Nautical terms
39 of 469 terms in Weather & sea. Show all
B
- Beaufort scale (Beaufort wind force scale) Weather & sea
-
A 0 to 12 scale for estimating wind force from what you see on the water, from Force 0 (calm, mirror-like sea) to Force 12 (hurricane, 64 knots or more). For example, Force 4 is 11 to 16 knots with numerous whitecaps; Force 8, 34 to 40 knots, is a gale.
C
- Chop (Choppy) Weather & sea
-
Short, breaking waves. A "choppy" sea is covered with them, even when the waves are not high.
- Coastal waters forecast (Marine forecast, CWF) Weather & sea
-
The NWS forecast for coastal and Great Lakes waters, issued four times a day by local offices, giving wind speed and direction, wave heights, visibility and weather, plus any advisories and warnings.
- Cold front (Front) Weather & sea
-
The leading edge of a colder air mass pushing in and replacing warmer air. A front in general is the boundary between two air masses of different temperature.
D
- Dense fog advisory Weather & sea
-
An NWS marine advisory for fog cutting visibility to a regionally set limit, never more than 1 nautical mile.
- Dew point Weather & sea
-
The temperature to which air must cool to become saturated. Cool the air further and the moisture condenses as dew, or over water as fog.
- Drift (Drift of current) Weather & sea
-
The speed of a current, in knots; also the distance a boat is carried by current and wind. A boat left drifting moves with wind and current without control.
E
- Ebb (Ebb current, Ebb tide) Weather & sea
-
The tidal current flowing away from shore or down a river or estuary, as the tide goes out.
F
- Fetch Weather & sea
-
The stretch of open water over which the wind blows from one direction, and its length. Wind blowing over a long fetch builds bigger seas than the same wind over a short one.
- Flood (Flood current, Flood tide) Weather & sea
-
The tidal current flowing toward shore or up a river or estuary, as the tide comes in.
- Fog (Advection fog, Radiation fog) Weather & sea
-
A cloud of tiny water droplets at the surface cutting visibility below about half a nautical mile. Advection fog forms when moist air moves over colder water; radiation fog forms on calm, clear nights and usually clears after sunrise.
G
- Gale warning Weather & sea
-
An NWS warning of sustained winds or frequent gusts of 34 to 47 knots (39 to 54 mph), forecast or occurring, not directly tied to a tropical cyclone.
- Gust Weather & sea
-
A sudden, brief increase in wind speed followed by a lull. NWS uses the term when peaks and lulls differ by 10 knots or more.
H
- High water (High tide, HW) Weather & sea
-
The highest level reached by a rising tide. The actual level can be raised or lowered by wind and weather as well as the predicted tide.
- Hurricane force wind warning Weather & sea
-
An NWS warning of sustained winds or frequent gusts of 64 knots (74 mph) or more that are not directly tied to a tropical cyclone. A hurricane itself brings a hurricane warning instead.
L
- Low water (Low tide, LW) Weather & sea
-
The lowest level reached by a falling tide. Wind, pressure and river flow can make it lower or higher than predicted.
N
- Neap tide (Neap tides) Weather & sea
-
Tides with a smaller-than-average range, twice a month when the moon is at first and last quarter: lower highs and higher lows.
- NOAA Weather Radio (NWR, WX channels) Weather & sea
-
Round-the-clock NWS broadcasts on seven VHF frequencies (162.40 to 162.55 MHz), with marine forecasts along the coasts and Great Lakes. Boaters can receive them on the weather channels of a VHF radio.
During severe weather NWS interrupts the routine broadcast with warnings.
R
- Range of tide (Tidal range) Weather & sea
-
The difference in height between a high water and the next low water. It varies from place to place and through the month, being largest at spring tides and smallest at neap tides.
S
- Sea breeze (Land breeze) Weather & sea
-
A breeze blowing from the sea onto land, usually by day when the land is warmer than the water. At night it may be replaced by a land breeze blowing offshore.
- Set (Set of current) Weather & sea
-
The direction toward which a current is flowing (the opposite of how wind direction is given). In navigation, "set and drift" describe how a current is pushing your boat.
- Significant wave height (SWH, Seas (forecast)) Weather & sea
-
The average height of the highest one-third of the waves, roughly what an experienced observer would estimate. It is what "seas 3 to 5 ft" means in a marine forecast; individual waves will sometimes be much higher.
NWS gives an example: with a significant wave height of 10 ft, about 1 wave in 1,000 is 19 ft or more.
- Slack water (Slack) Weather & sea
-
The time when a tidal current is at or near zero, especially when it reverses direction.
The time of slack water relative to high and low tide varies from place to place.
- Small craft advisory (SCA) Weather & sea
-
An NWS advisory for coastal and Great Lakes waters when winds or seas are expected to be hazardous to small boats, or ice is present. The wind and sea thresholds differ by region, and there is no legal definition of a "small craft": any boat those conditions could endanger is one.
NWS regional thresholds include: Southern region (Georgia to Texas) sustained winds 20 to 33 knots or seas 7 ft or more; Western (Washington to California) 21 to 33 knots or waves over 10 ft; Central (Minnesota to Ohio) 22 to 33 knots or waves over 4 ft. Check your local NWS office.
- Small craft should exercise caution (SCEC) Weather & sea
-
A precautionary statement in NWS marine forecasts alerting mariners with small, weather-sensitive boats to conditions that are building but below advisory level.
- Special marine warning (SMW) Weather & sea
-
An NWS warning for short-lived hazards on the water, usually lasting up to 2 hours: thunderstorm winds or gusts of 34 knots or more, hail 3/4 inch or larger, or waterspouts. It is often issued for a squall line or strong front.
- Spring tide (Spring tides) Weather & sea
-
Tides with a larger-than-average range, twice a month around new and full moon: higher highs and lower lows. The name has nothing to do with the season.
- Squall Weather & sea
-
A sudden strong wind that builds and dies away fairly quickly, often with rain, thunder and lightning. A squall line is a band of thunderstorms moving together.
- Storm surge Weather & sea
-
A rise (or drop) in sea level caused by a storm's strong winds and low pressure, on top of the normal tide. It is most dangerous when it arrives at high water with onshore winds.
- Storm warning Weather & sea
-
An NWS warning of sustained winds or frequent gusts of 48 to 63 knots (55 to 73 mph), forecast or occurring, not directly tied to a tropical cyclone.
- Sustained wind Weather & sea
-
The wind speed averaged over at least one minute. Warning and advisory thresholds are usually based on sustained wind or frequent gusts.
- Swell Weather & sea
-
Waves that have traveled away from the area where the wind made them. Swell is smoother and more regular than local wind waves, with a longer period, and it can arrive from a distant storm.
T
- Tidal current (Tidal stream, Current) Weather & sea
-
The horizontal flow of water caused by the tide. The timing of slack water and strongest current does not always line up with high and low tide and varies by place, so use current predictions, not only tide tables.
- Tide Weather & sea
-
The regular rise and fall of the water caused mainly by the pull of the moon and sun. Tide is the up-and-down movement; the sideways flow it causes is the tidal current. Most US coasts see two highs and two lows about every 24 hours 50 minutes.
dco.uscg.mil · oceanservice.noaa.gov · tidesandcurrents.noaa.gov
V
- Veer (Veering, Back, Backing) Weather & sea
-
Wind veers when its direction shifts clockwise (for example from south to west) and backs when it shifts counterclockwise, in the Northern Hemisphere.
W
- Waterspout Weather & sea
-
A rotating column of air over water, hanging from a cumulus or thunderstorm cloud and reaching the surface; some are tornadoes over water. NWS issues special marine warnings for them.
- Wave period (Period) Weather & sea
-
The time in seconds between two wave crests passing the same point. Marine forecasts often give it alongside wave height; swell has a longer period than local wind waves.
- Whitecap (Whitecaps) Weather & sea
-
A wave crest in open water that breaks and shows white foam because the wind is adding energy faster than the wave can carry it. Scattered whitecaps appear at about Force 3 on the Beaufort scale.
- Wind waves (Sea, Seas, Wind sea) Weather & sea
-
Waves raised by the local wind, still in the area where they are forming. They tend to be steeper, shorter and more irregular than swell.
General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.