4.11 Marine Weather & Meteorology

Why Masters Study Weather

Navigation General expects you to read the sky, the forecast, and a surface chart well enough to cancel, delay, or reroute a passenger trip before the first spray comes over the bow. Meteorology on Q162 is practical: pressure, wind, fronts, fog, seas, and how those products appear in marine forecasts. You are not training to be a National Weather Service meteorologist; you are training to be the last person who says "we go" or "we don't."

Highs, Lows, and Wind in the Northern Hemisphere

Atmospheric pressure is the weight of the air column. On surface charts, isobars connect equal pressure.

Northern Hemisphere (NH) surface wind rules (Buy Ballot / geostrophic sense as tested):

• Around a high: winds flow clockwise and outward
• Around a low: winds flow counterclockwise and inward

(In the Southern Hemisphere the rotation sense reverses — U.S. Near Coastal Master exams assume NH logic.)

Practical deck tip (NH): with your back to the wind, the low is roughly to your left. That rough field rule helps when a chart is not in your hand and the barometer is falling.

Barometer and tendency

A falling barometer often signals approaching low pressure or a front; a rising barometer often signals improving or building high pressure. Rate of fall matters: a rapid drop warns of a stronger system. Log pressure on longer coastal runs so you see tendency, not only a single snapshot.

Fronts

A front is the boundary between air masses of different temperature and moisture.

• Cold front: colder air advances under warmer air. Passage often brings a sharp wind shift (commonly veering in the NH), a line of showers or thunderstorms, possible squalls, then cooler, clearer air behind. Move fast; the worst is often near the front.
• Warm front: warmer air overruns colder air. Approach often shows lowering layered clouds, steadier rain or drizzle, reduced visibility, and a more gradual wind/temperature change.
• Occluded and stationary fronts: appear on weather maps when systems mature or stall; expect prolonged cloud and precipitation patterns as the map describes.

For small passenger vessels, cold-front squall lines are classic go/no-go killers: sudden wind, steep short seas in shallow water, lightning, and panicked passengers. If the forecast shows a strong cold front during your charter window, plan delay or cancel rather than "maybe skirt it."

Sea Breeze and Land Breeze

Local thermal winds form along coasts on fair-weather days:

• Sea breeze (day): land heats faster than water → lower pressure over land → wind blows from sea toward land. Afternoon sea breezes can build 10–20 kn in some regions and stack short chop against an ebb at an inlet.
• Land breeze (night): land cools faster → wind blows from land toward sea, often lighter.

These are not the same as the large-scale gradient wind around highs and lows, but they add vectorially to the gradient wind. A calm morning forecast can become a lively afternoon on the nose when the sea breeze fills in.

Fog Types Mariners Must Name

Fog is a cloud on the surface — visibility collapse that triggers Rule 19 conduct and sound signals.

Advection fog is the career fog for near-coastal masters: it can roll in with a breeze and stay. Radar, AIS, reduced speed, sound signals, and passenger delay are the tools — not hope.

Reading Weather Charts and Marine Reports

Master-level sources include:

• NWS marine forecasts (coastal waters, nearshore, offshore zones)
• NOAA Weather Radio (continuous marine broadcasts — see next section)
• Surface analysis charts: highs, lows, fronts, isobars
• Prognostic charts: forecast positions of systems
• Wave / sea state products and local buoy observations
• VHF WX channels and commercial weather apps that mirror official text

When you read a forecast, extract: wind direction and speed, gusts, seas/swell (height and period if given), visibility/fog, precipitation/thunderstorms, and timing of frontal passages. Compare the valid period to your departure and return. A "tonight" small craft headline does not clear a 1400 departure if the afternoon sea breeze already exceeds your vessel's comfort and COI common-sense limits.

Cloud Basics for Mariners

Clouds are free remote sensors:

• Cumulonimbus (Cb): towering storm clouds — lightning, downbursts, hail, sudden wind shifts. Avoid. Do not take passengers under building Cb "for the photos."
• Cumulus: fair-weather puffs can grow into towers as the day heats; watch vertical development.
• Stratus / nimbostratus: layered gray; rain and low visibility possible.
• Cirrus thickening and lowering can signal an approaching warm front or large system hours ahead.

Combine sky clues with the barometer and the written forecast. Sky alone can lie in complex coastal terrain; forecast alone can lag what you see building over the hills.

Wind, Fetch, and Seas

Seas (wind waves) grow with:

1. Wind speed
2. Duration (how long the wind blows)
3. Fetch — the distance of open water over which the wind blows

A 25 kn wind across a short fetch lake may raise only a steep, short chop. The same 25 kn across 100 miles of open fetch builds larger, more powerful seas. Near-coastal masters also face swell generated far away that arrives under light local wind — passengers get sick, bars break, and boarding ramps become dangerous even when "it doesn't feel windy at the dock."

Shallow water steepens waves; opposing current (ebb against swell at an inlet) creates standing seas that can break on the bar. Weather + current + tide is one problem, not three separate hobbies.

Master's weather briefing template

1. Synoptic: high/low/front positions and 24-hour movement
2. Local wind: gradient + sea breeze timing
3. Seas/swell vs route and bar
4. Visibility/fog probability
5. Thunderstorm/squall risk
6. Decision: go / delay / cancel / modify itinerary

Weather literacy is passenger safety. Q162 asks the definitions; the dock asks for your judgment.