TOPIC-5-Tropical-Revolving-Storm

Tropical Revolving Storms (TRS)

Tropical Revolving Storms (TRS) are intense tropical depressions over large sea areas. They're circular atmospheric vortices with gale-force winds spiraling inwards (Anti-Clockwise in NH, Clockwise in SH).

Dangers of TRS

• Violent winds and high seas cause damage to ships.

• Significant danger to shipping; requires study.

Naming Conventions

WMO maintains rotating lists of names for each Tropical Cyclone basin. Names of deadly cyclones are retired.

Advantages of Naming

• Short names reduce errors.

• Facilitates storm information exchange.

History of Naming

• Early storms named arbitrarily.

• Mid-1900s: Feminine names used.

• Later: Alphabetical lists, male names included.

Retiring Names

• Names of deadly storms are retired.

• Decided by WMO Tropical Cyclone Committees.

• Examples: Mangkhut, Irma, Maria, Haiyan, Sandy, Katrina, Mitch, Tracy.

Procedure for Naming Tropical Cyclones

Strict procedure by Tropical Cyclone Regional Body during meetings.

• Five bodies: ESCAP/WMO Typhoon Committee, WMO/ESCAP Panel, RA I, RA IV, RA V.

• Hurricane Committee determines names for six years.

• Names from members (National Meteorological and Hydrological Services).

• Similar procedures across regions.

Key Principles

• Not named after persons.

• Alphabetical sequence not primary.

• Familiar names for understanding and recall.

• Facilitates disaster risk awareness and reduction.

Storm Division

Navigable Semicircle

• Left of movement in NH (right in SH).

• Weaker winds, relatively better for navigation.

Dangerous Semicircle

• Right of movement in NH (left in SH).

• Strongest winds and heavy seas.

Structure of a Tropical Cyclone

1. The Eye or Vortex

• Calm, central area of lowest pressure.

• Diameter: 4-30 miles (avg. 10 miles).

• Characterized by light winds and fair weather.

• Weather: Calm, but seas extremely violent.

• Little precipitation, sometimes blue sky.

• Lowest surface pressure region.

• In severe cyclones, appears as a circular hole.

2. The Eye-Wall

• Inner ring of hurricane-force winds, 4-30 miles wide.

• Winds blow circularly up to 130 knots, gusts to 150 knots.

• Steep pressure gradient.

• Dense clouds, tall thunderstorms, heavy rains, strongest winds.

• Changes indicate storm intensity changes.

3. The Outer Storm Area

• Surrounds eye-wall, 50-800 miles diameter (avg. 500 miles).

• Strong winds (force 6-7), pressure gradient less steep.

• Wind indraft angle about 45º, decreasing to 0º in eye wall.

• Cirrus clouds point to storm center.

• Excellent visibility, except in showers.

Life Cycle of a Tropical Cyclone

• Usually spans about 9 days (2-3 to 20+ days).

1. Formation

• Dependent on six favorable environmental conditions in the Inter Tropical Convergence Zone.

• Gains energy from latent heat via thunderstorm activity.

• Heat engine powered by gravity and rotation.

2. Premature Stage

• Convection becomes more organized.

• Strengthening occurs simultaneously.

• Minimum surface pressure drops rapidly.

• Gale-force winds develop.

• Circulation center well-defined, eye may form.

• Distinctive spiral banding pattern on satellite/radar.

• Devastating effects on coastline in small area.

3. Mature Stage

• Intensifies if conditions favorable.

• Most dangerous stage, severe.

• About half of cyclones reach this stage.

• Circulation and gales increase markedly.

• Highly organized, symmetrical cloud fields with distinct eye.

• Lasts about a day at maximum intensity if conditions remain favorable.

4. Decay Stage

• Warm core destroyed.

• Central pressure increases, winds weaken.

• Decay occurs rapidly under unfavorable conditions.

• Heavy/medium rain may be present.

Factors Associated with Decay:

• Movement over land.

• Movement over cooler sea temperatures.

Seasons of Greatest Frequency of TRS

• North Atlantic: June-November, Hurricane.

• Eastern North Pacific: June-October, Hurricane.

• Western North Pacific: April-December, Typhoon.

• North Indian Ocean: February-October, Cyclone.

• South Indian Ocean: December-March, Cyclone.

• South Pacific and Australian Area: December-April, Hurricane/Willy-Willy.

• TRS do not occur between latitudes 5°N and 5°S.

Nomenclature According to Intensity (WMO)

• Tropical Disturbances: Low-pressure area, no strong winds.

• Tropical Depression: Rotary circulation, wind speed < 34 knots.

• Tropical Storm/Severe Tropical Storm: Lower pressure, wind speed 34-63 knots.

• Hurricane/Typhoon/Cyclone: Much lower pressure, sustained winds ≥ 64 knots.

Recipe for a Cyclone

• Ocean temperature ≥ 26ºC.

• Area of low pressure.

• Warm, moist, unstable air.

• Coriolis Force (5º-20º N/S).

Cyclone Seasons

• South Pacific: November-April.

• Northeast Pacific: June-November.

• Northwest Pacific: All year.

• North Atlantic: June-November.

• North Indian basin: April-December.

Signs of an Approaching Cyclone

• Long low swell

• Extensive high cirrus clouds

• Change of $3 hPa$ or more below average pressure

• Marked change in wind direction and speed

Finding the Direction of the Storm

• Face the wind; center is ~90º to your left in SH and to your right in the NH.

Hurricane Force Wind

• Averaging 64 knots.

• Storms categorized by wind speed.

Naming and Identification

• TRS given a name once identified.

• Tropical Cyclone warning centers keep lists.

• Two centers in South Pacific (Brisbane and Fiji).

• Naming aids identification.

Dissipation

• TRS need warm water.

• Dissipate over land/cooler waters, becoming tropical storms/depressions.

• Act as ‘pressure valves’ releasing heat from the ocean.

Movement

• In SH, wind blows clockwise spiraling inwards.

• General track is South Westerly, may change to South East.

Formation

• Cloud mass develops over warm seas ($≈ 80^\circ$).

• Moisture and warm air rise, reducing pressure.

• Atmospheric moisture condenses into thunderclouds.

• Cold air rushes in, spiraling upwards with force.

• Winds rotate, forming a circle up to 2000 km across.

• Sustained by warm, moist air.

Main Causes

• Rising warm air from equatorial seas.

• Condensation forms clouds and releases heat, leading to thunderstorms.

Impacts

• Extreme weather: high winds, thunder, lightning, rain.

• Damage to infrastructure and loss of life.

• Flooding, storm surges.

Factors Affecting Hurricane Movement

Hurricane Propagation

• Movement from one location to another.

• Steered by global winds.

Steering Factors

• Trade Winds: Steer westward.

• High-Pressure Systems: Curve northward.

• Subtropical Ridge: Influences Atlantic hurricanes.

  • Eastward high: Northeastward.

  • Westward high: Affects Florida, Cuba, Gulf of Mexico.

• Beta Drift: Northwestward due to Coriolis force.

• Weak Environmental Wind Field: Slows down, stalls.

• Mid-Latitudes: Recurve to the right, accelerating.

• Jet Stream: Accelerates storm, especially over warm currents.

Other Processes

• Vertical Wind Shear: Tilts wind field.

• Fujiwhara Effect: Orbit cyclonically around a point.

• Land Interaction: Disrupts circulation, reforms on other side.

• Unpredictable Movements: Loops, turns, curves.

Contributing Factors

Intensity factors of a Revolving Storm

• Instability: Near ITCZ.

• Humidity: Over western parts of tropical oceans.

• Latitude: Wind strength increases toward Equator.

• Temperature: Water temperature ≥ $27^\circ$.

Measuring Hurricanes

Saffir-Simpson Hurricane Intensity Scale

• Five categories by wind speed.

• Major hurricanes (3-5) cause significant damage.

Tropical Cyclone Track Forecasting

• Predicts track over five days, every 6-12 hours.

• Uses various meteorological tools.

• Signs: increasing swell, cloudiness, falling pressure, increasing tides, squalls, rain.

• Depends on determining positions of pressure areas and predicting movement. Computer models used.

Factors That Can Strengthen Tropical Cyclones

• Sea temperatures > $79^\circ$F ($26^\circ$C)

• Low vertical wind shear

• Warm moist air

• Ocean area along track

Factors That Can Weaken Tropical Cyclones

• Cooler Sea temperatures < $79^\circ$F ($26^\circ$C)

• High vertical wind shear

• Dry air

• Land masses along track

Dangerous Circle

Reasons for Naming the Dangerous Semicircle

• Winds stronger; cyclone's speed and wind field additive.

• Sailing ship carried into path.

• Storm likely to cross ship's course if recurves.

• NH: right of center; SH: left.

Clouds and Precipitation

• Clouds form over mountain tops.

• Large-area precipitation requires cloud water/ice to grow large enough to fall.

• Cloud is a visible aggregate of water droplets/ice crystals.

Characteristics of a TRS

• Size, wind, pressure, and eye.

Cause

*Warm (over $27^\circ$ Celsius)

• Warm moist air rises.

• Condenses to make clouds and rain.

• Releases energy, producing winds.
  *Storms are fueled by damp air when they reach land, dry air is being sucked up and they loose energy.

• Winds driven by Earth's spin.

• Vortex up to $1,000 km$ wide.

• Wind speeds up to $200 km$ per hour.

Origin, Movement, and Life Span

• Travel westward, inclined towards pole, then curve eastward ~ latitudes 20°N/S.

• Point of recurvature: westward changes to eastward.

• General direction: west, northwest, north, northeast (NH); west, southwest, south, southeast (SH).

• Dissipate or join extra-tropical cyclones.

• Originate between $5^\circ$ & $20^\circ$, travel between W and WNW in the NH and between W and WSW in the SH at ~12 knots.
  *Curve away from equator; curve to N then NE (NH); curve to S then SE (SH).

Terms Used in Connection with Tropical Revolving Storms

• Storm Field: Region covered by winds.

• Track: Route already passed.

• Path: Predicted route.

• Vertex/Cod: Westernmost point when recurving.

• Vortex/Center/Eye: Windless area, lowest pressure.

• Right-Hand Semicircle: Dangerous semicircle (NH).

• Left-Hand Semicircle: Navigable semicircle (NH).

• Trough Line: Dividing falling and rising pressure areas.

• Bar of the Storm: Advancing edge.

• Angle of Indraft: Angle of wind direction with isobars.

Typical Cloud Formations

• Cirrus, cirrostratus, altostratus, stratocumulus, cumulonimbus, and scuds.

• Main energy source: latent heat from condensation.

Path of a TRS

• Recurving travels around oceanic high.

• Speed increases after recurving (~15-20 knots).

• Some storms don't curve and dissipate quickly after crossing the coast.

• Occasional storms remain stationary or loop for four days.

Determining the Semicircle

• Determine true wind direction.

• Veering (right): right-hand semicircle; backing (left): left-hand semicircle.

• True in both hemispheres.

• Heave to if unsure of storm's movement.

• Continuously check storm's movement.

Navigable and Dangerous Semicircles in Detail

• Navigable Semicircle: Left in NH, right in SH; weaker winds.

• Dangerous Semicircle: Right in NH, left in SH; strongest winds.

• In dangerous semicircle, ship is driven into storm's center if not maneuvered.

Signs of Approach of a TRS

• 1. Warning and Alerting Messages: Monitor Radio/Telex/NAVTEX.

• 2. Swell: Indicates early warning; approaches from storm's direction.

• 3. Atmospheric Pressure: Barometer falls below 3 mb; expect TRS. Barometer must be corrected for accuracy.

• 4. Wind: Variation and increasing speed indicate approach.

• 5. Clouds: Colorful sky at sunrise/sunset; cirrus, altostratus, cumulus.

• 6. Visibility: Exceptionally good visibility may exist.

• 7. Radar: Fair warning ~100 miles prior; eye may be seen.

• Take action before visible on radar.

Sequence of Events During Hurricane Development

Approaching a Coastal Area (Category 2 example: wind speed 96-110 mph)

• 96 hours before landfall: No signs.

• 72 hours: Swell increases to ~6 feet, waves every 9 seconds.

• 48 hours: Clear sky; Swell is about nine feet, coming in every eight seconds.

• 36 hours: Barometer falling slightly; wind ~11 mph; swell ~13 feet, every seven seconds; cirrus clouds. Hurricane watch issued.

• 30 hours: High overcast; barometer falling at .1 millibar/hour; winds ~23 mph. The ocean swell is beginning to be obscured; small whitecaps appear.

• 24 hours: Low clouds
  Hurricane is given out for all ships to not get close, hurricane warnings, wind picks up at 34 mph. Wind-driven waves are covered in whitecaps, and streaks of foam begin to ride over the surface. Storm surge

• 18 hours: Low clouds thicker, rain squalls, gusty winds; barometer steadily falling; winds ~46 mph.

• 12 hours: Rain squalls frequent, winds don't diminish; lower cloud ceiling; barometer falling at 1 millibar/hour; wind ~74 mph. Heavy storm surge.

• 6 hours: Constant rain, 92 mph wind; barometer falling 1.5 millibar/hour; storm surge above high tide mark.

• 1 hour: Heavier rain, flooding; winds ~104 mph; barometer falling at 2 millibar/hour; storm surge covers roads.

After the Storm

• 1 hour after landfall: Dark sky, winds and rain return; storm surge retreats; barometer rising at 2 millibar/hour.

• 6 hours: Rains continue, winds ~92 mph; storm surge retreats.

• 12 hours: Rain in squalls, winds diminish; cloud ceiling rising; barometer rising at 1 millibar/hour; wind ~69 mph.

• 24 hours: Clouds break, overcast seen again; barometer rising by .2 millibar/hour; wind ~34 mph.

• 36 hours: Overcast broken, cirrus clouds disappear; barometer rising slightly; winds ~11 mph.

Actions in TRS

At Port

• Double moorings.

• Engine standby.

• All persons onboard.

• LSA at standby.

• Rig lifeline.

• No slack tanks.

• Secure hatches.

• Lower and secure derricks.

• Adequate fenders.

At Anchorage

• Go to sea or shift to safe anchorage.

• Drop both anchors with cables in water.

• Keep Engine standby.

• All persons to be onboard.

• Keep all LSA at the standby position.

• Rig lifeline at fore and aft.

• No slack tanks.

• All hatches should be securely battened down.

• All derricks should be lowered and secured.

• All bridge equipment and navigational lights should be in standby mode

At Sea

• Dangerous quadrant: Proceed fast, wind 1-4 points on stbd bow (NH), port bow (SH); alter course as wind veers (NH), backs (SH); pressure rises. If insufficient sea room, heave to with wind on stbd bow (NH), port bow (SH).

• Path of storm/navigable semicircle: Proceed fast, wind ~4 points on stbd quarter (NH), port quarter (SH); alter course as wind backs (NH), veers (SH); pressure rises.

Determining the Approximate Bearing of Approaching TRS

• Determine storm's location and direction relative to vessel.

• Maritime Weather Broadcast provides info.

• Recheck via observation.

• Locate center by radar within range.

• Employ Buys Ballot’s Law:

  • Northern Hemisphere: Face wind; center is 10-12 points to right (~200 miles away), 8 points when near.

  • Southern Hemisphere: Center to left.

Important Characteristics of a TRS

• Smaller size than temperate depressions

• Form near ITCZ

• Nearly circular isobars

• No fronts occur

• Very steep pressure gradient

• Great intensity

Actions to Be Taken When in the Vicinity of a Tropical Revolving Storm

1. Transmit priority message to coastal radio station with storm information.

2. Place the ship in a position of safety by:

   • Determine center bearing and distance.

   • Determine the semicircle the ship is in.

   • Plot probable path and danger area for two-day movement.

   • Maneuver the ship accordingly.

In the Absence of Reliable Sources:

• Estimate center bearing/distance via pressure, wind direction/force, and Buys Ballot’s Law.

  • Northern Hemisphere:

  • 5 mb below normal: center ~12 points to right, ~200 miles away (wind force ~6); ~100 miles (wind force ~8).

  • 10 mb below normal: center ~10 points to right, wind increasing.

  • 20 mb from normal the center is about 8 points of the compass to your right and already close to your position so that the wind force would now approach hurricane force

  • Southern Hemisphere:

  • Procedure same, but center to left.

  • Storms on western South Indian Ocean have large indraft angles.

Correct Avoidance Procedure of a TRS

• 1-2-3 Rule onboard

  • Plot the current and forecast 24 Hour storm position and forecast radius of 34 knots wind.

  • Using Compass extend the radius of the forecast 24 hours 34 knots wind area by 100 nm.

  • Draw tangents relative to the direction of the storm from the 34 knots radius of the current position of storm to the outermost radius at the 24 hour forecast position .The area between this is the Danger Area and is to be avoided

• Use the same procedure for 48 & 72 hours forecast position, however to draw the outermost circle use 200 nm and 300 nm as radius respectively.

Mariner’s 1-2-3 Rule

• A guideline to avoid a tropical storm or hurricane’s path.

Guidelines:

• 34 Knots is chosen as the critical value because as wind speed increases to this speed, sea state development approaches critical levels resulting in decreasing ship maneuverability

• Also, the state of the sea outside of the radius of 34 KT winds can also be significant enough as to limit course and speed options available to the mariner and must also be considered when avoiding hurricanes.

1-2-3 Rule

Single most important aid in accounting for hurricane forecast tract errors (FTE)

• The rule is derived from the latest 10-year average FTE associated with hurricanes in North Atlantic

1-2-3 rule establishes a minimum recommended distance to maintain from a hurricane in the Atlantic.

• Mariners in the Pacific can use this rule as a guide.

Larger buffer zones should be established in situations with higher forecast uncertainly, limited crew experience, decreased vessel handling, or other factors set by the vessel master.

• Rule does not account for sudden and rapid intensification of hurricanes.

Messages Required to Be Sent as per SOLAS

According to SOLAS Chapter V – 1/7/02 Regulation 5:

• Meteorological services and warnings: Contracting Governments undertake to carry out, in cooperation, meteorological

Tropical Revolving Storms (TRS) are intense tropical depressions over large sea areas, characterized by circular atmospheric vortices with gale-force winds spiraling inwards.

Dangers of TRS:

• Violent winds and high seas damage ships.
• Pose a significant threat to shipping, requiring careful study.

Naming Conventions:

• WMO maintains rotating lists of names for each Tropical Cyclone basin.
• Names of deadly cyclones are retired to avoid confusion.

Naming Advantages:

• Short names reduce errors and facilitate storm information exchange.

Naming History:

• Early storms were named arbitrarily, with feminine names used in the mid-1900s, later followed by alphabetical lists including male names.

Retiring Names:

• Names of deadly storms are retired by WMO Tropical Cyclone Committees, exemplified by names like Mangkhut, Irma, Maria, Haiyan, Sandy, Katrina, Mitch, and Tracy.

Naming Procedure:

• Strict procedure by Tropical Cyclone Regional Body during meetings, involving five bodies such as ESCAP/WMO Typhoon Committee.
• Hurricane Committee determines names for six years, sourced from members' National Meteorological and Hydrological Services.
• Procedures are similar across regions.

Key Principles:

• Storms are not named after persons.
• Alphabetical sequence is not primary; familiar names are preferred for understanding and recall, promoting disaster risk awareness and reduction.

Storm Division:

• Navigable Semicircle: Left of movement in NH (right in SH) with weaker winds.
• Dangerous Semicircle: Right of movement in NH (left in SH) with strongest winds and heavy seas.

Structure of a Tropical Cyclone:

1. The Eye or Vortex
   • Calm, central area of lowest pressure (4-30 miles diameter).
   • Light winds, fair weather, but extremely violent seas.
   • Little precipitation, sometimes blue sky.
2. The Eye-Wall
   • Inner ring of hurricane-force winds (4-30 miles wide).
   • Winds up to 130 knots, gusts to 150 knots.
   • Steep pressure gradient, dense clouds, heavy rains.
3. The Outer Storm Area
   • Surrounds eye-wall, 50-800 miles diameter.
   • Strong winds (force 6-7), less steep pressure gradient.
   • Cirrus clouds point to storm center.

Life Cycle of a Tropical Cyclone:

1. Formation
   • Dependent on six favorable environmental conditions in the Inter Tropical Convergence Zone.
   • Gains energy from latent heat via thunderstorm activity.
2. Premature Stage
   • Convection becomes more organized; strengthening occurs.
   • Minimum surface pressure drops rapidly.
   • Gale-force winds develop, circulation center well-defined.
3. Mature Stage
   • Intensifies if conditions favorable; most dangerous stage.
   • Highly organized, symmetrical cloud fields with distinct eye.
4. Decay Stage
   • Warm core destroyed; central pressure increases, winds weaken.
   • Decay occurs rapidly under unfavorable conditions like movement over land or cooler sea temperatures.

Seasons of Greatest Frequency of TRS:

• North Atlantic: June-November (Hurricane).
• Eastern North Pacific: June-October (Hurricane).
• Western North Pacific: April-December (Typhoon).
• North Indian Ocean: February-October (Cyclone).
• South Indian Ocean: December-March (Cyclone).
• South Pacific and Australian Area: December-April (Hurricane/Willy-Willy).
• TRS do not occur between latitudes 5°N and 5°S.

Nomenclature According to Intensity (WMO):

• Tropical Disturbances: Low-pressure area, no strong winds.
• Tropical Depression: Rotary circulation, wind speed

Recipe for a Cyclone:

• Ocean temperature ≥ 26ºC.
• Area of low pressure.
• Warm, moist, unstable air.
• Coriolis Force (5º-20º N/S).

Cyclone Seasons:

• South Pacific: November-April.
• Northeast Pacific: June-November.
• Northwest Pacific: All year.
• North Atlantic: June-November.
• North Indian basin: April-December.

Signs of an Approaching Cyclone:

• Long low swell
• Extensive high cirrus clouds
• Change of $3 hPa$ or more below average pressure
• Marked change in wind direction and speed

Finding the Direction of the Storm:

• Face the wind; center is 90º to your left in SH and to your right in the NH.

Hurricane Force Wind:

• Averaging 64 knots.
• Storms categorized by wind speed.

Naming and Identification:

• TRS given a name once identified.
• Tropical Cyclone warning centers keep lists.
• Two centers in South Pacific (Brisbane and Fiji).
• Naming aids identification.

Dissipation:

• TRS need warm water; dissipate over land/cooler waters, becoming tropical storms/depressions; act as ‘pressure valves’ releasing heat from the ocean.

Movement:

• In SH, wind blows clockwise spiraling inwards; general track is South Westerly, may change to South East.

Formation:

• Cloud mass develops over warm seas ($≈ 80^\circ$).
• Moisture and warm air rise, reducing pressure.
• Atmospheric moisture condenses into thunderclouds.
• Cold air rushes in, spiraling upwards with force.
• Winds rotate, forming a circle up to 2000 km across.
• Sustained by warm, moist air.

Main Causes:

• Rising warm air from equatorial seas; condensation forms clouds and releases heat, leading to thunderstorms.

Impacts:

• Extreme weather: high winds, thunder, lightning, rain; damage to infrastructure and loss of life; flooding, storm surges.

Factors Affecting Hurricane Movement:

• Hurricane Propagation: Movement from one location to another, steered by global winds.

Steering Factors:

• Trade Winds: Steer westward.
• High-Pressure Systems: Curve northward.
• Subtropical Ridge: Influences Atlantic hurricanes.
  • Eastward high: Northeastward.
  • Westward high: Affects Florida, Cuba, Gulf of Mexico.
• Beta Drift: Northwestward due to Coriolis force.
• Weak Environmental Wind Field: Slows down, stalls.
• Mid-Latitudes: Recurve to the right, accelerating.
• Jet Stream: Accelerates storm, especially over warm currents.

Other Processes:

• Vertical Wind Shear: Tilts wind field.
• Fujiwhara Effect: Orbit cyclonically around a point.
• Land Interaction: Disrupts circulation, reforms on other side.
• Unpredictable Movements: Loops, turns, curves.

Contributing Factors:

• Intensity factors of a Revolving Storm:
  • Instability: Near ITCZ.
  • Humidity: Over western parts of tropical oceans.
  • Latitude: Wind strength increases toward Equator.
  • Temperature: Water temperature ≥ $27^\circ$.

Measuring Hurricanes:

• Saffir-Simpson Hurricane Intensity Scale: Five categories by wind speed; major hurricanes (3-5) cause significant damage.

Tropical Cyclone Track Forecasting:

• Predicts track over five days, every 6-12 hours, using various meteorological tools.
• Signs: increasing swell, cloudiness, falling pressure, increasing tides, squalls, rain.
• Depends on determining positions of pressure areas and predicting movement; computer models used.

Factors That Can Strengthen Tropical Cyclones:

• Sea temperatures > $79^\circ$F ($26^\circ$C)
• Low vertical wind shear
• Warm moist air
• Ocean area along track

Factors That Can Weaken Tropical Cyclones:

• Cooler Sea temperatures

Dangerous Circle:

• Reasons for Naming the Dangerous Semicircle:
  • Winds stronger; cyclone's speed and wind field additive.
  • Sailing ship carried into path.
  • Storm likely to cross ship's course if recurves.
  • NH: right of center; SH: left.

Clouds and Precipitation:

• Clouds form over mountain tops; large-area precipitation requires cloud water/ice to grow large enough to fall; cloud is a visible aggregate of water droplets/ice crystals.

Characteristics of a TRS:

• Size, wind, pressure, and eye.
  • Cause: Warm (over $27^\circ$ Celsius).
    • Warm moist air rises, condenses to make clouds and rain, releases energy producing winds.
    • Storms are fueled by damp air when they reach land, dry air is being sucked up and they lose energy.
  • Winds driven by Earth's spin; vortex up to $1,000 km$ wide; wind speeds up to $200 km$ per hour.

Origin, Movement, and Life Span:

• Travel westward, inclined towards pole, then curve eastward ~ latitudes 20°N/S.
  • Point of recurvature: westward changes to eastward.
  • General direction: west, northwest, north, northeast (NH); west, southwest, south, southeast (SH).
  • Dissipate or join extra-tropical cyclones.
  • Originate between $5^\circ$ & $20^\circ$, travel between W and WNW in the NH and between W and WSW in the SH at ~12 knots.
    • Curve away from equator; curve to N then NE (NH); curve to S then SE (SH).

Terms Used in Connection with Tropical Revolving Storms:

• Storm Field: Region covered by winds.
• Track: Route already passed.
• Path: Predicted route.
• Vertex/Cod: Westernmost point when recurving.
• Vortex/Center/Eye: Windless area, lowest pressure.
• Right-Hand Semicircle: Dangerous semicircle (NH).
• Left-Hand Semicircle: Navigable semicircle (NH).
• Trough Line: Dividing falling and rising pressure areas.
• Bar of the Storm: Advancing edge.
• Angle of Indraft: Angle of wind direction with isobars.

Typical Cloud Formations:

• Cirrus, cirrostratus, altostratus, stratocumulus, cumulonimbus, and scuds; main energy source: latent heat from condensation.

Path of a TRS:

• Recurving travels around oceanic high; speed increases after recurving (~15-20 knots); some storms don't curve and dissipate quickly after crossing the coast; occasional storms remain stationary or loop for four days.

Determining the Semicircle:

• Determine true wind direction; veering (right): right-hand semicircle; backing (left): left-hand semicircle; true in both hemispheres; heave to if unsure of storm's movement; continuously check storm's movement.

Navigable and Dangerous Semicircles in Detail:

• Navigable Semicircle: Left in NH, right in SH; weaker winds. Dangerous Semicircle: Right in NH, left in SH; strongest winds. In dangerous semicircle, ship is driven into storm's center if not maneuvered.

Signs of Approach of a TRS:

1. Warning and Alerting Messages: Monitor Radio/Telex/NAVTEX.
2. Swell: Indicates early warning; approaches from storm's direction.
3. Atmospheric Pressure: Barometer falls below 3 mb; expect TRS; barometer must be corrected for accuracy.
4. Wind: Variation and increasing speed indicate approach.
5. Clouds: Colorful sky at sunrise/sunset; cirrus, altostratus, cumulus.
6. Visibility: Exceptionally good visibility may exist.
7. Radar: Fair warning ~100 miles prior; eye may be seen; take action before visible on radar.

Sequence of Events During Hurricane Development:

• Approaching a Coastal Area (Category 2 example: wind speed 96-110 mph):
  • 96 hours before landfall: No signs.
  • 72 hours: Swell increases to ~6 feet, waves every 9 seconds.
  • 48 hours: Clear sky; Swell is about nine feet, coming in every eight seconds.
  • 36 hours: Barometer falling slightly; wind ~11 mph; swell ~13 feet, every seven seconds; cirrus clouds issued.

After the Storm:

• 1 hour after landfall: Dark sky, winds and rain return; storm surge retreats; barometer rising at 2 millibar/hour.
  -6 hours: Rains continue, winds ~92 mph; storm surge retreats.
  -12 hours: Rain in squalls, winds diminish; cloud ceiling rising; barometer rising at 1 millibar/hour; wind ~69 mph.

Actions in TRS:

• At Port: Double moorings, engine standby, all persons onboard, LSA at standby, rig lifeline, no slack tanks, secure hatches, lower and secure derricks, adequate fenders.
• At Anchorage: Go to sea or shift to safe anchorage; drop both anchors with cables in water; keep engine standby; all persons to be onboard; keep all LSA at the standby position; rig lifeline at fore and aft; no slack tanks; all hatches should be securely battened down; all derricks should be lowered and secured; all bridge equipment and navigational lights should be in standby mode.
• At Sea: Dangerous quadrant: Proceed fast, wind 1-4 points on stbd bow (NH), port bow (SH); alter course as wind veers (NH), backs (SH); pressure rises. If insufficient sea room, heave to with wind on stbd bow (NH), port bow (SH). Path of storm/navigable semicircle: Proceed fast, wind ~4 points on stbd quarter (NH), port quarter (SH); alter course as wind backs (NH), veers (SH); pressure rises.

Determining the Approximate Bearing of Approaching TRS:

• Determine storm's location and direction relative to vessel; Maritime Weather Broadcast provides info; recheck via observation; locate center by radar within range; employ Buys Ballot’s Law:
  • Northern Hemisphere: Face wind; center is 10-12 points to right (~200 miles away), 8 points when near.
  • Southern Hemisphere: Center to left.

Important Characteristics of a TRS:

• Smaller size than temperate depressions, form near ITCZ, nearly circular isobars, no fronts occur, very steep pressure gradient, great intensity.

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