tropical revolving storm

What Actions vessel should take to avoid Tropical Revolving Storm?

Most of the seafarers especially, the Officers of watch will be aware of TRS or Tropical revolving storm. We have also discussed in detail regarding Tropical Revolving Storm you can read it here. However, as far as this article is concerned, let us look at the signs of a TRS, so that you can make better appraisal of the situation and actions that can be taken to avoid TRS.

Warning signs of an approaching TRS

  1. Storm Warnings: Weather Reports from Meteorological observation stations provide satellite images, position and pressure of the storm centre and also the probable direction of movement of the storm. Hence, it is of utmost importance that the vessel does not miss any single report regarding TRS. Once the report is obtained it is best practise to plot the position of the TRS and the position of the Ship on the Routing chart with Date and Time to monitor the progress of the storm and Vessel.
  2. Swell: Due to very violent winds near the eye wall of TRS, swells are generated and send out in radial direction. Swell travels much faster than the speed of the TRS. Swell travels thousand miles and hence it can be experienced by a vessel at thousand miles away. Swell usually is the first indication of an TRS in the vicinity. If the you can observe the direction of swell properly you can get a rough idea regarding the centre of the Storm.
  3. Atmospheric Pressure: TRS is developed from a depression, an area of low pressure. So if a vessel is in or near an area of TRS the pressure will drop steadily. An TRS is suspected in vicinity if the aneroid barometric pressure (corrected for index error and height above sea level) drops below 3 mb below normal. A TRS is confirmed if the foregoing conditions are met and the barometric pressure falls more than 5 mb below normal.
  4. Wind: Wind speed and directions are usually constant in the tropics but if an appreciable change in the direction or strength of the wind indicates a Tropical Revolving Storm (TRS) in vicinity.
  5. Weather: Cirrus clouds in bands of filaments are aligned towards the direction of the storm centre. Also, threatening appearance of dense, heavy clouds on the horizon is seen. Sometimes peculiar dark red or copper colour of sky is seen at sunset before a TRS. Frequent lighting may be seen.
tropical revolving storm

Action required when TRS is confirmed

It is of vital importance to avoid passing close to storm. Now, how do we define close? Well passing within 80 miles of the centre of the storm can be considered as close, but each company makes guidelines on how close can a vessel be of a Storm or how much distance a vessel needs to keep from a storm for safety, well these guidelines takes priority and should be followed for the safety of the vessel. So lets start with what action you should take, to decide the best course of action if a storm is in the vicinity two things need to be known, the bearing of the centre of the storm and the path of the storm.
  1. Obtain the bearing of the centre of the storm: This is easy to do, the observer should face the true wind, the centre of the storm will be 8 to 12 points (090o to 115o) on your right in the North Hemisphere and similarly to left in the South Hemisphere when the storm centre is 200 miles away (Buy’s Ballots Law). That is to say when the barometer has fallen about 5 mb and the wind has increased to about force 6. As a rule, the nearer they are to the centre, the closer the angle to 90o.
  2. Ascertain which side of the semicircle the vessel lies: Below observations are for an stationary observer. The observer should note at least two readings for wind and constant interval but at least 2 hours interval is preferred this is to give time for veering or backing and to wee out errors. If the wind veers (clock wise change of wind) then the vessel is in Right Hand side of the Semi Circle (RHSC) and if wind is backing (anti-clockwise change of wind), than the vessel is at Left-hand semicircle (LHSC). This holds good for both NH and SH.
  3. Avoiding action for TRS: Any avoiding action should aim to keep the vessel well out of the storm centre.
  • Action, if the vessel is in North Hemisphere: If the wind is veering the ship is in the dangerous quadrant, so the ship should proceed with all available speed with the wind 10o-45o on starboard bow, depending on the speed of the vessel. As the wind veers the ship should alter course to starboard thereby tracing a course relative to the storm. If the wind remains steady from one direction or nearly steady so that the vessel is in the path of the storm then the wind should be brought well on to the starboard quarter and the ship should proceed with all available speed.
  • Action, if the vessel is in South Hemisphere: If the wind is backing the ship is in the dangerous semicircle, so the ship should proceed with all available speed with the wind between 10o-45o on the port bow, depending on her speed. As the wind backs the ship should alter course to course to port thereby tracing a course relative to the storm. If the wind remains steady from one direction or nearly steady so that the vessel is in the path of the storm then the wind should be brought well on to the port quarter and the ship should proceed with all available speed.
In NH, Vessel in Dangerous Quadrant, wind on Starboard Bow. Vessel in Path or Navigable Semicircle, wind on Starboard Quarter. In SH, Vessel in Dangerous Quadrant, wind on Port Bow. Vessel in Path or Navigable Semicircle, wind on Port Quarter.
tropical revolving storm If insufficient room to run when in the navigable semicircle and it is not practicable to seek shelter, the ship should heave-to with the wind on her starboard bow in the North Hemisphere or on her port bow in the south hemisphere.
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If a vessel is at harbour when a tropical revolving storm is approaching it is preferable to proceed to sea provided their is sufficient time to avoid the worst of the storm. Riding out a tropical storm in a harbour or anchorage is an unpleasant and potentially hazardous experience. Other methods are available to avoid Tropical revolving storm or TRS which are mentioned below for reference.

Mariner’s 1-2-3 Rule guidelines for avoiding Hurricanes at Sea

Mariner’s 1-2-3 rule, also known as the Danger Rule, is an guideline mariners should follow to avoid an tropical storm or hurricane’s path. In order to help account for the inherent errors in hurricane forecasting, a few guidelines should be used by the mariner in order to limit the potential of a close encounter between ship and storm. 34 Knots Rule 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 manoeuvrability. 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 This is the single most important aid in accounting for hurricane forecast tract errors (FTE). Understanding and use of this rule should be mandatory for any vessel navigating near a hurricane. The rule is derived from the latest 10-year average FTE associated with hurricanes in North Atlantic. While this rule was derived in the North Atlantic, it is a good technique to use in any tropical cyclone basin. The 1-2-3 rule establishes a minimum recommended distance to maintain from a hurricane in the Atlantic, as it was derived from Atlantic tropical cyclone date.  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 otherfactors set by the vessel master. The rule does not account for sudden and rapid intensification of hurricanes that could result in an outward expansion of the 34 KT wind field. Also,the rule does not account for the typical expansion of the wind field as a system transitions from hurricane to extratropical gale/ storm.
1-2-3 Thumb Rule
1 – 100 miles error radius for 24hrs forecast 2 – 200 miles error radius for 48hrs forecast 3 – 300 miles error radius for 72hrs forecast
tropical revolving storm
1-2-3 Rule Explained above
  How to use 1-2-3 Rule onboard
  1. Plot the current and forecast 24 Hour storm position and forecast radius of 34 knots wind.
  2. Using Compass extend the radius of the forecast 24 hours 34 knots wind area by 100 nm.
  3. 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.
  4. 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.
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Safety Sector Method for keeping vessel clear of TRS

The material given below is an extract from the book ‘HMSO’; Meteorology for Mariners; and is meant to give guidelines for navigating in the vicinity of T.R.S. In order to be on guard for an erratic movement in the path of a tropical “revolving storm”,  it is as well to plot a ‘danger area’ on the chart as an added precaution. How is sector Method done? From the reported position of the centre of the storm, lay off its track and the distance it is expected to progress in 24 hours. From the reported centre, lay off two lines 40o on either side ofthe track.  With the centre of the stormas centre and the estimated progress in 24 hours as radius, describe an arc tocut the two lines on either side of the track. This will embrace the sector into which the storm centre may be expected to move within the next 24 hours. In taking avoiding action, provided there issufficient sea room, the mariner would do well to endeavor to get his ship outside this sector as early as possible. If, after a few hours, the direction of the storm is reference to the new estimated path of the storm and action taken to get out of the sector. The most difficult situation is encountered when the ship finds herself at or near the point of curvature of the storm. In such cases all efforts must be made to avoid crossing ahead of the storm,  and to stay clear of the area into which the storm may turn after recurving. Tracks given in the sailing directions for previous storms, are a good guide to the possible movement of the storm, but reports must be taken at least every 6 hours. Example: A ship in a position A at midnight steaming 180o T at 20 knots receives a report of tropical storm to the south of her with centre at Hmoving north-north-westwards at 6 knots. Sector 1 is drawn but no action is taken at this time since if the storm continues on its course the ship will pass more than 200 nautical mile away from the centre. Six hours later, when the ship is at B, the storm is reported to be centered at Hand moving northwards at 10 knots.
tropical revolving storm
Use of Safety Sector for keeping a ship clear of a Tropical Storm (North Hemisphere)
Sector 2 is drawn and it is apparent that if the storm continues on this path, the closest approach could be 150 nautical mile or considerably less. Speed is therefore reduced to 15 knots and the plot maintained. At 1200, with ship at C, the storm is reported at Hnow moving north-north-eastwards and having accelerated to 12 knots. Sector  3 is drawn and from the plot it is now apparent that if the ship continues on her southerly course she will steam into dangerous proximity to the storm. Heaving to at this stage will only allow the storm to draw closer to the ship;  therefore a bold alteration of course to 250oT is made and speed increased to 20 knots to clear thestorm field. At  1800 with ship at D, the  storm is reported at H4 moving north-eastwards at 15 knots and Sector 4 is drawn. Even if the path of the storm should change to a northerly direction the closest approach now is not likely to be less than 200 nautical miles. To ensure an adequate margin of safety the ship maintains a course of 250oT until midnight and then reverts to her original course of 180o T or an amended southerly course to make her destination. It will be seen from the diagram that the safety sector is merely a rule-of-thumb method of keeping clear of the storm field. Its effectiveness depends on the reception of radio reports giving the position of the storm centre and its progress, and its accuracy on the assumption that the storm will not alter course more than 40o without being detected. If no reports of the position and progress of the storm centre are received, it will be impossible to plot a sector and the mariner must be guided by his own observations and those received from other ships in the vicinity, and by careful attention to the ‘Practical Rules for Avoiding Tropical Storms’.      


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