Meteorology and Weather Routeing for Ships

Prerequisites

You should already know the weather basics from the RYA Day Skipper and Coastal Skipper/Yachtmaster courses: the Beaufort scale, highs and lows, warm and cold fronts, sea breezes, the shipping forecast, and how to read a simple synoptic chart. You should also know the OOW's restricted-visibility duties from the bridge watchkeeping lesson, and the GMDSS equipment carried on a merchant ship.

For the Officer of the Watch (Unlimited) certificate (STCW Regulation II/1), meteorology is part of the navigation competence "plan and conduct a passage and determine position". You will be asked about it in the MCA written and oral examinations (see MSN 1856 and the MCA oral syllabus in MIN 653 for the current scope).

Learning Objectives

• Read a surface synoptic chart and relate isobars, fronts and pressure tendency to wind and weather at the ship.
• Describe the life of a mid-latitude depression and the weather sequence as it passes.
• Explain how tropical revolving storms (TRS) form, where and when they occur, and how to detect one.
• Find which semicircle of a TRS the ship is in, and apply the avoidance rules in both hemispheres.
• Recognise ice hazards and the reporting duties under SOLAS.
• Name the types of fog at sea and predict when they will form.
• Use NAVTEX, SafetyNET and other forecast sources, and know their limits.
• Explain the principles of weather routeing and the dangers of heavy weather for large ships.
• Make and report ship weather observations.

Why Weather Matters on a Big Ship

A 300 m ship is not immune to weather. Heavy weather damages cargo and hatch covers, loses containers overboard, and makes engines race and overload. Parametric rolling has rolled large container ships through 30° and more, collapsing stacks of containers. Fog remains a factor in many collisions. A TRS can sink any ship. The OOW is the person on the bridge who sees the barometer fall, notices the long swell, and reads the NAVTEX first. Your job is to recognise the signs early and tell the master in time to act.

The Synoptic Chart

A surface synoptic chart shows mean sea-level pressure as isobars, with fronts, highs and lows marked. The analysis shows the situation at a given time (usually 0000, 0600, 1200 or 1800 UTC); a prognosis (forecast chart) shows the expected situation 24, 48 or more hours ahead. Ships receive them by HF radiofacsimile, email or satellite data services.

Reading the isobars

• Spacing gives wind strength. Closer isobars mean stronger winds. A geostrophic wind scale on the chart converts spacing to wind speed for the latitude.
• Direction. At sea the surface wind blows roughly along the isobars but crosses them towards low pressure by about 10° to 20° (more over land, because of friction). In the northern hemisphere the wind blows anticlockwise round a low and clockwise round a high; in the southern hemisphere, the opposite.
• Buys Ballot's law. In the northern hemisphere, stand with your back to the true wind and low pressure is on your left. In the southern hemisphere it is on your right.

Pressure features

Pressure tendency

The barometer's change over the last three hours is more useful than its value. The UK Met Office uses these terms:

A fall of more than 6 hPa in three hours means a gale is likely. In the tropics, allow for the regular daily (diurnal) pressure wave of about 3 hPa, with highs near 1000 and 2200 local time and lows near 0400 and 1600. Without that correction a normal afternoon fall can look like a warning, and a real warning can be masked.

Mid-Latitude Depressions

Formation and life

Depressions in mid-latitudes form on the polar front, the boundary between cold polar air and warm sub-tropical air. A wave develops on the front, pressure falls at its tip, and the circulation draws warm air poleward (the warm front) and cold air equatorward behind (the cold front). The wedge of warm air between them is the warm sector. The cold front moves faster than the warm front and eventually catches it, lifting the warm air off the surface to form an occlusion. The depression then usually slows and fills.

Depressions in the North Atlantic usually move east or north-east at 15 to 30 knots, broadly in the direction of the isobars in the warm sector, but some deepen explosively ("weather bombs", a fall of 24 hPa or more in 24 hours) and some stall.

The diagram below shows a northern hemisphere depression and the weather in each sector.

Click to zoom

The weather sequence

For a ship on the equatorward side of the centre (the usual case in the English Channel or Western Approaches), as a depression passes:

1. Ahead of the warm front: high cirrus thickens to altostratus, the barometer falls steadily, the wind backs a little and freshens, rain starts and becomes continuous, and visibility drops.
2. At the warm front: the wind veers (perhaps from south to south-west), the rain eases, and the fall of the barometer slows or stops.
3. Warm sector: mild, humid, low cloud, drizzle, and poor visibility; sea fog is common over cold water.
4. At the cold front: a sharp veer (south-west to north-west), heavy rain, squalls, and sometimes thunder. The barometer rises quickly.
5. Behind the cold front: cold air, cumulus and showers, good visibility between showers, gusty winds.

On the poleward side of the centre the wind backs steadily (east, then north-east, then north) instead of veering, and the fronts may not be felt as clearly.

Why it matters to the OOW

The sequence gives warning. A falling barometer, backing wind and thickening cloud tell you a front is coming hours before the rain. Note them in the log, compare them with the forecast, and tell the master if the weather is developing faster or worse than forecast.

Tropical Revolving Storms

What and where

A TRS is an intense, compact low that forms over warm tropical oceans. Its winds can exceed 120 knots, it can produce seas over 15 m, and it brings torrential rain and storm surges on coasts. In the centre is the eye, typically 10 to 30 miles across, with light winds and sometimes clear sky, surrounded by the eye wall where the strongest winds blow.

Local names are:

TRS form only where:

• the sea surface temperature is at least about 26 to 27°C over a deep layer;
• the latitude is more than about 5° from the equator, so that the Coriolis effect can start the rotation (there are none on the equator, and none in the South Atlantic in normal years);
• the wind shear between low and high levels is small.

They are classified by sustained wind: tropical depression (below 34 knots), tropical storm (34 to 63 knots), and hurricane, typhoon or severe cyclone (64 knots and over, Force 12).

Typical track

In both hemispheres a TRS usually moves westward at first at about 10 to 15 knots, drifting away from the equator. Many then recurve: in the northern hemisphere they turn north and then north-east; in the southern hemisphere south and then south-east. After recurving they often speed up. Tracks are irregular, so never assume a storm will follow the textbook path.

Warning signs

Modern forecasts from the regional warning centres (for example the US National Hurricane Center and the Japan Meteorological Agency) give positions, tracks and wind radii, broadcast through NAVTEX and SafetyNET. But forecasts can be late or wrong, and you must know the physical signs:

• Swell: a long, low swell from an unusual direction, which can arrive many hundreds of miles ahead of the storm. Its direction points roughly to the storm.
• Barometer: a pressure 3 hPa or more below the normal for the time of year and place, after correcting for the diurnal variation, should make you suspect a TRS; 5 hPa or more below normal makes one nearly certain (The Mariner's Handbook, NP100).
• Sky: cirrus in bands converging towards the storm, then a dark wall of cumulonimbus (the bar of the storm).
• Wind: a change in the steady trade wind direction, or the wind increasing with heavy rain squalls.

Finding the centre

In the northern hemisphere, face the true wind: the centre lies about 90° to 135° to your right (often stated as about 10 points). In the southern hemisphere it lies to your left. Plot this bearing and update it as the wind changes.

The semicircles

Divide the storm by its track. Looking along the direction the storm is moving:

• Northern hemisphere: the right-hand side is the dangerous semicircle, and the left-hand side is the navigable semicircle.
• Southern hemisphere: the left-hand side is dangerous, and the right-hand side navigable.

The dangerous semicircle has stronger winds, because the storm's forward speed adds to the wind there, and its winds tend to blow a ship ahead into the path of the storm. The front (advance) quadrant of the dangerous semicircle is the worst place of all, and it is also where the storm may recurve towards you. In the navigable semicircle the winds are weaker and tend to push a ship towards the rear of the storm. "Navigable" means less dangerous, not safe.

The diagram below shows both hemispheres.

Click to zoom

Which semicircle are you in?

Watch the true wind direction over time (allow for your own course changes by stopping or holding a steady course while you observe):

Avoidance rules

The rules below are for power-driven ships, as given in The Mariner's Handbook and standard textbooks. They aim to take the ship away from the centre and, above all, out of the dangerous front quadrant. Begin early: the best action is to stay well clear, keeping at least 250 miles from the centre where you can, and not to cross ahead of the storm's track.

Northern hemisphere:

• Dangerous semicircle: bring the wind 10° to 45° (1 to 4 points) on the starboard bow and proceed at the best practical speed. As the wind veers, alter course to starboard to keep it on the same relative bearing. If the ship cannot make headway, heave to with the wind on the starboard bow.
• Navigable semicircle: bring the wind well on the starboard quarter and proceed at best speed, noting the course. Alter course to port as the wind backs.
• In the path, ahead of the centre: bring the wind well on the starboard quarter and proceed at best speed into the navigable semicircle, then act as for the navigable semicircle.
• Behind the centre: keep clear of the storm's track, and avoid steering in the direction the storm is heading.

Southern hemisphere: the same, with port and starboard exchanged. In the dangerous (left-hand) semicircle bring the wind on the port bow and alter to port as the wind backs; in the navigable semicircle or the path, put the wind on the port quarter.

The diagram below applies the northern hemisphere rules to three ships.

Click to zoom

The 1-2-3 rule

Forecast track positions have errors that grow with time. A widely taught planning rule (from US practice) adds 100 miles to the forecast 34-knot wind radius at 24 hours, 200 miles at 48 hours and 300 miles at 72 hours, and treats the result as the area to avoid. Forecast accuracy has improved since the rule was devised, but it remains a sensible, cautious margin. Check your company's heavy weather and TRS procedures for the margins they require.

Reporting a TRS

SOLAS Chapter V, Regulation 31, requires the master of every ship meeting a tropical storm, or winds of Force 10 or more for which no storm warning has been received, to report it by all available means to ships nearby and to the competent authorities (via a coast station). Regulation 32 lists the information: a statement that a TRS has been encountered, time (UTC) and date, position, barometer reading (corrected) and tendency, true wind direction and force, state of sea and swell (with direction, period and height), and the ship's true course and speed. While under the influence of the storm, further reports should be sent, preferably at hourly intervals but in any case at least every three hours. These messages are sent as safety (SECURITE) messages.

Ice

Hazards

• Icebergs from the Greenland glaciers drift south past Newfoundland on the Labrador Current, most numerous from about February to July. The International Ice Patrol, run by the US Coast Guard under SOLAS, monitors them and publishes the limit of known ice.
• Growlers and bergy bits: small pieces broken from bergs, low in the water and hard to see or detect by radar, especially in sea clutter. They can hole a ship.
• Sea ice (pack ice): frozen sea water, in the Baltic, Gulf of St Lawrence, the Arctic and Antarctic, and in winter in the Sea of Okhotsk and elsewhere.
• Ice accretion: spray freezing on the superstructure, which raises the centre of gravity and can capsize smaller ships. The risk is high when the air temperature is below about −2°C with strong winds, and severe at lower temperatures and higher winds.

Signs of ice nearby

• A sudden fall in sea temperature (though this is not reliable on its own).
• Ice blink: a white glare on the underside of cloud over pack ice.
• An unusual calm: a sudden lack of swell suggests ice to windward.
• Radar echoes that come and go in the clutter.

Duties

When ice is reported on or near the track, SOLAS requires the master to proceed at a moderate speed at night or to alter course to pass well clear. Report dangerous ice encountered (type of ice, position, time UTC) under SOLAS V/31. Post extra lookouts, use both radars on appropriate ranges, and reduce speed in poor visibility so that the ship can stop in the distance a growler can be seen. Ships in polar waters must comply with the IMO Polar Code.

Fog at Sea

Fog forms when air is cooled to its dew point, so that its water vapour condenses. The OOW can predict it by comparing the air temperature (dry bulb) with the dew point worked out from the wet bulb: as they close to within about 1°C, fog becomes likely.

The diagram below shows the three types most met at sea.

Click to zoom

• Advection fog (sea fog): warm, moist air moves over colder sea and is cooled from below. It can cover large areas and persist even in fresh winds. Common on the Grand Banks, where the Gulf Stream meets the Labrador Current; off the UK in spring and early summer when the sea is still cold; and in the warm sectors of depressions.
• Radiation fog: forms over land on clear, calm nights as the ground cools. It drifts out over harbours, estuaries and narrow coastal waters, and usually clears by late morning as the sun warms the land.
• Sea smoke (steam fog): very cold air flowing over relatively warm water, as off ice edges and Arctic coasts in winter. Usually shallow, but it can be dense.
• Frontal fog: rain falling into cold air near a warm front saturates it and reduces visibility.

When visibility falls, the restricted-visibility duties apply at once: COLREG Rules 6, 19 and 35, plus MGN 315 and MGN 369 practice (inform the master, post a dedicated lookout, engines on standby, radar plotting, safe speed, sound signals, hand steering where appropriate).

Forecasts and Warnings

NAVTEX

NAVTEX is the GMDSS system for broadcasting maritime safety information (MSI) to ships in coastal waters, to roughly 250 to 400 miles from the transmitter.

• 518 kHz: the international service, in English.
• 490 kHz: national services, which may be in the local language.
• 4209.5 kHz: HF NAVTEX in some areas.

Each message has a header with the station letter, a subject indicator and a serial number. The main subject indicators are:

The receiver cannot be set to reject A, B, D and L. Program in the stations for the area, check the receiver at the start of each watch, and read every new message.

SafetyNET and other satellite services

Beyond NAVTEX range, MSI is broadcast by the Enhanced Group Call (EGC) service: Inmarsat SafetyNET (and now Iridium SafetyCast, recognised in the GMDSS). The world is divided into 21 NAVAREAs for navigational warnings and matching METAREAs for weather. Ocean forecasts and storm warnings for each METAREA are broadcast on schedule, and the ship's EGC receiver prints or displays them.

Other sources

• Radiofacsimile (HF weather fax): surface analyses, prognoses, wave charts, ice charts.
• GRIB files and weather apps: raw computer model output. They are useful for trends but have no forecaster's judgement. They can under-forecast gusts, local effects and the intensity of tropical storms, and they cannot replace official warnings.
• Shore-based routeing services and the company's weather department.
• Own observations: barometer, wind, sky, sea and swell. Always compare the forecast with what you see.

Weather Routeing

Climatic routeing

At the planning stage, use climatic data to choose a route for the season: Ocean Passages for the World (NP136), routeing charts (monthly charts of winds, currents, ice limits and storm tracks), and the Load Line Convention zones and seasonal areas, which set the draught limits by area and time of year. Great-circle routes in winter in the North Atlantic or North Pacific run into the main storm tracks; a longer route further south may be faster and safer.

Ship routeing services

Many companies contract a weather routeing service. It uses forecast wind, waves, currents and ice together with the ship's performance curves (speed loss in waves from different directions) to recommend a route, updated during the voyage. The aim may be least time, least fuel, least risk of damage, or a fixed arrival time with minimum fuel.

Routeing advice is advice. The master keeps full responsibility for the ship's safety and decides whether to follow it. The OOW should know the current routeing recommendation and report when the weather differs from it.

Heavy weather dangers for large ships

IMO guidance to masters on avoiding dangerous situations in adverse weather and sea conditions (MSC.1/Circ.1228) describes these hazards:

• Synchronous rolling: the wave encounter period is close to the ship's natural roll period, so each wave adds to the roll.
• Parametric rolling: in head or following seas, when the encounter period is about half the roll period and the wavelength is similar to the ship's length, the ship's stability varies as the crest passes amidships and then the ends. Rolling can build up within a few cycles. Large container ships and car carriers with fine hull forms and wide flare are most at risk.
• Surf-riding and broaching in following and quartering seas, when the ship's speed is close to the wave speed.
• Reduced stability on a wave crest amidships.
• Slamming and green water in head seas, damaging the bow, hatch covers and deck cargo.

The remedy in each case is to change the encounter period: alter course, alter speed, or both. Check the ship's guidance (some ships carry onboard polar diagrams showing dangerous speed and heading combinations).

Ship Weather Observations

Ships are the main source of surface weather data over the oceans. SOLAS V/5 asks governments to encourage ships to collect and report weather data, and many ships volunteer through the WMO Voluntary Observing Ships (VOS) scheme, recruited in the UK by the Met Office.

Observations are made at the main synoptic hours (0000, 0600, 1200 and 1800 UTC) and, if possible, at the intermediate hours (0300, 0900, 1500 and 2100 UTC). A full observation includes:

• Pressure, read from a precision aneroid barometer and corrected to mean sea level, plus the three-hour tendency.
• Air temperature (dry bulb) and wet bulb, from a screen on the windward side, away from heat sources.
• Sea temperature (from an engine intake or a bucket sample).
• True wind direction and speed, worked out from the apparent wind and the ship's course and speed.
• Visibility, present and past weather, and cloud type and cover.
• Wind waves and swell: direction, period and height.
• Ice, and ice accretion if present.

The data is coded (or entered in electronic logging software) and sent, normally free of charge to the ship, to the national weather service. Your observation helps the next forecast for every ship in the area.

Worked Example: A TRS in the North Atlantic

A bulk carrier is at 18° N, 60° W on 10 September, course 270°, 13 knots, bound for the Gulf of Mexico. The noon NAVTEX and SafetyNET carry a hurricane warning: centre at 13° N, 54° W, moving 285° at 12 knots, hurricane-force winds within 40 miles, 34-knot winds within 150 miles.

1. Plot it. The storm is about 460 miles to the south-east and moving west-north-west. Its forecast track passes about 200 miles south of the ship's present position, and crosses the ship's intended track further west.
2. Apply the 1-2-3 rule. At 24 hours the forecast centre is about 290 miles further on. Avoidance radius = 150 + 100 = 250 miles. At 48 hours: 150 + 200 = 350 miles around a point about 580 miles on. Both circles cover the ship's planned route.
3. Find the semicircle. The ship is on the right-hand (north) side of the storm's track, ahead of the centre: the dangerous semicircle, in the front quadrant, the worst position.
4. Act early. Continuing west at 13 knots would keep the ship just ahead of a storm moving the same way at 12 knots. The master alters course to the north at full speed to open the distance from the track, accepting a delay, and informs the company and the routeing service.
5. Monitor. The OOW records the corrected barometer and the swell each hour. That evening a long swell arrives from the south-east and the barometer is 4 hPa below normal for the time of day. The wind, north-east at first, veers to east and freshens: confirmation that the ship is in the dangerous semicircle. The OOW calls the master.
6. Avoidance rule. With the wind veering and freshening, the master brings the wind about 30° on the starboard bow, which puts the ship's head about north-east, away from the track, at best speed, and alters further to starboard as the wind veers. The ship sends a TRS report under SOLAS V/31 because it meets stronger winds than the warning predicted.

Common Mistakes

• Ignoring the barometer's trend and watching only its value. Log it every hour in deteriorating weather.
• Forgetting the diurnal variation in the tropics.
• Mixing up the hemispheres. Write the rules for the hemisphere you are in on the bridge.
• Calling the navigable semicircle "safe". It is only less dangerous.
• Trying to cross ahead of a TRS to save time.
• Trusting GRIB files over official warnings, especially for tropical storms.
• Assuming radar will detect growlers. It often will not.
• Expecting fog only in calm weather. Advection fog can come with Force 5 or 6.
• Treating routeing advice as an order rather than information for the master's decision.

Summary

• Isobar spacing gives wind strength; at sea the wind crosses isobars inward by 10° to 20°. Buys Ballot: back to the wind, low on the left in the northern hemisphere.
• Pressure tendency over three hours is the key warning. A fall of over 6 hPa in three hours suggests a gale.
• A depression brings a predictable sequence: falling glass and backing wind ahead of the warm front, veer and drizzle in the warm sector, sharp veer and squalls at the cold front.
• TRS form over warm seas away from the equator. Warning signs: swell, a barometer 3 hPa or more below normal, cirrus, changing trade winds.
• Northern hemisphere: dangerous semicircle on the right of the track; wind veering means dangerous; put the wind on the starboard bow. Southern hemisphere: everything reversed.
• Navigable semicircle: wind on the quarter (starboard in the north, port in the south).
• Report TRS, Force 10 winds without warning, and dangerous ice under SOLAS V/31.
• Fog: advection (warm air over cold sea), radiation (land at night), sea smoke (cold air over warm sea).
• NAVTEX on 518 kHz covers coastal waters; SafetyNET covers the oceans by METAREA.
• Weather routeing balances time, fuel and risk; the master decides. Beware synchronous and parametric rolling.

Check Your Understanding

1. In the northern hemisphere you stand with your back to the true wind. Where is the low pressure?

Answer: On your left (and slightly ahead), by Buys Ballot's law. In the southern hemisphere it would be on your right.

1. The barometer has fallen 4.5 hPa in the last three hours. How is this described, and what does it suggest?

Answer: Falling quickly (3.6 to 6.0 hPa in three hours). A deepening or approaching depression, with strong winds likely.

1. Describe the change in wind and weather at a cold front in the northern hemisphere.

Answer: A sudden veer (typically south-west to north-west), heavy rain and squalls, perhaps thunder, then a quick rise in pressure, colder air, showers and better visibility.

1. Give three conditions needed for a TRS to form.

Answer: Sea surface temperature of about 26 to 27°C or more; latitude more than about 5° from the equator; low vertical wind shear. A pre-existing disturbance helps.

1. In the northern hemisphere the wind is steadily veering and increasing. Which semicircle are you in, and what should a power-driven ship do?

Answer: The dangerous semicircle. Bring the wind 10° to 45° on the starboard bow, proceed at best speed, and alter to starboard as the wind veers. If unable to make way, heave to with the wind on the starboard bow.

1. In the southern hemisphere the wind direction stays steady while it strengthens and the barometer falls. Where are you and what action is recommended?

Answer: In or near the path ahead of the centre. Bring the wind well on the port quarter, proceed at best speed into the navigable semicircle (on the right of the track in the southern hemisphere), then act as for the navigable semicircle.

1. What information must a TRS report under SOLAS V/32 contain?

Answer: A statement that a TRS has been encountered; time (UTC) and date; position; corrected barometer reading and tendency; true wind direction and force; state of sea and swell; and the ship's true course and speed.

1. Why are growlers particularly dangerous?

Answer: They are small and low in the water, so they are hard to see, especially at night or in fog, and often do not show on radar among sea clutter, yet they are heavy enough to hole a ship.

1. Which NAVTEX message categories cannot be rejected, and on which frequency is the international service broadcast?

Answer: A (navigational warnings), B (meteorological warnings), D (search and rescue and piracy) and L (further navigational warnings). The international service is on 518 kHz.

1. What is parametric rolling, and what can the OOW do about it?

Answer: Large rolls building up in head or following seas when the wave encounter period is about half the natural roll period and the wavelength is close to the ship's length, because stability changes as crests pass. Change the encounter period by altering course and/or speed, and call the master.

Further Reading

• R.M. Frampton and P.A. Uttridge, Meteorology for Seafarers (Brown, Son & Ferguson)
• UKHO NP100, The Mariner's Handbook
• UKHO NP136, Ocean Passages for the World
• SOLAS consolidated edition (IMO), Chapter V Safety of Navigation and Chapter IV Radiocommunications
• ICS Bridge Procedures Guide, 6th edition (International Chamber of Shipping, 2022)
• MGN 369 (M+F) Amendment 1, navigation practices in restricted visibility
• IMO Model Course 7.03, Officer in Charge of a Navigational Watch
• IMO Model Course 1.25, GMDSS General Operator's Certificate