National Weather Service United States Department of Commerce

Marine Definitions

Montauk Point NY to Sandy Hook NJ out 20 nautical miles
including Long Island Sound and New York Harbor


Beach Erosion: The carrying and wearing away of beach materials, such as sand or rocks, by wave action, currents tides or wind.

Beaufort Scale: A scale that indicates the wind speed using the effect wind has on certain familiar objects.

"Bomb": Indicates an expected rapid intensification of a cyclone (low pressure) with surface pressure expected to fall by at least 24 millibars in 24 hour.

Coastal Flooding: The inundation of land areas along the ocean coast caused by sea waters over and above normal tidal action that may be originate from the ocean front, back bays, sounds, or other bodies of ocean water.

Coastal Flood Warning: Issued when tidal flooding or storm induced flooding is occurring, imminent or highly likely along coastal areas within approximately the next 12 hours.

Coastal Flood Watch: Issued when conditions are favorable for tidal flooding or storm induced flooding along coastal areas within approximately 12 to 36 hours.

Coastal Waters: The marine area, including bays, harbors and sounds extending from the coast to 20 nautical miles offshore.

Crest: Highest point in a wave.

Fathom: Unit of depth in the ocean equal to 6 feet.

Fetch: The marine area in which waves are generated by winds consisting of a near constant direction and speed.

Freezing Spray: An accumulation of freezing water droplets on a vessel caused by a combination of the wind, water, air temperature and vessel temperature.

Gale Warning: Issued for sustained winds or frequent gusts of 34 to 47 knots, not associated with a tropical system. 

Heavy Surf: Large waves breaking on or near the shore resulting from swells spawned by a distant storm.

Heavy Surf Advisory: Issued for heavy surf that poses a threat to life and/or property.

High: An area of high pressure usually accompanied by anticyclonic (clockwise) outward flow.

High Seas: That portion of the Atlantic Ocean which extends from 20 to 40 nm off the East Coast to 35 W.

Hurricane Force Wind Warning: Issued for sustained winds or frequent gusts of 64 knots or greater, not associated with a tropical systems.

Knot: Is equal to 1 nautical miles per hour or 1.15 statue miles (the length of 1 minute latitude) per hour.

Land Breeze: A wind that blows from the land towards a body of water. Also known as an offshore breeze. It occurs when the land is colder than the water.

Lightning: Any form of visible electrical discharges produced by thunderstorms.

Low: An area of low pressure usually accompanied by cyclonic (counterclockwise) inward flow.

Marine Weather Statement: Issued to provide mariners with details on significant or potentially hazardous conditions.

Mean Lower Low Water: A tidal datum which corresponds to the average mean of the lower low tide heights of a mixed tide observed over a specific 19 year period.

Nautical Mile (nm): Unit of distance used for marine navigation and is equal to 1.15 statue miles.

Neap Tide: Occurs at the first and third quarters of the moon.

Offshore Waters: The portion of the ocean, gulfs and seas beyond the coastal waters extending to a specified distance from the coastline (usually 100 nm), a specified depth, or covering an area defined by specific latitude and longitude points.

Rip Currents: Are narrow channels of water flowing seaward from the beach through breaking waves in the surf zones. They can rapidly carry a person into deeper waters.

SeaBreeze: A wind that flows from a sea or ocean towards a land mass. Also known as an onshore breeze. It occurs when the land is warmer than the water.

Significant Wave Height: is the wave height that is measured and forecast. It is the average wave height of the highest 1/3 of all individual waves, which corresponds closely to the visually observed mean wave height. Generally, it is assumed that individual wave heights can be described using a Rayleigh distribution.

  • Example: Significant Wave Height = 10 ft
  • 1 in 10 waves to be larger than 11 ft
  • 1 in 100 waves to be larger than 16 ft
  • 1 in 1000 waves to be larger than 19 ft

Small Craft Advisory: Issued for sustained winds or frequent gusts of 25 to 33 knots and/or seas 5 feet or higher. 

Special Marine Warning: Issued to convey information regarding hazardous convective activity or other short-term weather events occurring over the water or which may impact those on the water. For brief or sudden occurrence of sustained winds or frequent gusts 34 knots or greater. Usually in association with thunderstorms and/or waterspouts.

Spring Tide: Tides of increased range or tidal currents of increased speed that occur semimonthly as a result of the new moon or the full moon.

Squall: A sudden wind increase characterized by a duration of minutes and followed by a sudden decrease in winds (an increase in wind speed of 16 knots or more and sustained at 20 knots or more for at least 1 minute).

St. Elmo's Fire: The glow on a masthead produced by an extreme buildup of electrical charge. Unprotected mariners should immediately move to shelter when this phenomena occurs. Lightning may strike the mast within five minutes after it begins to glow.

Storm Surge: The difference between the astronomical tide and the observed tide.

Storm Tide: The actual water level resulting from the astronomical tide plus the storm surge.

Storm Warning: Issued for sustained winds or frequent gusts 48 to 63 knots, not associated with a tropical system. 

Swell: Wind generated waves that have traveled out of their area of origin. Swells exhibit smoother, more regular and uniform crests and longer wave periods than wind waves. Swell direction is the direction from which the swells are propagating.

Swell Direction: Is the direction from which the swells are propagating.

Surf: The waves in the area between the shoreline and the outermost limit of breakers.

Tidal Piling: Abnormally high water levels caused by an accumulation of successive incoming tides that do not allow the water to completely drain due to opposing strong winds and/or waves/swells.

Tide: The periodic rise and fall of water resulting from the gravitational interactions between the Earth, Moon and Sun.

Tide Anomaly: Actual water level minus the tide table predictions.

Tide Cycle: The periodic changes in the intensity of tides caused primarily by the varying relations between the Earth, Moon and Sun.

Tropical Weather Terms - Tropical Prediction Center

Trough: Lowest point in a wave.

Waterspout: A tornado over water - a rapidly rotating column of air extending from the a cumulonimbus cloud with a circulation that reaches the surface of water.

Wave Height: Height from crest to trough.

Wave Length: Distance from crest (trough) to the next crest (trough).

Wave Period: Time between consecutive crests (troughs) to pass a fixed point.

Wind Wave: Local waves of short period generated from the action of wind only on the water surface.


Top of Page | Home Page