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Below are some common terms and definitions used in our web site and in the field of metocean studies and forecasting.


The measurement of depths of water in oceans, seas, and lakes; also information derived from such measurements.


A float, particularly a floating object moored to the bottom to mark a channel, anchor, shoal, rock, etc.

Central Pressure Index (CPI)

The estimated minimum barometric pressure in the eye (approximate center) of a particular hurricane. The CPI is considered the most stable index to intensity of hurricane wind velocities in the periphery of the storm. The highest winds are associated with storms having the lowest CPI.

Continental Shelf

The zone bordering a continent and extending from the low water line to the depth (usually about 180 meters) where there is a marked or rather steep descent toward a greater depth.


A line on a map or chart representing points of equal elevation with relation to a datum. It is called an isobath when connecting points of equal depth below a datum and an isobar when used to represent atmospheric pressure.

Deep Water

Water so deep that surface waves are little affected by the ocean bottom. Generally, water deeper than one-half the surface wavelength is considered deep water.


In wave forecasting, the length of time the wind blows in nearly the same direction over the fetch (generating area).


A circular movement of water formed on the side of a main current. Eddies may be created at points where the main stream passes projecting obstructions or where two adjacent currents flow counter to each other.


In meteorology, usually the "eye of the storm" (hurricane/typhoon); the roughly circular area of comparatively light winds and fair weather found at the center of severe tropical cyclones.


The area in which seas are generated by a wind having a fairly constant direction and speed. Sometimes use synonymously with fetch length. Also generating area.

Fetch Length

The horizontal distance (in the direction of the wind) over which a wind generates seas or creates a wind setup.

Generation of Waves

(1) The creation of waves by natural or mechanical means; (2) The creation and growth of waves caused by a wind blowing over a water surface for a certain period of time. The area involved is called the generating area or fetch.

Gravity Wave

A wave whose velocity of propagation is controlled primarily by gravity. Water waves more than 2 inches long are considered gravity waves. Waves longer then 1 inch and shorter than 2 inches are in an indeterminate zone between Capillary and Gravity Waves.

Group Velocity

The velocity of a wave group. In deep water, it is equal to 1/2 the velocity of the individual waves within a group.

Hindcasting, Wave

The use of historical synoptic wind fields to calculate characteristics of waves that probably occurred at some past time.


An intense tropical cyclone in which winds tend to spiral inward toward a core of low pressure, with maximum surface wind velocities that equal or exceed 33.5 m/s (75 mph or 65 knots) for several minutes or longer at some points. Tropical Storm is the term applied if maximum winds are less than 33.5 m/s.

Joint Industry Project (JIP)

A project where multiple companies pool resources and data to produce a comprehensive study where all participants share in the results.


The unit of speed used in navigation equal to 1 nautical mile (6,076.115 feet or 1,852 meters) per hour.

Length of a Wave

The horizontal distance between similar points on two successive waves measured perpendicularly to the crest.

Monochromatic Waves

A series of waves generated in a laboratory; each wave has the same length and period.

Nautical Mile

The length of a minute of arc, 1/21,600 of an average great circle of the Earth. Generally, one minute of latitude is considered equal to one nautical mile. Equal to 1,852 meters or 6,076.115 feet.

Radius of Maximum Winds

Distance from the eye of a hurricane - where surface and wind velocities are zero - to the place where surface wind speeds are maximum.

Refraction (of water waves)

(1) The process by which the direction of a wave moving in shallow water at an angle to the contours are changed. The part of the wave advancing in shallower water moves more slowly than that part still advancing in deep water, causing the wave crest to bend toward alignment with the underwater contours; (2) The bending of wave crests by currents.

Sea State

Description of the sea surface with regard to wave action. Also called the state of sea.

Shallow Water

Commonly, water of such a depth that surface waves are noticeably affected by bottom topography. It is customary to consider water depths less than 1/2 the surface wavelength as shallow water.

Significant Wave

A statistical term relating to the 1/3 highest waves of a given wave group and defined by the average of their heights and periods. Note that the composition of the higher waves depends upon the extent to which the lower waves are considered. Experience indicates that a careful observer who attempts to establish the character of the higher wave will record values which approximately fit the definition of the significant wave height.

Significant Wave Height

The average height of the 1/3 highest waves of a given wave group. Note that the composition of the higher waves depends upon the extent to which the lower waves are considered. In wave record analysis, the average height of the highest 1/3 of a selected number of waves, this number being determined by dividing the time of record by the significant period.

Significant Wave Period

An arbitrary period generally taken as the period of the 1/3 highest waves within a given group. Note that the composition of the higher waves depends upon the extent to which the lower waves are considered. In wave record analysis, this is determined as the average period of the most frequently recurring of the larger well-defined waves in the record of study.

Storm Surge

A rise above normal water level on the open coast due to the action of wind stress on the water surface. Storm surge resulting from a hurricane also includes that rise in level due to atmospheric pressure reduction as well as that due to wind stress.


Wind-generated waves that have traveled out of their generating area. Swell characteristically exhibits a more regular and longer period and has flatter crests than waves with their fetch (Seas).

Synoptic Chart

A chart showing the distribution of meteorological conditions over a given area at a given time. Popularly called a weather map.

Tropical Cyclone

See Hurricane

Wave Forecasting

The theoretical determination of future wave characteristics, usually from observed or predicted meteorological phenomena.

Wave Spectrum

In ocean wave studies, a graph, table, or mathematical equation showing the distribution of wave energy as a function of wave frequency.

Wave Steepness

The ratio of the wave height to the wavelength

Wind Waves

(1) Waves being formed and built up by the wind; (2) Loosely, any wave generated by wind.



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