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NWS Charleston, SC - Marine Definitions

Marine Terminology:

  • Breaker...A wave that has become too steep to be stable. As water becomes shallower waves become steeper. When the steepness (ratio of height to length of wave) exceeds 1/7, the laws which govern surface-wave motion are no longer satisfied and the crest outpaces the body of the wave to form a turbulent white mass called a breaker. There are three types: spilling-break gradually over distance; plunging-peak up, curl over with tremendous mass, and then break with a crash; surging-do not spill or plunge, but surge up the beach face.

  • Chart Datum...The tidal datum to which soundings on a navigation chart are referred. In order to provide a factor of safety, some level lower than mean sea level is generally selected, such as mean low water or mean lower low water.

  • Ebb/Flood Current...Movement of a tidal current away/towards the coast or tidal estuary.

  • Fathom...Unit of depth in the ocean equal to six feet. 1000 fathoms = (approx.) 1 nautical mile = 6076 feet.

  • Fetch...(Also called generating area.) The area in which ocean waves are generated by the wind. It is generally influenced by the coastline, frontal boundaries, or areas of wind curvature/divergence. For an increase in the fetch length expect an increase in the wave height (amplitude).

  • Ground Swell...Swell as it passes through shallow water. It is characterized by a marked increase in height in water shallower than 1/10 the wavelength. To the seaman, ground swell is an indication of shoal water; to the shore-dweller it is often an indication of approaching bad weather.

  • Gulf Stream...A warm, well-defined, swift, relatively narrow, ocean current which originates where the Florida and the Antilles currents begin to curve eastward from the continental slope off Cape Hatteras, North Carolina. East of the Grand Banks the Gulf stream meets the cold Labrador current. It continues eastward across the Atlantic ocean.

  • Land Breeze...A coastal breeze blowing towards the sea, caused by the sea being warmer than the adjacent land. Therefore, it usually peaks during the night.

  • Lunar/Solar Day...Time required for the earth to rotate once with respect to the moon (24h50min.)/ Time required for earth to rotate once with respect to the sun (24hours).

  • Marine Forecast...A forecast for a specific oceanic and/or coastal area, of weather elements of particular interest to maritime transportation. Elements included are: wind, visibility, general weather state, and any warnings.

  • Marine Weather Observation (Ship observation)...The weather as observed from a ship at sea, usually taken in accordance with procedures set by the World Meteorological Organization (WMO). The following elements are usually included: total cloud amount; wind direction and speed; visibility; weather; pressure; temperature; cloud layer data; pressure tendency; sea-water temperature; dew-point temperature; state of the sea (waves); and sea ice. Also included are the date/time/position/ship name/course/speed.

  • Mean High Water (MHW)...The average level of all high water (mean sea level) at a place over a 19-year period.

  • Mean Low Water (MLW) ...the average level of low water (low tide) at a place over a 19-year period. An approximation of this low water datum is used as chart datum for the Atlantic Coast of the United States.

  • Mean Lower Low Water (MLLW)... The average level of the lower low water of the day over a 19-year period for a given site. An approximation othis level (lower low water datum) is used as chart datum for the Pacific coast of the United States.

  • Mean Sea Level (MSL)...The average height of the sea surface, based upon the hourly observation of tide height>t on the open coast or in adjacent waters which have free access to the sea. Observations in the United States of mean sea level are taken as the average height of the sea surface for all stages of the tide over a 19-year period. At the Custom House in downtown Charleston a tide gauge takes readings every hour. These recorded tide levels are kept on station for several years.

  • Nautical mile...The length of one minute of arc along any great circle on the earth's surface. Since this actual distance varies slightly with latitude, a nautical mile by international agreement is defined as 1852 meters...or 6076 feet (1.15 miles).

  • Northeaster (Nor'easter)...A cyclonic storm of the east coast of North America, so called because the winds over the coastal area are from the northeast. They may occur at any time of year but are most frequently violent from September-April. Northeast storms usually develop in the lower-middle latitudes (30-40N) within 100 miles east or west of the coastline. They progress generally north/northeastward...reaching maximum intensity near New England. They nearly always bring precipitation, and frequently winds of gale force to the affected coastal regions.

  • Red Tide...A growth of single celled plant-like animals in surface waters in such quantities as to color the sea red and kill fish.

  • Rip Current...A strong water-surface current of short duration flowing seaward from the shore; the return movement of water piled up on the shore by incoming waves and wind. It usually appears as a visible band of agitated water; and, with the outward movement concentrated in a limited band, its velocity is increased. A rip current is often miscalled a "rip tide". To swimmers, the phenomenon is known as "undertow".

  • Sea Breeze...A coastal breeze blowing towards the land, caused by the land being warmer than the adjacent sea. Thus, it usually peaks during the afternoon.

  • Sea Fog...Fog that is caused by moist air in transport over a cold body of water.

  • Swell...Ocean waves that have traveled out of their fetch (area of origin). Swell characteristically exhibits a more regular and longer period, and has flatter crests than waves within their fetch.

  • Tidal Current...The horizontal movement of water associated with the rise and fall of the tide. Coastline curvature will influence the nature of the current.

  • Tidal Wave...The wave motion of the tides. Also...an unusually high water level along shore. Refers to a storm surge or Tsunami.

  • Tidal Wind...A very light breeze which occurs in calm weather in inlets where the tide sets strongly. It blows onshore with rising tide and offshore with ebbing tide. The tidal wind can allow for big temperature ranges from the coast to just a few miles inland when the surface winds over inland sections are calm.

  • Tide...The periodic rising and falling of the earth's oceans. It results from the tide-producing forces of the moon and sun acting upon the rotating earth. This disturbance actually propagates as a wave along the surface of the waters of the earth. Sometimes, the horizontal movement of water along coastlines is called a "tide", but this is the tidal current. Tide refers to the vertical wave-like movement.

  • Tide-Producing Force...The slight local difference between the gravitational attraction of two astronomical bodies and the centrifugal force that holds them apart. These forces are exactly equal and opposite at the center of gravity of either bodies, but since gravitational attraction is inversely proportional to the square of the distance, it varies at each point on the surface. Therefore, gravitational attraction predominates at the surface point nearest to the other body, while centrifugal repulsion predominates at the surface point farthest from the other body. So there are two regions on the surface where tide producing forces are maximum, and normally there are two tides each lunar and solar day.

  • Tide Tables...Annual tabulations of daily predictions of the times and heights of high and low tide (high and low water respectively) at various places. Such tables are constructed from astronomical data and from the results of harmonic analysis of previous observations at the desired point. They are compiled and issued by national hydrographic authorities such as NOAA's National Ocean Service (NOS). The heights in tide tables usually are measured from chart datum rather than mean sea level.

Local Watch/Warning/Advisory Criteria:

Charleston Harbor:
  • Small Craft Should Exercise Caution...Sustained winds of 15-20 knots - *No longer issued as of December 3, 2007*
  • Small Craft Advisory...Sustained winds (or frequent gusts) of 20-33 knots expected, usually within 24 hours
  • Gale Watch...Sustained winds (or frequent gusts) of 34-47 knots possible within 36 hours
  • Gale Warning...Sustained winds (or frequent gusts) of 34-47 knots expected, usually within 12 hours
  • Storm Watch...Sustained winds (or frequent gusts) of 48-63 knots possible within 36 hours
  • Storm Warning...Sustained winds (or frequent gusts) of 48-63 knots expected, usually within 12 hours
  • Hurricane Force Wind Watch...Sustained winds (or frequent gusts) of at least 64 knots expected, usually within 36 hours, and not associated with a tropical cyclone
  • Hurricane Force Wind Warning...Sustained winds (or frequent gusts) of at least 64 knots expected, usually within 12 hours, and not associated with a tropical cyclone
SC (out 20 nm) and GA (out 60 nm) Coastal Waters:
  • Small Craft Should Exercise Caution...Sustained winds of 20 knots and/or 5 foot seas - *No longer issued as of December 3, 2007*
  • Small Craft Advisory...Sustained winds (or frequent gusts) at least 25 knots and/or 6 foot seas, usually within 24 hours
  • Gale Watch...Sustained winds (or frequent gusts) of 34-47 knots possible within 36 hours
  • Gale Warning...Sustained winds (or frequent gusts) of 34-47 knots expected, usually within 12 hours
  • Storm Watch...Sustained winds (or frequent gusts) of 48-63 knots possible within 36 hours
  • Storm Warning...Sustained winds (or frequent gusts) of at least 48 knots expected, usually within 12 hours
  • Hurricane Force Wind Watch...Sustained winds (or frequent gusts) of at least 64 knots expected, usually within 36 hours, and not associated with a tropical cyclone
  • Hurricane Force Wind Warning...Sustained winds (or frequent gusts) of at least 64 knots expected, usually within 12 hours, and not associated with a tropical cyclone
Additional Marine Products:
  • Special Marine Warning...Issued for all zones when potentially hazardous conditions (winds at least 34 knots and/or waterspouts and/or 3/4 inch or larger hail) are expected, usually within 2 hours.
  • Dense Fog Advisory...Issued for all zones except AMZ370 when visibilities of 1/2 nautical mile or less are expected or already occurring.

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National Weather Service
Charleston Weather Forecast Office
5777 South Aviation Avenue
Charleston, S.C. 29406-6162
(843) 744-0303

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Page last modified: 05/01/2010 10:05 PM
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