring Barometric Pressure for TRS Detection:
- Normal Pressure Readings:
- At sea level, normal atmospheric pressure is around 1013 millibars (mb). Significant deviations from this value can indicate changing weather conditions.
- Falling Pressure:
- One of the earliest indicators of an approaching TRS is a gradual and continuous fall in barometric pressure. The lower the pressure, the closer the storm is. TRS are associated with areas of very low pressure at the center (the eye of the storm).
- How to Use: Regularly check and log the barometer’s readings. If pressure falls consistently over several hours or days, this is a strong indication that a low-pressure system, potentially a TRS, is approaching.
3. Identifying Pressure Trends:
- Slight Pressure Drop: A small, gradual fall in pressure (a few millibars over 24 hours) may indicate a general weather disturbance, but not necessarily a TRS.
- Significant Drop in Pressure:
- A steady, significant fall in pressure over a short period (e.g., 10 mb or more over several hours) is a typical sign of a TRS’s approach. In the early stages, the pressure drop may be gradual, but as the TRS nears, the rate of pressure fall will accelerate.
- Example: As a TRS approaches, barometric pressure can drop rapidly, reaching values well below 1000 mb, with values near the storm’s center sometimes falling to around 950 mb or lower.
4. Correlating Pressure Drop with Wind Shifts:
- Wind Changes: In conjunction with falling pressure, shifts in wind direction and speed are common signs of a TRS. The wind will start to veer (shift clockwise in the Northern Hemisphere or counterclockwise in the Southern Hemisphere) as the storm approaches. These changes in wind patterns, along with the barometer’s pressure readings, help pinpoint the TRS’s location relative to the vessel.
- Accelerating Pressure Fall: As a TRS nears, the pressure drop becomes more rapid. When this happens in tandem with veering winds, the ship is likely within or near the dangerous semicircle of the TRS, where winds are strongest.
5. Observing the Eye of the Storm:
- Sudden Pressure Rise: If a vessel is near or within the eye of the TRS, the barometer will show an abrupt pressure rise after the lowest point is recorded. The eye of the storm is a calm region with low pressure, but as the storm moves past, the pressure rises quickly, and the wind direction shifts dramatically.
Steps for Using an Aneroid Barometer During a TRS:
- Regular Monitoring:
- Record the barometric pressure regularly (every hour or two, especially if the weather is deteriorating). Keeping a log allows you to track the pressure trend and detect any rapid drops.
- Look for a Gradual Drop in Pressure:
- A steady, continuous fall in pressure is the first warning sign of an approaching TRS. Note the rate of this fall to estimate the proximity and intensity of the storm.
- Assess Pressure Falls in Combination with Wind Shifts:
- If you observe a falling barometer combined with veering winds (clockwise shift in the Northern Hemisphere or counterclockwise in the Southern Hemisphere), it strongly indicates the presence of a TRS and immediate action should be taken.
- Monitor the Rate of Pressure Fall:
- The faster the pressure drops, the closer the TRS is. A rapid fall in pressure indicates that the storm is intensifying or that the ship is nearing the storm’s center.
- Use the Barometer to Judge Your Position Relative to the TRS:
- If the pressure is falling rapidly, you may be in the dangerous semicircle.
- The wind veering and the barometer dropping significantly suggest you are on the storm’s right-hand side in the Northern Hemisphere or left-hand side in the Southern Hemisphere.
- React to Pressure Changes After the Eye Passes:
- If the barometer shows a sudden rise in pressure after the lowest point, this indicates that the storm’s eye has passed, and the other side of the TRS will bring a sudden change in wind direction and renewed dangerous conditions.
Tropical Revolving Storm (TRS): Vertex
In the context of a Tropical Revolving Storm (TRS), the vertex is the point where the initially westward track of the storm begins to curve (or re-curve) due to the influence of mid-latitude westerlies and the Coriolis effect. This point marks the transition of the storm’s movement from predominantly westward (driven by trade winds) to a more northerly or northeasterly direction.
Characteristics of the Vertex:
- Location:
- The vertex usually occurs at the point where the storm moves into the higher latitudes (generally around 20° to 30° north or south of the equator) and starts to be affected by the mid-latitude westerlies.
- Turning Point:
- At the vertex, the TRS recurves or changes direction from moving primarily westward to moving poleward (northward in the Northern Hemisphere and southward in the Southern Hemisphere).
- After reaching the vertex, the storm is typically pushed northeast in the Northern Hemisphere or southeast in the Southern Hemisphere due to the stronger westerly winds in mid-latitudes.
- Coriolis Effect:
- The Coriolis force plays a significant role in causing the TRS to curve toward higher latitudes at the vertex. As the storm moves away from the equator, the Coriolis effect increases, causing the storm to deviate from its original westward track.
- Steering by Westerlies:
- Once the storm reaches the vertex, it starts to be steered by the westerlies, which are the prevailing winds in the mid-latitudes that blow from west to east.
Action to Take When a Hurricane is Approaching: Bound from Amsterdam to the West Indies:
If you are bound from Amsterdam to the West Indies and receive a weather bulletin and visual warning indicating a hurricane moving northeast, with its center expected to pass over or near your position, the following steps must be taken to ensure the safety of the ship, crew, and cargo. The action plan will depend on your position relative to the hurricane, but the goal is always to avoid the dangerous quadrant of the hurricane.
- Determine Your Position Relative to the Hurricane
The first step is to determine your location relative to the hurricane by applying Buys Ballot’s Law. In the Northern Hemisphere, if you stand with your back to the wind:
- The center of the low-pressure system (hurricane) will be on your right-hand side and slightly behind you.
Dangerous and Navigable Semicircles
- The right-hand side of the hurricane’s path is the dangerous semicircle. In this region, winds are stronger, and the sea is more violent due to the combined effect of the storm’s movement and rotational wind speed.
- The left-hand side is the navigable semicircle, where the wind and sea conditions are comparatively less severe.
Your priority is to avoid the dangerous semicircle and move to a safer part of the storm’s periphery, if not entirely away from the storm.
- Actions Based on Position Relative to the Storm
If You Are in the Dangerous Semicircle (right side of the storm):
- Action: Immediately alter course to starboard (right) to put the wind on your starboard quarter (behind and to the right).
- Reason: By keeping the wind on your starboard quarter, you move away from the hurricane’s path while keeping it on your left side. This helps you to navigate toward the safer part of the storm and avoid the most dangerous winds and sea conditions.
If You Are in the Navigable Semicircle (left side of the storm):
- Action: Alter course to port (left) to put the wind on your starboard bow (front right).
- Reason: By steering into the storm with the wind on your starboard bow, you can maintain a safer position within the navigable semicircle, away from the storm center. This will help you avoid being drawn into the more dangerous areas of the storm.
- Reduce Speed and Maintain Distance from the Hurricane Center
- Reduce Speed: Slow down the vessel to minimize damage from high seas and strong winds. This reduces the strain on the ship’s structure, the cargo, and its equipment.
- Maintain Safe Distance: Your goal is to increase the distance between the ship and the hurricane’s center by moving toward the safer semicircle (navigable side). Avoid sailing into areas where the storm is expected to intensify.
- Secure the Ship and Crew
- Secure Cargo: Ensure that all cargo and loose items are properly secured to prevent damage or shifting due to heavy rolling and pitching caused by large waves.
- Close Hatches: Make sure all hatches, doors, and ventilators are tightly closed to prevent water ingress during rough seas.
- Prepare Emergency Equipment: Make sure all lifesaving equipment, such as life rafts, lifeboats, and communication systems, are ready and functional in case of an emergency.
- Crew Preparedness: Inform the crew of the weather conditions and ensure they are stationed appropriately for any necessary actions or evacuations.
- Maintain Constant Weather Monitoring
- Weather Updates: Continue to receive weather bulletins and monitor the storm’s position, movement, and intensity. Use satellite or radar systems to track the storm and adjust your course accordingly.
- Alternative Route Planning: Depending on the hurricane’s path, consider rerouting your voyage entirely to avoid entering storm-prone regions. Keep updating the ship’s route based on the latest weather reports.
- Communicate with Relevant Authorities
- Notify Authorities: Inform the ship’s company and relevant maritime authorities of the ship’s current position, intended course change, and actions taken to avoid the hurricane.
- Seek Guidance: Depending on the situation, seek additional guidance from weather routing services or port authorities if necessary.
Reasons for These Actions:
- Avoiding the Most Dangerous Conditions:
- The dangerous semicircle of a hurricane is where the winds are strongest, and the seas are roughest.
- Steering to the safer navigable semicircle will reduce the ship’s exposure to the most extreme conditions.
- Minimizing Risk to Ship and Cargo:
- By avoiding the center of the hurricane and reducing speed, you minimize the risk of damage to the vessel and its cargo.
- Large waves and high winds can cause structural damage or capsizing if not handled properly.
- Ensuring Crew Safety:
- Securing the vessel and preparing the crew helps to ensure their safety in case the conditions worsen.
- A well-prepared crew is essential for avoiding accidents or injuries during storm conditions.
- Efficient Route Planning:
- Constantly monitoring the hurricane’s path and adjusting the course accordingly allows you to navigate safely and efficiently, avoiding delays while ensuring the ship’s safety.
Isobaric pattern of a well-developed TRS:-
The isobaric pattern of a Tropical Revolving Storm (TRS) represents the lines of equal atmospheric pressure that surround the storm’s center. The diagram provides a visual understanding of how pressure gradients within the storm create its characteristic cyclonic wind patterns. Here’s a breakdown of the key features:
1. Concentric Isobars
- Isobars are lines of equal atmospheric pressure. In a TRS, the isobars form concentric circles around the eye, which is the low-pressure center of the storm.
- Tightly Packed Isobars: Near the eye of the storm, the isobars are tightly packed, indicating a steep pressure gradient. This steep gradient is responsible for the strongest winds in the storm, which are typically found in the eye wall (the area surrounding the calm eye).
- Widely Spaced Isobars: As you move outward from the center, the isobars become more widely spaced, indicating a weaker pressure gradient and diminishing wind speeds.
2. Eye of the Storm
- The eye is the center of the TRS, where the pressure is lowest, and the weather is typically calm. The air here is descending, leading to clear skies and light winds.
3. Cyclonic Wind Circulation
- In the Northern Hemisphere, the winds around the TRS rotate counterclockwise due to the Coriolis effect. The arrows in the diagram represent this cyclonic motion.
- In the Southern Hemisphere, the winds would rotate clockwise.
4. Wind Speed and Direction
- The strongest winds occur in the region just outside the eye, known as the eye wall. The wind speeds decrease as you move further from the center because the pressure gradient weakens.
- The cyclonic circulation of the winds around the TRS results in spiraling wind patterns, with air moving toward the low-pressure center but being deflected by the Coriolis force.
