Marine meteorology for navigators

In short

Marine meteorology is the study of atmospheric pressure, wind, moisture, cloud, visibility and the resulting sea state as they affect a ship at sea, together with the observing and warning services that supply the data. A deck officer uses it to decide a safe route, to avoid tropical revolving storms and heavy swell, to anticipate fog and reduced visibility, and to protect cargo, hull and crew. It is examined in B.Sc Nautical Science Navigation papers and in the DG of Maritime Administration (formerly DG Shipping) oral and written examinations for Second Mate and Mate/Master, where storm avoidance and chart interpretation are standard questions. Employers — Shipping Corporation of India, Great Eastern Shipping, Essar, Synergy, Anglo-Eastern, Fleet Management, Maersk Line India and the Indian Register of Shipping — test it in cadet and officer interviews. The Indian framework is the Merchant Shipping Act, 2025 (which replaced the 1958 Act from 15 March 2026), SOLAS Chapter V, and IMD services including RSMC New Delhi for the North Indian Ocean.

Explained

Atmosphere, pressure and the synoptic picture

• Layers: troposphere (weather lives here; depth about 8 km at the poles to 16 km at the equator, temperature falling about 6.5 °C/km), then the tropopause and stratosphere (ozone, temperature rises).
• Atmospheric pressure: force per unit area of the air column; SI unit the pascal, reported in hectopascals (hPa), numerically equal to the old millibar. Standard mean sea level pressure = 1013.25 hPa.
• Mean sea level reduction: a shipboard reading must be corrected for index error and for height of the barometer above the waterline (about +0.11 hPa per metre near the surface, i.e. roughly 1 hPa per 9 m). A mercury barometer also needs temperature and latitude (gravity) corrections; an aneroid/precision aneroid barometer needs only index and height.
• Barometric tendency: the change over the past 3 hours, reported with the trace shape from the barograph. Rapid fall means a deepening low approaching; rapid rise behind a front means a squally, gusty cold-air stream.
• Diurnal variation: in the tropics pressure peaks near 1000 and 2200 local time and dips near 0400 and 1600, range about 1–3 hPa. A reading 3 hPa or more below the diurnal-corrected normal for the place and date is a classic warning that a tropical storm lies within a few hundred miles.
• Synoptic chart: simultaneous observations plotted and analysed — isobars (equal pressure), highs, lows, troughs, ridges, cols, fronts. Close isobars mean a steep pressure gradient and strong wind.
• Pressure systems: low (depression/cyclone) — inflow, convergence, rising air, cloud and rain; high (anticyclone) — outflow, subsidence, clear skies but often haze or fog in winter.

Winds: forces, laws and local systems

• Forces on air: pressure gradient force (drives the wind from high to low), Coriolis force (deflects to the right in the Northern Hemisphere, left in the Southern, zero at the equator), centrifugal force on curved flow, and surface friction.
• Geostrophic wind: straight, frictionless, balanced flow parallel to the isobars. Gradient wind includes curvature — subgeostrophic around a low, supergeostrophic around a high. At the surface friction turns the wind across the isobars into the low by about 10°–20° over sea and 25°–45° over land, and cuts its speed.
• Buys Ballot's Law: in the Northern Hemisphere, stand with your back to the wind and the low pressure lies to your left (reverse in the Southern Hemisphere). For a tropical storm, facing the wind the centre bears about 90°–135° to the right in the NH, to the left in the SH.
• Veering = wind direction changing clockwise (e.g. SW to W); backing = anticlockwise. Direction is always the direction the wind blows from, in degrees true.
• Beaufort scale: wind force 0–12 estimated from the appearance of the sea; it is a wind scale, not a wave scale. Douglas scale rates sea (wind-driven) and swell separately.
• Global circulation: Hadley, Ferrel and polar cells; trade winds, doldrums/ITCZ, horse latitudes (subtropical highs), westerlies, roaring forties, polar easterlies.
• Monsoon: differential heating of the Asian landmass reverses the flow. Southwest monsoon — onset over Kerala around 1 June, strong SW winds, heavy rain and high swell in the Arabian Sea and Bay of Bengal, withdrawal from September; northeast monsoon — dry NE winds from October to December, bringing rain to Tamil Nadu and the southeast coast. Ports such as Mangalore and Ratnagiri restrict operations in the SW monsoon fair-weather/foul-weather seasons.
• Local winds: sea breeze (day, from sea to land, onset mid-morning, may reach force 4–5 and veers with Coriolis), land breeze (night, weaker), anabatic/katabatic valley winds, and named winds (Bora, Mistral, Sirocco, Harmattan, Loo, Norwester/Kalbaisakhi of Bengal).

Moisture, clouds, visibility and fog

• Humidity: absolute humidity (mass of vapour per m³); specific humidity; relative humidity = actual vapour pressure ÷ saturation vapour pressure at that temperature, as a percentage. Dew point is the temperature to which air must cool at constant pressure for saturation. Measured with a Mason's (wet-and-dry-bulb) hygrometer in a screen or sling psychrometer.
• Lapse rates: dry adiabatic 9.8 °C/km, saturated adiabatic about 5–6 °C/km, environmental (observed) varies. Environmental lapse rate greater than the adiabatic → unstable air (cumuliform cloud, showers, squalls, good visibility between showers); less → stable (stratiform cloud, drizzle, poor visibility); inversion → very stable, trapping haze and smoke, and causing super-refraction of radar/visual horizon.
• Cloud families: high (cirrus, cirrocumulus, cirrostratus — ice), middle (altostratus, altocumulus, nimbostratus), low (stratus, stratocumulus, cumulus, cumulonimbus). Cumulonimbus brings thunderstorms, hail, line squalls, microbursts and waterspouts. Cloud amount is logged in oktas.
• Fog types at sea:
  • Advection fog — warm moist air moves over a colder sea; the sea-surface temperature is below the air's dew point. The main sea fog (Grand Banks, Arabian Sea in the pre-monsoon, Gulf in winter).
  • Radiation fog — land cools at night; drifts out over harbours and estuaries (Mumbai, Kandla, Haldia in winter).
  • Frontal fog — rain falling into cold air at a warm front saturates it.
  • Steam/arctic sea smoke — very cold air over a much warmer sea.
  • Mixing fog — two nearly saturated air masses of different temperature mix.
• Rule of thumb: compare sea-water intake temperature with the air dew point every watch; when SST falls below the dew point, expect fog, and comply with COLREG Rule 19 and SOLAS requirements for safe speed.
• Visibility codes: fog < 1,000 m (dense fog < 50 m), mist/haze 1–2 km, moderate 2–5 km, good > 5 km.

Air masses, fronts and mid-latitude depressions

• Air masses take the temperature and humidity of their source: maritime tropical (mT), continental tropical (cT), maritime polar (mP), continental polar (cP), arctic/antarctic. They modify as they travel.
• Fronts: boundary between two air masses. Warm front — warm air rides over cold, gentle slope (1:100–1:150), sequence cirrus → cirrostratus → altostratus → nimbostratus, continuous rain, veering wind, falling then steadying pressure, poor visibility. Cold front — cold air undercuts warm, steep slope (1:50), cumulonimbus, line squall, sharp veer, pressure rise, sudden temperature drop, clearing with good visibility. Occluded front — cold front overtakes warm, mixed characteristics. Stationary front — little movement.
• Frontal (extratropical) depression life cycle: frontal wave on the polar front → open wave with warm sector → mature deepening low → occlusion → decay. Diameter 1,000–3,000 nm, moves typically 20–30 kt, usually eastward in the westerlies; the warm sector gives the worst visibility and the cold sector the heaviest swell and gusts.
• Secondary lows, troughs and polar lows can be more violent than the parent system; a trough gives a wind shift and squalls with no closed centre.
• Differences from a tropical storm: frontal lows have fronts, a cold core, broad wind fields and strengthen over cold water in high latitudes; tropical storms are warm-cored, frontless, small and far more intense.

Tropical revolving storms (TRS) and avoidance

• Formation requirements: sea-surface temperature above about 26.5 °C to a depth of 50–60 m, latitude roughly 5°–20° (enough Coriolis), weak vertical wind shear, pre-existing disturbance, mid-level moisture. Names: cyclone (Indian Ocean), typhoon (NW Pacific), hurricane (Atlantic/NE Pacific), willy-willy (Australia).
• Structure: eye (10–50 nm across, calm, light winds, confused sea), eye wall (strongest winds and rain), spiral rain bands, outflow cirrus aloft. Diameter 100–500 nm — far smaller than a frontal low. Storm surge is the chief killer in the Bay of Bengal because of the shallow, funnel-shaped head of the bay.
• Movement: away from the equator, generally westward at 10–15 kt in low latitudes, then recurving poleward and eastward near the point of recurvature (vertex); speed increases after recurvature and the storm may become extratropical.
• North Indian Ocean seasons: two peaks — pre-monsoon April–June and post-monsoon October–December. The Bay of Bengal sees roughly four times as many systems as the Arabian Sea.
• IMD classification by 3-minute sustained wind: Depression 17–27 kt; Deep Depression 28–33 kt; Cyclonic Storm 34–47 kt; Severe Cyclonic Storm 48–63 kt; Very Severe Cyclonic Storm 64–89 kt; Extremely Severe Cyclonic Storm 90–119 kt; Super Cyclonic Storm 120 kt and above. Names come from the WMO/ESCAP panel list of 13 member countries.
• Signs of an approaching TRS: a long, low swell from an unusual direction (travels faster than the storm, the earliest sign); pressure 3 hPa or more below normal after diurnal correction; unusually clear air then cirrus converging, altostratus, bands of cumulonimbus; wind increasing and steadily changing direction; heavy oppressive weather.
• Locating the centre: apply Buys Ballot's Law; the path is found by taking bearings of the centre at intervals — a steady bearing with falling pressure means the storm is heading for the ship.
• Semicircles (Northern Hemisphere): the right-hand semicircle relative to the track is dangerous — storm wind and forward motion add, and the wind tends to blow the ship into the path; the left is the navigable semicircle. Reversed in the Southern Hemisphere. The forward sector of the dangerous semicircle is the dangerous quadrant.
• Action (NH): in the dangerous semicircle, put the wind on the starboard bow (about 010°–045° relative), proceed at best speed and the wind will veer; in the navigable semicircle, put the wind on the starboard quarter (about 130°–180° relative) and run, altering to keep it there as the wind backs. In the SH use the port bow and port quarter respectively, with the left semicircle dangerous. If the centre is ahead and in doubt in the NH, heave to on the starboard tack.
• Clearance: aim to keep at least 200 nm from the centre of a developing storm and more (250–300 nm) from a mature one, allowing for forecast error with the 1-2-3 rule. IMO guidance to masters on avoiding dangerous situations in adverse weather and sea conditions (MSC.1/Circ.1228) covers synchronous and parametric rolling, broaching and high wave groups.
• Reporting: SOLAS Chapter V obliges the master to report by all means a tropical storm encountered, winds of force 10 or more with no storm warning received, dangerous ice, derelicts and sub-freezing air temperatures with gale conditions — a danger message to nearby ships and the competent authority. This is enforced in India through the Merchant Shipping Act, 2025 and its rules.

Ocean currents, sea, swell and ice

• Currents are driven by wind, density (thermohaline) differences and the Coriolis effect; surface drift is roughly 2–3% of wind speed, deflected about 20°–45° from the wind. Major gyres are clockwise in the NH and anticlockwise in the SH.
• Indian Ocean peculiarity: the North Indian Ocean currents reverse with the monsoon — the Southwest Monsoon Current flows east in the summer and the Northeast Monsoon Current west in the winter; the Somali Current reverses and upwells strongly in the SW monsoon. Routeing and ETA calculations must use seasonal routeing charts and pilot charts, not a fixed current.
• Waves: height depends on wind speed, fetch and duration. Sea is the locally generated wave field; swell is the regular, longer-period remnant travelling out of the generating area. Significant wave height (Hs) is the mean of the highest one-third of waves; the maximum probable wave in a storm is about 1.6 to 2.0 × Hs.
• Rogue/abnormal waves occur where swell meets an opposing current — the Agulhas Current off South Africa is the classic case, and the route guidance is to stay inshore of the 200 m line or well outside the current.
• Ship response: synchronous rolling when the wave encounter period equals the natural roll period; parametric rolling when it is about half the roll period in head or following seas (container ships); broaching and loss of stability in following/quartering seas; pooping. The remedy is to change course and/or speed to shift the encounter period.
• Ice: icebergs (glacial), sea ice, growlers and bergy bits; the limit of maximum iceberg drift is charted and the North Atlantic is covered by the International Ice Patrol (SOLAS Chapter V). Topside icing from freezing spray with air below about −2 °C and strong winds raises KG and can capsize a small vessel; relevant to Indian ships trading to north Russia, Canada and the Antarctic.

Observing, Indian services and weather routeing

• Shipboard instruments: precision aneroid barometer and barograph; dry- and wet-bulb thermometers in a marine screen; sea temperature from the engine-room intake or a canvas bucket; anemometer (relative wind — convert to true); cloud, visibility and sea/swell by estimation; rain gauge on some ships.
• Observation and coding: synoptic hours 0000, 0600, 1200, 1800 UTC; the ship synoptic report is coded in FM 13 SHIP and is sent free of charge through the coast station. India's Voluntary Observing Ships (VOS) fleet is run by IMD, which supplies and calibrates the instruments and issues the Ship Weather Code book and logbook.
• India Meteorological Department: RSMC New Delhi is the WMO Regional Specialised Meteorological Centre for tropical cyclones over the North Indian Ocean. Area Cyclone Warning Centres at Chennai, Mumbai and Kolkata and Cyclone Warning Centres at Visakhapatnam, Bhubaneswar, Ahmedabad and Thiruvananthapuram issue coastal and port warnings; Port Meteorological Offices at the major ports brief masters. INSAT-3D/3DR imagery, Doppler weather radars and buoys of INCOIS (Hyderabad) feed the analysis; INCOIS issues ocean state forecasts and high-wave/swell-surge alerts.
• Dissemination to ships (GMDSS maritime safety information): NAVTEX on 518 kHz (English) and 490/4209.5 kHz for coastal areas out to about 250–400 nm; Inmarsat SafetyNET (EGC) for the high seas; HF radiofacsimile and radiotelephony; internet and e-mail routeing products in port and via VSAT. India is the issuing service for the marine forecast area METAREA/NAVAREA VIII (N), and the national NAVAREA coordinator function sits with the Indian Navy's National Hydrographic Office, Dehradun. Exact station lists and schedules change — always verify in the current Admiralty List of Radio Signals Vol. 3 and Indian Notices to Mariners.
• Weather routeing: choosing the least-time or least-damage track using forecast winds, waves and currents, subject to limiting values of wave height, roll and speed loss. Done onboard with routeing charts and forecasts, or by a shore service. Benefits: fuel saving (directly relevant to the IMO CII and EEXI regime), less hull and cargo damage, better ETA reliability. Limitation: forecast skill falls off beyond about 5 days, and the master remains responsible for the route under SOLAS Chapter V Regulation 34 (voyage planning).

Key points

• Isobar spacing - the closer the isobars, the steeper the pressure gradient and the stronger the wind.
• Buys Ballot's Law - back to the wind, low pressure on the left in the Northern Hemisphere, on the right in the Southern.
• Veer/back - veering is a clockwise change of wind direction, backing anticlockwise; wind is always named by where it comes from.
• Barometer height correction - add about 0.11 hPa per metre of barometer height above sea level to get MSL pressure.
• Tropical diurnal range - 1-3 hPa; a 3 hPa shortfall against normal is a TRS alarm, not a normal fluctuation.
• TRS birth conditions - SST above about 26.5 °C, latitude 5°-20°, low wind shear; they never form on the equator because Coriolis is zero.
• Dangerous semicircle - the right-hand side of the track in the NH, the left in the SH; the forward half of it is the dangerous quadrant.
• NH avoiding action - dangerous semicircle: wind on the starboard bow, full speed; navigable semicircle: wind on the starboard quarter and run.
• Long low swell - the earliest and most reliable visual sign of a distant tropical storm, because swell outruns the system.
• IMD cyclone grades - Cyclonic Storm at 34 kt, Very Severe at 64 kt, Super Cyclonic Storm at 120 kt (3-minute mean wind).
• North Indian Ocean season - April to June and October to December, with the Bay of Bengal far more active than the Arabian Sea.
• Advection fog - forms when sea-surface temperature is below the dew point of the air flowing over it; compare SST and dew point each watch.
• Instability - environmental lapse rate exceeding the adiabatic gives cumuliform cloud, showers and squalls; inversions trap haze and distort the radar horizon.
• Cold front - sharp veer, squall, pressure rise and clearing; warm front - slow veer, continuous rain, poor visibility.
• Hs and maximum wave - the highest wave in a storm may be twice the significant wave height.
• Synchronous and parametric rolling - change course or speed to break the match between wave encounter period and roll period.
• Monsoon current reversal - North Indian Ocean surface currents reverse twice a year; use seasonal routeing charts for passage planning.
• SOLAS danger message - a master must report a tropical storm, force 10 winds without warning, dangerous ice or derelicts.

Formulas and examples

Example 1 — true wind from apparent wind. A ship steers 000° (T) at 12 kt; the masthead anemometer gives the apparent wind from 045° at 20 kt. Find the true wind.

• Apparent air motion is towards 225°: E-component = 20 sin 225° = −14.14 kt; N-component = 20 cos 225° = −14.14 kt.
• Ship's velocity: E = 0, N = +12 kt.
• True air motion = sum: E = −14.14 kt, N = −2.14 kt → speed = √(14.14² + 2.14²) = √204.6 = 14.3 kt.
• Direction of motion = 180° + arctan(14.14/2.14) = 180° + 81.4° = 261.4° → wind comes from 081°.
• Answer: true wind from 081° (T) at 14.3 kt. The ship's own northerly motion made the wind appear both stronger and more northerly.

Example 2 — geostrophic wind at low latitude. Pressure falls 2 hPa in 100 km at 20°N; ρ = 1.2 kg/m³.

• f = 2 × 7.292 × 10⁻⁵ × sin 20° = 1.4584 × 10⁻⁴ × 0.3420 = 4.99 × 10⁻⁵ s⁻¹.
• dp/dn = 200 Pa ÷ 1 × 10⁵ m = 2.0 × 10⁻³ Pa/m.
• V = 2.0 × 10⁻³ ÷ (1.2 × 4.99 × 10⁻⁵) = 33.4 m/s ≈ 65 kt.
• Because f is small in low latitudes, the same gradient drives a much stronger wind than in temperate latitudes; the balance breaks down within about 5° of the equator, and friction plus curvature reduce the real surface wind to roughly 70–80% of this.

Example 3 — relative humidity, dew point and fog risk. Dry bulb 30.0 °C, wet bulb 26.0 °C, station pressure 1010 hPa. (e_s = 42.4 hPa at 30 °C, 33.6 hPa at 26 °C, 29.8 hPa at 24 °C, 31.7 hPa at 25 °C.)

• e = 33.6 − (6.6 × 10⁻⁴ × 1010 × 4.0) = 33.6 − 2.67 = 30.9 hPa.
• RH = 30.9 / 42.4 = 72.9 %.
• Dew point: e_s = 30.9 hPa lies between 24 °C and 25 °C → 24 + (30.9 − 29.8)/(31.7 − 29.8) = 24 + 0.58 = 24.6 °C.
• If the sea-water intake temperature then reads 23 °C — below the dew point — expect advection fog: reduce to safe speed, post a look-out, sound fog signals and use radar (COLREG Rule 19).

Example 4 — swell as a storm warning. A long swell of period 12 s is observed from 150°, with no local wind to match.

• L = 1.56 × 12² = 224.6 m; C = 1.56 × 12 = 18.7 m/s = 18.7 × 1.944 = 36.4 kt; group speed = 18.2 kt.
• The swell energy travels at about 18 kt, far faster than a storm moving at 10–12 kt, so it arrives first; with pressure 4 hPa below normal, suspect a system to the SE and plot the centre by Buys Ballot's Law.
• If the ship's beam is 32 m and GM is 1.2 m, T_roll ≈ 0.8 × 32 / √1.2 = 25.6 / 1.095 = 23.4 s — a tender ship; a 12 s encounter period is close to half the roll period, so watch for parametric rolling in head or following swell and alter course or speed.

Example 5 — 1-2-3 rule clearance. A cyclone's 34-kt wind radius is 150 nm and the 24-hour forecast position is 400 nm from the ship's intended track.

• Danger circle at 24 h = 150 + 100 = 250 nm radius; at 48 h = 150 + 200 = 350 nm; at 72 h = 150 + 300 = 450 nm.
• The track passes 400 nm from the 24-hour position — clear by 150 nm — but at 72 hours the 450 nm circle would engulf it, so the ship must divert now, into the navigable semicircle, or wait in port. At 15 kt the ship makes 360 nm a day, so the diversion must be started at least two days ahead.

Where it is used

• Bridge watchkeeping and voyage planning: daily weather routeing decisions, appraisal under SOLAS V Reg 34, heavy-weather speed reduction, ballast and lashing decisions — the core work of a Third/Second Mate on Indian-flag and foreign-going ships.
• Cyclone-season operations on the Indian coast: port closures and sailing orders at Paradip, Visakhapatnam, Chennai and Haldia on IMD cyclone alerts; offshore supply and rig-move windows in the Mumbai High and KG basin.
• Monsoon trade: coastal and Gulf trades, dredgers, tugs and offshore units work to fair-weather/foul-weather windows set by the SW monsoon; Lakshadweep and Andaman passenger services follow IMD swell alerts.
• Observing and forecasting services: IMD Voluntary Observing Ships, Port Meteorological Offices, RSMC New Delhi and INCOIS ocean-state forecasts; nautical officers also move into these and into Indian Navy/Coast Guard met roles.
• Marine insurance, P&I and casualty investigation: heavy-weather damage, cargo shifting and general-average claims are argued on logbook weather entries, barograph traces and hindcast data; surveyors with the Indian Register of Shipping use them.
• Shore-based fleet performance and chartering: weather-routeing providers and charter-party "good weather" performance claims (speed and consumption warranties) depend on wind force and sea state records.

Interview questions

• Q: State Buys Ballot's Law and its use. A: In the Northern Hemisphere, with your back to the wind the low pressure is on your left (the right in the Southern Hemisphere); it gives the approximate bearing of a storm centre from a single wind observation.
• Q: Why does a tropical revolving storm not form on the equator? A: The Coriolis force is zero there, so the converging air cannot acquire the rotation needed to organise a closed circulation.
• Q: Which is the dangerous semicircle and why? A: The right-hand semicircle relative to the track in the NH (left in the SH), because the storm's forward speed adds to the wind and the wind tends to carry the vessel into the storm's path.
• Q: You are in the navigable semicircle in the North Indian Ocean. What action do you take? A: Bring the wind on the starboard quarter, about 130°–180° relative, proceed at best safe speed and alter course to port as the wind backs, keeping it there until clear.
• Q: What is the earliest sign of a distant tropical storm? A: A long, low swell from an unusual direction, because swell travels faster than the storm and arrives well before the cloud and pressure fall.
• Q: How much must the barometer fall before you suspect a TRS in the tropics? A: About 3 hPa or more below the normal for that place and time after correcting for the diurnal variation, with the wind rising and steadily shifting.
• Q: How do you anticipate fog at sea? A: Compare the sea-water temperature with the dew point of the air; when the sea is colder than the dew point in a moist airflow, advection fog is likely.
• Q: Difference between sea and swell? A: Sea is the irregular wave field raised by the local wind; swell is the longer-period, more regular wave that has travelled out of its generating area.
• Q: What corrections apply to a shipboard aneroid barometer? A: Index (calibration) error and height correction of about 0.11 hPa per metre above sea level; mercury barometers additionally need temperature and latitude corrections.
• Q: What weather reports is a master obliged to send? A: Under SOLAS Chapter V, danger messages on tropical storms, winds of force 10 or more with no warning received, dangerous ice, derelicts and sub-freezing temperatures with gale conditions — plus routine synoptic observations if the ship is in a voluntary observing fleet.
• Q: How does a cold front differ from a warm front on passage? A: A cold front gives a sharp veer, a squall, heavy short-lived rain, rising pressure and clearing with good visibility; a warm front gives slow veering, continuous rain, falling then steady pressure and poor visibility in the warm sector.
• Q: Which Indian body issues cyclone warnings for the Bay of Bengal, and how do they reach your ship? A: The India Meteorological Department, with RSMC New Delhi and the Area Cyclone Warning Centres at Chennai, Mumbai and Kolkata; they reach the ship by NAVTEX, Inmarsat SafetyNET, radiofacsimile and coast radio stations.

Common mistakes

• Confusing the semicircle with the hemisphere — the dangerous semicircle is defined relative to the storm's track, not relative to the ship, and the required manoeuvre is a mirror image in the Southern Hemisphere.
• Reporting the apparent (relative) wind as the true wind, or forgetting that a wind "from 045°" is a vector towards 225° when solving the triangle.
• Treating the Beaufort force as a sea-state scale, or quoting a wave height where the question asks for wind force.
• Omitting the height correction when reducing the barometer to mean sea level, and ignoring the tropical diurnal variation before declaring an abnormal fall.
• Comparing air temperature with sea temperature to predict fog instead of comparing the dew point with the sea temperature.
• Keeping a fixed set-and-drift for the North Indian Ocean when the monsoon currents reverse seasonally, giving a badly wrong ETA.
• Plotting one bearing of the storm centre and assuming a track; the track needs successive bearings, and a steady bearing with falling pressure means the storm is coming straight at the ship.

Related topics

• Chartwork and passage planning · Electronic navigation: radar, ARPA, ECDIS and GNSS · Ship stability and heavy-weather handling · Cargo care and securing in heavy weather · Oceanography and ocean currents · GMDSS and maritime safety information.