Glossary

Marine weather glossary

Marine weather has its own vocabulary. Here are the terms that appear on every NWS coastal, offshore, and Great Lakes forecast — decoded.

The warning tiers

Small Craft Advisory — winds 20-33 kt, seas 4-7 ft (varies by area).
Gale Warning — winds 34-47 kt.
Storm Warning — winds 48-63 kt.
Hurricane Force Wind Warning — winds 64+ kt.
Special Marine Warning — short-fuse warning for thunderstorms, waterspouts, dangerous weather within 2 hours.
Marine Weather Statement — non-warning information.

Significant wave height (Hs) — average of highest 1/3 of waves. What forecasts report.
Maximum wave height — approximately 1.6× Hs.
Rogue wave — wave over 2× Hs. Real, occasional, dangerous.
Swell — waves from distant storm. Long period, smooth.
Wind wave / sea — waves from local wind. Short period, choppy.
Sea state — combined swell + wind wave.
Period — seconds between wave crests. Long period = more energetic swell.

True wind

Wind relative to the ground.

Apparent wind

Wind relative to the moving boat. What sails feel.

Beaufort scale

Wind speed scale from 0 (calm) to 12 (hurricane). Still used in marine forecasts.

Fetch

Distance over which wind blows across water. Long fetch = big waves.

Squall

Sudden strong wind, often with rain. Can be very local.

Katabatic

Wind flowing down slopes into the water. Can be extreme near steep coastlines.

Land breeze

Overnight wind from land to sea.

Sea breeze

Daytime wind from sea to land.

Rip current — narrow, fast current flowing seaward. Leading cause of surf zone drownings.
Undertow — Different from rip current: broad backwash zone that pulls you toward shore's bottom, then out.
Tidal current — horizontal water motion from tides.
Storm surge — abnormal rise of water above tide during storm.
Seiche — standing wave in enclosed body of water (Great Lakes, harbors).
Tsunami — long-period wave from seafloor displacement.

Advection fog — warm moist air over cold water. Common in New England summer.
Sea smoke — very cold air over warm water. Common in winter.
Radiation fog — overnight cooling of moist air over land. Drifts over water at dawn.
Frontal fog — associated with weather front passage.

November witch — historically deadly Great Lakes storm season (Nov).
Meteotsunami — atmospheric-pressure-induced wave. Can be tsunami-sized on Great Lakes.
Steamboat weather — historic term for shipping-safe conditions.
Ice cover — critical for winter shipping, aviation, and fishing.

NWS marine forecasts use standard phrasing: