Frequently Asked Questions

Updated on 2 March 2026

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Marine Frequently Asked Questions

  • What are the obligations for national institution for marine meteorological services?
  • What is the Douglas scale and is it appropriate terminology ?
  • What is the Beaufort scale ? What is Sea State ?
  • What is the GMDSS ?
  • What is Code 41 (or Inmarsat Code-41) ?
  • What is swell ?
  • What is a storm surge ?

What are the obligations of the national institutions for marine meteorological services?

Obligations of National Meteorological Services in the implementation of Marine Meteorological Services (MMSs) are detailed in the Manual on Marine Meteorological Services (WMO-No.558). It provides detailed information on
- international obligations of national meteorological agencies to provide MMS for the high seas
- international obligations of national meteorological agencies to provide MMS for coastal and off-shore areas
- international obligations regarding services for main ports and harbour areas

What is the Douglas scale and is it appropriate terminology?

References to Douglas Scale appear more regularly in the maritime literature. The following terminology is recommended for use, such as supplying weather information and forecasts for shipping, publications, pilots, etc.:

For the length of swell waves:
Short 0 - 100 m
Average 100 - 200 m
Long over 200 m

For the height of swell waves:
Low 0 - 2 m
Moderate 2 - 4 m
Heavy over 4 m

For the height of sea waves:
Calm (glassy) 0
Calm (rippled) 0 - 0.1 m
Smooth (wavelets) 0.1 - 0.5 m
Slight 0.5 - 1.25 m
Moderate 1.25 - 2.5 m
Rough 2.5 - 4 m
Very rough 4 - 6 m
High 6 - 9 m
Very high 9 - 14 m
Phenomenal over 14 m

In all cases, the exact bounding length or height is included in the lower category, i.e. a sea of four meters is described as rough. When the state of the sea surface is so confused that none of the above descriptive terms can be considered appropriate, the term “confused” should be used. The above-mentioned information is described in detail in the WMO publication No. 8 (Guide to Meteorological Instruments and Methods of Observation) - part II, Chapter 4 (Marine Observations).

What is the Beaufort scale? What is Sea State?

The sea state basically specifies wave height. Wave height depends on local winds but on remote winds also (swell). 

The Beaufort scale specifies wind speed.

As far as weather observation is concerned, while sea state reporting remains a legal international practice, with modern in situ observing techniques we try to avoid using sea state or Beaufort scale as we prefer direct readings from appropriate instruments (e.g. wave buoys and anemometers respectively). Details about sea state and Beaufort can be found in the WMO Manual on Codes, No. 306, part A, Alphanumeric codes (look for the WMO Code table 3700 (i.e. Sea State) and in Section E for the Beaufort scale).

As far as forecasts and warnings are concerned, WMO does not recommend the use of sea state as it recommends using wave height; however, national meteorological services may be using sea state (e.g. Douglas scale) in some cases instead of wave height but this is not a formal international practice.

What is the GMDSS?

The GMDSS is the Global Maritime Distress and Safety System. It is an integrated communications system using satellite and terrestrial radiocommunications to ensure that no matter where a ship is in distress, aid can be dispatched. This System ensures also the provision of Maritime Safety Information (MSI), both meteorological and navigational information, on a global basis at sea. More information is being provided on the JCOMM GMDSS.

What is Code 41 (or Inmarsat Code-41)?

In the framework of the GMDSS, meteorological observations from ships – regarded as Maritime Safety Information – can be sent free of charge for the ships if sent via the Inmarsat satellite data telecommunication system using transmission code 41. The cost of transmission is paid by the meteorological service of the receiving country. The list of Inmarsat Land Earth Stations accepting Code 41 is available here.

What is swell?

A Swell is any system of water waves that has left its generating area (source: International Meteorological Vocabulary, WMO-No. 182). It is a series of surface gravity waves that are not generated by the local wind. Swell waves often have a long wavelength and a narrower range of frequencies and directions. Information on practical applications for swell and wave forecasting can be found in the Guide to Wave Analysis and Forecasting (WMO-No.702).

What is a storm surge?

A storm surge is a difference between the actual water level under influence of a meteorological disturbance (storm tide) and the level which would have been attained in the absence of the meteorological disturbance (i.e. astronomical tide). (source: International Meteorological Vocabulary, WMO - No. 182). Storm surges are caused primarily by high winds pushing on the water surface. Further information can be found in the Guide to Storm Surge Forecasting (WMO-No. 1076)

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