Marine Glossary

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sea

Besides its more common use as a body of salt water more or less confined by land, in marine weather its primary meaning has to do with waves, that is, wind waves generated or sustained by winds within their fetch, as opposed to swell.

There is not, however, universal consistency in the use of this term, and it can in some cases be important to understand exactly what is meant in a particular usage. Sometimes the term "sea" or "seas" is used to refer to the "combined seas" meaning wind waves and swell, or more generally just the nature of the sea surface itself without making such a distinction. In this usage, sea and sea state are synonymous.

In WMO terminology, seas means combined seas, whereas in some US references "seas" are meant to refer to wind waves as distinguished from swells. Clearly it is valuable to use terminology that removes this uncertainty. The de facto usage of the word in many cases is "combined seas" or actual sea state. Hence such terms as smooth sea (only ripples); a short sea (short, irregular or broken waves); or confused or chaotic sea (highly disturbed surface with waves coming from many directions and of different heights). A cross sea, on the other hand, refers to waves running across the swells, which is more a reference to seas as waves.

A sea may be designated head sea, beam sea, quartering sea, or following sea, when the predominant wave direction is 0°, 90°, 45°, or 180° relative to the vessel's course.

See also swell and RES-2 on sea state. See G112 for an illustration of wave terminology.

Fully developed seas is a phrase that means the sea state has not been limited by fetch or duration for the given wind speed.