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Marine Weather Terms

2nd Mate

times asked: 5; times asked
questions: 3; questions
last asked: 2025; last asked

The slides, written out

AI-drawn illustration. Simplified to explain the idea, not to scale.

Slide 1 · MARINE

MARINE WEATHER TERMS EVERY OFFICER SHOULD KNOW STORM SURGE • SIGNIFICANT WAVE HEIGHT • BORE TIDE DEW POINT TEMPERATURE Understanding these concepts improves voyage planning, safety, and watchkeeping decisions. STORM SURGE SIGNIFICANT WAVE HEIGHT BORE TIDE DEW POINT TEMPERATURE

Slide 2 · A) STORM SURGE

A) STORM SURGE DEFINITION An abnormal rise of sea level generated by a storm, caused mainly by strong winds and low atmospheric pressure. KEY FACT Approximate sea-level rise: 1 cm for every 1 hPa pressure drop. OPERATIONAL IMPACT • Coastal flooding • Unsafe anchorages • Port approach hazards OFFICER'S FOCUS Monitor: • Storm track • Landfall position • Predicted surge levels HOW STORM SURGE FORMS STRONG WINDS PUSH WATER TOWARDS COAST LOW PRESSURE (SEA LEVEL RISE) Wind piles water up against the coast and low pressure allows sea level to rise further. B) SIGNIFICANT WAVE HEIGHT (Hs) DEFINITION Average height of the highest one-third of waves in a sea state. WHY IT MATTERS This is the standard wave parameter used in marine forecasts. OPERATIONAL IMPACT • Cargo securing • Ballast planning • Ship handling decisions OFFICER'S FOCUS Always assess vessel response against forecast Hs. WHAT Hs MEANS HIGHEST ONE-THIRD OF WAVES MEAN SEA LEVEL Individual waves may exceed the significant wave height, with occasional waves substantially larger than Hs.

Slide 3 · C) BORE TIDE

C) BORE TIDE DEFINITION A tidal wave travelling upstream against the river current when tidal flow is funneled into a narrowing estuary or river. COMMON LOCATIONS • Severn Estuary (UK) • Hooghly River (India) • Qiantang River (China) OPERATIONAL HAZARDS • Sudden yawing at anchor • Chain snatch loads • Strong local currents OFFICER'S FOCUS ✓ Warn bridge team ✓ Monitor anchor position ✓ Plan river transit timing carefully HOW A TIDAL BORE FORMS TIDE COMES IN FROM SEA BORE FRONT BORE RUSHES UPSTREAM AGAINST THE RIVER CURRENT The leading edge may appear as a steep breaking wave or series of waves, with height depending on local tidal range and channel geometry.

Slide 4 · D) DEW POINT TEMPERATURE

D) DEW POINT TEMPERATURE DEFINITION The temperature to which air must be cooled, at constant pressure and constant water vapour content, for saturation to occur. WHY OFFICERS MONITOR IT Condensation begins when air reaches its dew point. • Fog forms • Dew forms • Relative humidity reaches 100% FOG RISK INDICATORS Fog risk increases when: • Air temperature ≈ Dew point • Difference reduces to 1–2°C • Difference is falling RULE OF THUMB If the sea surface temperature is below the dew point of the overlying air, advection fog is likely to develop if other meteorological conditions are favourable. HOW DEW POINT AFFECTS FOG WARM, MOIST AIR COOLER SEA SURFACE AIR COOLS TO DEW POINT CONDENSATION & FOG FORMATION Moist air moving over a cooler sea surface cools to its dew point, causing tiny water droplets to form – reducing visibility and creating sea fog. OFFICER'S PRACTICAL APPLICATION MONITOR Dry bulb temperature (air temp.) Wet bulb temperature Dew point spread ASSESS Track decreasing spread between air temp. and dew point. ACT EARLY Be ready for fog signals, reduced speed, and extra lookout measures. Good seamanship starts with understanding the weather before it becomes a hazard.

In exams

Asked 5 times across 3 different questions, most recently in 2025. Asked most in 2nd Mate (5).

Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.

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