NWS Jacksonville » Marine Data » Marine Weather Glossary

Average wave period - The average of all the wave periods in the wave spectrum.

Breakers - Waves that break, displaying white water. Depends on wave steepness and bottom bathymetry.

Combined Seas - Combination of both wind waves and swell. Also called "Seas" in the Coastal Waters Forecast and is approximately equal to significant wave height.

Dominant Wave Period - Simply put, the wave period in the wave spectrum which has the greatest wave energy.  Keep in mind that the wave spectrum may contain several different wave periods, so the dominant period is just one characteristic of the wave spectrum, but it does indicate the wave period containing the most energetic wave. The dominant wave period could change in a matter of hours from a swell period of, say 11 seconds, down to a wind wave period of perhaps 4 seconds if either the 11 second wave weakened and dissipated or wind speeds increase quickly to generate stronger wind waves.

Gale Warning - Wind of 34 knots to 47 knots sustained or frequent gusts.

Group Velocity - The speed at which a particular wave front or swell train advances.

High Surf Advisory - Issued when seas of 7 feet or greater are affecting the coastline.

MAREP - (short for MArine REPort) A marine weather report from a vessel which is forwarded to the National Weather Service. Since there are very few weather buoys in the coastal waters, MAREPS are very useful in determining weather conditions for the marine forecaster. Contact the National Weather Service in Jacksonville if you are interested in this important weather collection program.

Predominant Wind - The wind that generates (or is expected to generate) the local component of the significant sea conditions across the forecast area. This is the wind included in all marine forecast products.

Rip Currents - Narrow channels of water flowing seaward from the beach through areas of breaking waves. To escape the pull of the current, swim parallel to shore, then diagonally toward shore away from the current.

Seas - Used to describe the combination or interaction of wind waves and swell in which the separate components are not distinguished. This includes the case when swell is negligible or is not considered in describing sea state. Specifically, seas = √(S2+W2) where S is the height of the swell and W is the height of the wind wave. When used, seas should be considered as being the same as the significant wave height.

Significant Wave Height - The average height of the highest 33% of all waves in a swell train or in a wave generating region. It also approximates the value an experienced observer would report if visually estimating sea height. This is the predicted height given in the Coastal Waters Forecast.

Small Craft Advisory - Issued for average wind speeds of 20 knots to 33 knots, and/or forecast seas of 7 feet or greater. Either condition must occur for at least 2 hours. (locally defined criteria)

Small Craft Should Exercise Caution - Issued for winds of 15 to 20 knots or combined seas of 6 feet. (locally defined criteria)

Special Marine Warning - A warning issued for 2 hours or less by the National Weather Service to warn boaters of any of the following that is not adequately covered by the existing Coastal Waters Forecast:

   1) thunderstorm or non-thunderstorm winds of 34 knots or more (39 mph)
   2) waterspouts, detected by radar or observed
   3) tornadoes moving from land to water

Storm Warning - Sustained or frequent winds of 48 knots or more.

Swell - Wind waves which have traveled beyond the wave generation region.

Waterspout - In general terms, a tornado occurring over water. Normally it refers to a small and relatively weak rotating column of air underneath a rapidly growing cumulonimbus cloud or towering cumulus clouds. Waterspouts are most common over tropical and subtropical waters.

Wave Period - Time, in seconds for the passage of successive wave crests.

Wave Steepness - Ratio of wave height to wavelength. Theoretical wave steepness maximum is around 1/7, after which the wave becomes unstable and breaks.

Wave Trough - The lowest part of the wave.

Whitecap - The breaking crest of a wave, usually white and frothy.

Wind Waves - Waves generated by the local wind blowing at the time of observation.

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