Tropical Revolving Storms: Formation and Warning Signs

What a TRS is and what it is called

In short: a tropical revolving storm (TRS) is an intense, compact low that forms over warm tropical sea. It is the most dangerous weather a ship can meet, so you must recognise it early and keep clear.

Read first: Pressure, Wind and the Coriolis Effect (topic 1), Humidity, Dew Point and Fog (topic 2), Depressions, Fronts and Synoptic Charts (topic 3), Weather Instruments and Observations (topic 4) and Weather Forecasts and Information at Sea (topic 5). This is topic 6 of Meteorology. Next: Tropical Revolving Storms: Semicircles and Avoiding Action.

• What it is: a deep low with winds spiralling in round a calm eye. Anticlockwise in the NH, clockwise in the SH (as for every low, see Pressure, Wind and the Coriolis Effect).
• How it differs from a temperate depression: a temperate low has fronts, is generally larger and moves faster. A TRS has no fronts, is smaller and more violent.
• Names by ocean (the name depends on where the storm is):

• The Met Office also calls a storm in the eastern Pacific a hurricane. In practice the typhoon is the western Pacific storm.
• Stages (Met Office): the storm is named when its sustained wind reaches 39 mph (34 kn): a tropical storm. At 74 mph (64 kn) it becomes a hurricane, typhoon or cyclone, depending on where it is. Below that it is a tropical depression.
• Trap: “hurricane-force wind” means Beaufort Force 12, 64 kn or more. It describes wind speed only. A winter gale in the North Atlantic can reach it without any TRS.

Where and why they form

In short: a TRS needs very warm sea and enough Coriolis effect to start spinning. That gives a narrow band of latitude on each side of the equator.

• Latitude: between about 5° and 30° N or S; they rarely develop within 5° of the equator (NOAA National Hurricane Center).
• Sea surface temperature: of at least 26.5 °C (NOAA National Hurricane Center). The warm, moist air rising off the sea is the storm’s fuel. This is why hurricanes never form near the UK: the sea there is far too cold (Met Office).
• Why not on the equator: the Coriolis effect is zero at the equator. Without it the air flows straight into the low and fills it, so no spin can start. Storms rarely develop within 5° of it, and the spin has opposite directions on the two sides.
• Over land or cold sea it loses its fuel and weakens. It can still bring very heavy rain far inland.
• Season: mostly late summer and autumn of each hemisphere, when the sea is warmest. Check the Routeing Chart for the month and the Sailing Directions for your area.
• On the bridge: when you plan a passage through the tropics in the season, the TRS risk is part of the passage plan, not something to think about once the barometer falls.

Tracks and recurvature

In short: a TRS first drifts west with the trade winds and towards the pole, then turns away and heads east. That turn is called recurvature.

• Northern Hemisphere: storms forming between 5° and 30° N typically move west (roughly WNW), and near 30° N they often turn and recurve to the NE (NOAA National Hurricane Center).
• Southern Hemisphere: the mirror image: roughly WSW, then recurves to the SE.
• Memory aid: “west and out, then east”: west first, away from the equator, then east.
• Not every storm recurves. Some keep going west into land; some stall or loop. Treat any forecast track as a best guess and keep plotting new positions.
• Why it matters: the track tells you where the storm will be when you get there. A storm “behind” you today may cross your route tomorrow after it recurves.
• Avoiding action (semicircles, wind on which bow) is in the next topic: Tropical Revolving Storms: Semicircles and Avoiding Action.

Warning signs: the barometer

In short: in the tropics pressure normally changes very little. A small, steady fall below the normal value for that place and month is a serious warning.

• Step 1, correct the reading: in the tropics the barometer rises and falls twice a day in a regular pattern (diurnal variation). Remove it first, or you may miss a real fall or chase a false one: the early fall is so gradual that the daily cycle can mask it (NOAA National Hurricane Center).
• Step 2, compare with the mean: the Sailing Directions list the average monthly pressure for each area.
• Step 3, read the difference:

• Memory aid: “3 mb = maybe, 5 mb = for sure”. These are rules of thumb: always check them against the forecasts and warnings.
• Rate of fall: a fall of more than 2 mb in 3 h is significant, above all if it happens in two 3-hour periods in a row. Near a hurricane the pressure can fall 10 mb in 3 h (general barometer rules of thumb).
• On the bridge: log the corrected pressure regularly. If it is 3 mb below the mean, tell the Master and step up the watch. At 5 mb, assume a storm is on the way and act.

Warning signs: swell, wind and sky

In short: the swell often arrives first, then the wind changes, then the sky and the air change. Put all the signs together; no single one is enough.

Swell

• A long, low swell is often the first sign. It can travel about 750 NM a day and be felt more than 3000 NM from the centre (NOAA National Hurricane Center).
• Look up-swell: its direction shows where the storm was when the swell was made. This gives you a first bearing of the storm.
• Watch the period: a long period and an odd direction compared with the local sea are the clue.

Wind

• In the trade-wind belt the wind is steady. In the NH you expect the ENE trade.
• A clear change in direction or strength from the usual trade is a warning sign.
• Later the wind freshens and becomes gusty as the storm gets closer.

Sky and weather

• With the storm 500–1000 NM away the sky may still look clear.
• At about 300–600 NM: thin, wispy high cirrus spreading away from the direction of the storm (NOAA National Hurricane Center).
• Nearer: the cirrus thickens into cirrostratus, then altostratus.
• Then: rain showers and thick cumulonimbus walls of the outer rain bands, when the centre may still be 200–400 NM away.
• Throughout: an oppressive, heavy, humid atmosphere.

Radar

• The radar shows the rain bands of the storm only when they are close. By then it is late to start avoiding action.
• Use it to confirm, never as the first warning.

Monitoring on passage: the twice-daily check

In short: in TRS season, on passage through a TRS area, do a TRS risk analysis twice a day. The aim is to see a storm coming days ahead, not hours.

• The risk-analysis form comes from NOAA’s Mariner’s Guide for Hurricane Awareness in the North Atlantic Basin (2000). It has now been replaced by NOAA’s Mariner’s Tropical Cyclone Guide (NHC, May 2023).
• What to check each time:
  1. Latest tropical storm warnings and advisories received (NAVTEX, SafetyNET, storm centres).
  2. Plot each storm’s position, past track and forecast track on the chart, with the time of each position.
  3. Distance and bearing from the ship to each storm now, and at the forecast times.
  4. Your own corrected pressure vs the mean, wind vs the normal trade, swell and sky.
  5. Decision: carry on, change route or speed, or seek shelter. Tell the Master.
• Plotting: keep the plot up to date with every new warning. Compare where the storm actually went with where it was forecast to go; a storm that is off its forecast track is a warning that the forecast is uncertain.
• On the bridge: brief the next watch on every storm in the area and the latest plot.

Sources of TRS information

In short: combine your own observations with official warnings and with the climate information in the publications.

• Own ship: corrected barometer, swell, wind, sky. The only source that tells you what is happening here, now.
• Official warnings (MSI): NAVTEX near the coast and Inmarsat-C SafetyNET on the ocean carry tropical storm warnings for your area. See Weather Forecasts and Information at Sea.
• Storm warning centres: each ocean has a regional centre that issues TRS advisories, e.g. the US National Hurricane Center for the North Atlantic and eastern Pacific.
• Publications for the climate: Sailing Directions (mean pressure, storm seasons), Routeing Charts (storm tracks per month), Ocean Passages of the World (NP136), NP100 The Mariner’s Handbook.
• Your duty to report: if you meet a tropical storm, send a danger message to ships nearby and the competent authorities (SOLAS V/31). Contents of the report: SOLAS V/32.