Question 1

Your examiner shows you a synoptic chart with widely-spaced isobars and a central pressure of 1025 hPa. What type of weather system is this and what conditions would you expect at sea in summer versus winter in UK waters?

Accepted Answer

ANTICYCLONE (High Pressure System): widely spaced isobars = light gradient = light winds; air subsides in the centre preventing cloud formation. Summer UK anticyclone conditions: light winds (Force 0-3), fine settled weather, low visibility due to haze (dust/pollen suspended in the stable air), sea fog possible if warm air lies over cold sea (North Sea fog in summer), flat calm sea, good sailing conditions but risk of becalming. Winter UK anticyclone conditions: light winds, clear skies, severe overnight frost (no cloud blanket to trap heat — rapid radiation cooling), dense radiation fog over land that drifts offshore on light winds (sea fog), bitterly cold, icing on exposed superstructure in freezing conditions, frost on decks creating slip hazard. Navigation significance: anticyclonic conditions bring high visibility and calm sea BUT summer haze and winter fog can both reduce visibility severely despite the otherwise fine weather. Anticyclone meteorology at OOW level covers both the theoretical understanding and the practical implications for navigation. UK summer anticyclone fog: the North Sea summer sea fog is one of the most persistent hazards for UK coastal shipping. The mechanism: warm continental air (heated over land in summer) moves over the relatively cold North Sea — the air is cooled below its dew point and fog forms. This is indistinguishable from the advection fog type — it persists regardless of sunshine and does not follow the diurnal pattern of radiation fog. Superstructure icing (winter): when air temperatures drop below 0°C and spray freezes on contact with the vessel — this adds top weight (raised centre of gravity) and can reduce stability dramatically. Vessels on winter North Sea, Norwegian coast, or Trans-Atlantic passages in winter must monitor superstructure ice accumulation. At OOW level, the examiner uses anticyclone questions to test whether candidates know that calm, high-pressure conditions do NOT automatically mean good visibility — and that superstructure icing is an anticyclonic winter hazard as much as a storm hazard.

Question 2

You are OOW in the North Atlantic. The wind is from the SW, force 6, and backing slowly to the south. The barometer has been falling steadily for 6 hours at 2 hPa/hour. Using Buys Ballot's Law, where is the centre of the depression and what do you expect next?

Accepted Answer

Buys Ballot's Law (Northern Hemisphere): stand with your back to the wind — the centre of lowest pressure (the depression) lies approximately 90° to 120° to your LEFT (and slightly forward). Wind from SW, standing with back to SW means facing NE — the depression centre lies to the NW or NNW of your position. Backing wind (SW to S) means the depression is approaching from the west or northwest — you are in the WARM SECTOR of an approaching depression (between the warm front and cold front). Expected sequence: (1) Barometer continues to fall; (2) Wind continues to back and freshen; (3) Warm front will pass with steady rain; (4) Wind veers to W-NW as cold front arrives; (5) Violent squalls, heavy rain, and rapid pressure rise as the cold front passes; (6) Barometer rises rapidly; (7) Wind veers to NW-N; clearing showers with good visibility. Buys Ballot's Law (1857): in the Northern Hemisphere, air flows anticlockwise around a low pressure centre (depression). Stand with your BACK to the wind — the low pressure centre is to your LEFT (approximately 8-10 compass points, which is roughly 90°-120°). The wind backing (anticlockwise change: SW → S → SE) in the Northern Hemisphere means the depression is approaching and you are on the warm sector side. Rate of barometric fall: 1 hPa/hour = steady approach; 2 hPa/hour = more rapid approach or deepening depression; 3+ hPa/hour = storm development. MCA Three Degrees Rule (a useful supplement): if the barometer falls 3 hPa in 3 hours — take heavy weather precautions. Southern Hemisphere: Buys Ballot reversed — stand with back to wind, low is to the RIGHT. Depression structure recall: ahead of warm front = NE veering to E-SE, overcast, steady rain, moderate swell; warm sector = S-SW, mild, drizzle; cold front = squalls, veering to NW, heavy rain, rapid clearing.

Question 3

On passage from the UK to the Grand Banks in June, you encounter fog. Your examiner asks you to identify the type of fog and explain why it forms here.

Accepted Answer

ADVECTION FOG — the dominant fog type on the Grand Banks. Mechanism: warm, moist air from the Gulf Stream (sea surface temperature 15-20°C) flows northward and encounters the cold Labrador Current (SST 2-8°C). The warm air is cooled below its dew point by contact with the cold sea surface, causing condensation — fog forms. Characteristics: persists regardless of time of day (unlike radiation fog which disperses with daytime heating); very dense, may last days; geographically fixed to the temperature boundary zone. Other fog types: Radiation Fog — over land, forms on calm clear nights as the land surface cools; disperses with morning sun; does not persist at sea. Sea Smoke (Arctic Sea Smoke) — cold dry air over relatively warm water; water vapour condenses immediately above the surface into low wisps — opposite mechanism to advection fog. Avoidance: route planning to avoid the Grand Banks fog zone (approximately 43°-47°N, 45°-55°W) in spring/summer; alternative routes further north or south. The Grand Banks off Newfoundland is one of the foggiest places on Earth — over 100 fog days per year. It is the classic textbook example of advection fog. The mechanism: warm humid air from the southwest (over the warm North Atlantic Drift/Gulf Stream) advects northward and meets the cold Labrador Current. The cold water chills the air below the dew point — fog forms. Unlike radiation fog (which is a land phenomenon forming on calm clear nights), advection fog has no diurnal cycle — it does not burn off in the morning. It persists until the air mass changes. Sea smoke: the reverse situation — very cold air (e.g., Arctic outbreak) over relatively warm sea water causes intense evaporation and immediate condensation above the surface. This looks dramatic (billowing "steam") but is typically shallow (under 2-3 metres). At OOW exam level, the candidate must distinguish the three main fog types: advection fog (horizontal movement of air mass); radiation fog (nocturnal cooling of land surface); sea smoke (cold air over warm sea). Coastal fog: a combination type where radiation fog forms over land at night and drifts offshore on the morning land breeze.