Sea state

Short description: General condition of the free surface on a large body of water

"State of the sea" redirects here. For the Venetian Empire, see Stato da Màr.

NOAA ship Delaware II in foul weather on Georges Bank. Sea State 5 and 8 range

In oceanography, sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, period, and spectrum. The sea state varies with time, as the wind and swell conditions change. The sea state can be assessed either by an experienced observer (like a trained mariner) or by using instruments like weather buoys, wave radar, or Earth observation satellites.[1]

The short-term statistics describing the sea state are determined for a time interval in which the sea state is considered to be constant. This duration has to be much longer than the individual wave period, but shorter than the period in which the wind and swell conditions can be expected to vary significantly. Typically, the sea state is assumed to be constant for 15-30 minutes.[1]

## Contents

* 1 WMO sea state code
* 2 Sea states in marine engineering
* 3 CCI Sea State
* 4 See also
* 5 Citations
* 6 General and cited references

## WMO sea state code

Winter, North Atlantic – Water over deck and hatches, storm with huge waves (1958)

The World Meteorological Organization (WMO) sea state code largely adopts the 'wind sea' definition of the Douglas Sea Scale. The direction from which the swell is coming should be recorded.{{Citation needed|date=October 2025}

graphic of WMO sea state codes, descriptions, and wave heights in meters WMO Sea State Code | Wave height | Characteristics

|
|

0 | 0 metres (0 ft) | Calm (glassy)
1 | 0 to 0.1 metres (0.0 to 3.9 in) | Calm (rippled)
2 | 0.1 to 0.5 metres (3.9 in to 1 ft 7.7 in) | Smooth (wavelets)
3 | 0.5 to 1.25 metres (1 ft 8 in to 4 ft 1 in) | Slight
4 | 1.25 to 2.5 metres (4 ft 1 in to 8 ft 2 in) | Moderate
5 | 2.5 to 4 metres (8 ft 2 in to 13 ft 1 in) | Rough
6 | 4 to 6 metres (13 to 20 ft) | Very rough
7 | 6 to 9 metres (20 to 30 ft) | High
8 | 9 to 14 metres (30 to 46 ft) | Very high
9 | Over 14 metres (46 ft) | Phenomenal
Character of the sea swell  | 0\. None

|

Low | 1\. Short or average
2\. Long
Moderate | 3\. Short
4\. Average
5\. Long
High | 6\. Short
7\. Average
8\. Long
| 9\. Confused

## Sea states in marine engineering

In engineering applications, sea states are often characterized by the following two parameters:

* The significant wave height H1/3 — the mean wave height of the highest third of the waves.[2]
* The mean wave period, T1.

In addition to the short-term wave statistics presented above, long-term sea state statistics are often given as a joint frequency table of the significant wave height and the mean wave period. From the long and short-term statistical distributions, it is possible to find the extreme values expected over a given number of years by fitting an extreme value distribution.[3] The extreme value distribution can then inform an engineer about the most extreme significant wave height to be expected during a lifetime of a ship or offshore structure. Using the response amplitude operators of the ship, the engineer can then use the extremes in significant wave height and wave period to calculate the most extreme loads that the ship should be able to withstand. Withstanding significant wave heights that happen on average once in 100 years or once in 1000 years is a common demand for design of ships and offshore structures.[4]

## CCI Sea State

The "Sea State" project within the ESA's Climate Change Initiative (CCI) program created an 18-year data set (2002–2020) covering various sea state-related Essential Climate Variables as measured by the Jason satellite series, other ocean-focused Earth observation satellites, and in situ sources.[5][6][7][8] In 2025, data from CCI Sea State, combined with the new SWOT satellite measurements, were used to describe a new record wave height of nearly 20 m in the open ocean.[9][10]

## See also

* Beaufort scale
* Cross sea
* Douglas sea scale

## Citations

1. ↑ 1.0 1.1 Holthuijsen, Leo H. (2007). Waves in oceanic and coastal waters. Cambridge: Cambridge University Press. ISBN 978-0-511-27021-5.
2. ↑ Munk, W.H. (1944). Proposed uniform procedure for observing waves and interpreting instrument records. La Jolla, California: Wave Project at the Scripps Institution of Oceanography.
3. ↑ Aarnes, Ole Johan; Breivik, Øyvind; Reistad, Magnar (2012). "Wave Extremes in the Northeast Atlantic" (in en). Journal of Climate 25 (5): 1529–1543. doi:10.1175/JCLI-D-11-00132.1. ISSN 0894-8755. https://journals.ametsoc.org/doi/10.1175/JCLI-D-11-00132.1.
4. ↑ Jonathan, Philip; Ewans, Kevin (2013). "Statistical modelling of extreme ocean environments for marine design: A review" (in en). Ocean Engineering 62: 91–109. doi:10.1016/j.oceaneng.2013.01.004. https://linkinghub.elsevier.com/retrieve/pii/S002980181300019X.
5. ↑ "Sea State" (in en). https://climate.esa.int/en/projects/sea-state/.
6. ↑ "CCI Sea State" (in en-GB). https://www.umr-lops.fr/en/Projects/Active-projects/CCI-Sea-State.
7. ↑ Laboratory, Plymouth Marine. "Sea State CCI". https://pml.ac.uk/projects/sea-state-cci/.
8. ↑ "CCI+ Sea State" (in en). https://www.dlr.de/en/eoc/research-transfer/projects-missions/cci-sea-state.
9. ↑ "Satellites reveal the power of ocean swell" (in en). https://www.esa.int/Applications/Observing_the_Earth/Space_for_our_climate/Satellites_reveal_the_power_of_ocean_swell.
10. ↑ Ardhuin, Fabrice; Postec, Taina; Accensi, Mickael; Piolle, Jean-François; Dodet, Guillaume; Passaro, Marcello; De Carlo, Marine; Husson, Romain et al. (2025-09-23). "Sizing the largest ocean waves using the SWOT mission". Proceedings of the National Academy of Sciences 122 (38). doi:10.1073/pnas.2513381122. PMID 40956893. PMC 12478040. https://www.pnas.org/doi/10.1073/pnas.2513381122.

## General and cited references

* Bowditch, Nathaniel (1938), American Practical Navigator, H.O. publication No. 9 (revised ed.), United States Hydrographic Office, OCLC 31033357
* Faltinsen, O. M. (1990), Sea Loads on Ships and Offshore Structures, [Cambridge University Press], ISBN 0-521-45870-6

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Physical oceanography

Waves|

* Airy wave theory
* Ballantine scale
* Benjamin–Feir instability
* Boussinesq approximation
* Breaking wave
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* Cross sea
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* Green's law
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* Iribarren number
* Kelvin wave
* Kinematic wave
* Longshore drift
* Luke's variational principle
* Mild-slope equation
* Radiation stress
* Rogue wave
* Rossby wave
* Rossby-gravity waves
* Sea state
* Seiche
* Significant wave height
* Soliton
* Stokes boundary layer
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Tides|

* Amphidromic point
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Landforms|

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Plate
tectonics|

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Ocean zones|

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Sea level|

* Deep-ocean Assessment and Reporting of Tsunamis
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Acoustics|

* Deep scattering layer
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* Sofar bomb
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Satellites|

* Jason-1
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Related|

* Argo
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