Marine meteorology: what you need for the exam

Atmospheric pressure and isobars

Standard atmospheric pressure at sea level is 1013 hPa (mbar). On a synoptic chart, pressure is shown by isobars — lines of equal pressure, usually at 4 hPa intervals. The wind blows parallel to the isobars (in the northern hemisphere with a slight inflow toward low pressure) and the closer they are, the stronger it is.

Buys-Ballot’s law (a practical rule for the northern hemisphere): stand with your back to the wind — the centre of low pressure is on your left. A quick way to gauge where a cyclone is moving relative to you.

A low-pressure area (L, cyclone) — bad weather: clouds, precipitation, strong winds. A high-pressure area (H, anticyclone) — usually clear, weak winds in summer, cold and fog in winter.

Fronts and cyclones

A mid-latitude cyclone is a structure with warm and cold fronts:

• Warm front (red semicircles on the chart): warm air rides over cold. Slow approach: cirrus → stratus → drizzle, wind veers clockwise, pressure falls. After it passes — warm, humid, visibility worsens.
• Cold front (blue triangles): cold air undercuts warm, lifting it sharply. Fast approach: powerful cumulonimbus, downpours, squalls, thunderstorms. After passing — sharp clearing, cooling, wind veers clockwise, pressure rises.
• Occluded front (combined symbols): the cold front catches up with the warm one, the warm sector is lifted up. Prolonged precipitation, the cyclone weakens.

For the sailor, the key sign of imminent worsening is a pressure drop of more than 2 hPa in 3 hours and a wind shift. A fast drop (4+ hPa in 3 hours) is a sign of an active cyclone — head for shelter.

Breezes and local winds

On clear summer days near the coast, breezes — daily-circulation winds — are at work:

• Sea breeze (by day): land heats faster than sea, an updraft and low pressure form over it, and air flows from sea to land. Starts around midday, peaks at 2–4 p.m., 3–5 m/s, up to 10 m/s in the tropics.
• Land breeze (at night): land cools faster than sea, the circulation reverses — wind from land to sea. Usually weaker than the sea breeze.

Knowing the breezes helps plan departures and returns: with a sea breeze it is easy to head out, with a land breeze — to come back.

Katabatic winds — cold air flowing down a high coast or mountain. They suddenly intensify at night and can reach 15–25 m/s. Typical in fjords, Adriatic straits (bora) and the Mediterranean (mistral, tramontane).

The Beaufort scale

The Beaufort scale (0–12) describes wind force by visual cues — useful when no instrument is at hand. Key points for the sailor:

• 0 (calm, 0–0.2 m/s): mirror water.
• 3 (gentle breeze, 3.4–5.4 m/s): small waves, crests start to appear. Comfortable sailing.
• 4 (moderate, 5.5–7.9 m/s): waves up to 1 m, scattered whitecaps. Working wind for a cruising yacht.
• 5 (fresh, 8.0–10.7 m/s): many whitecaps, spray. Reef the main.
• 6 (strong, 10.8–13.8 m/s): big waves, foam. For experienced sailors.
• 7 (near gale, 13.9–17.1 m/s): wave crests blown into foam. Coastal Skipper limit.
• 8 (gale, 17.2–20.7 m/s): storm warning. Get into shelter.
• 10+ (storm): life-threatening on a small yacht.

Forecasting from clouds and signs

Without internet, a sailor reads the weather from the sky:

• Cirrus clouds — wispy, high. 12–24 hours before a warm front.
• Cirrostratus — a veil that gives the sun a halo. The front is closer.
• Altocumulus “mackerel sky” — weather change within hours.
• Cumulonimbus “anvils” — thunderstorms, squalls. Move away from a squall line as fast as possible.
• Morning dew and fog hugging the water — a sign of stable good weather.

Working forecast sources: NAVTEX (automatic reception of storm warnings), GRIB files via a satellite terminal, national marine services (Météo France, UK Met Office, Russian Roshydromet).

Tides

Tides are caused by the gravitational pull of the Moon (the main contribution) and the Sun. Key concepts for the exam:

• Semi-diurnal tide: two high and two low waters per day (≈12 h 25 min between two highs).
• Spring tides: full moon and new moon — Moon and Sun act together, maximum tidal range.
• Neap tides: first and last quarter — minimum range.
• Rule of twelfths: over 6 hours of the cycle the water rises/falls by 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the full range.
• Tidal currents: shown on the chart as diamonds with a table of direction and speed for each hour relative to high water of a reference port.

For the sailor, tides decide whether you can enter a port, how much chain to pay out at anchor, and how to set course relative to the current. An hour’s mistake can land you on a sandbank.