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International VOS Scheme

» Introduction

Lieutenant Matthew Fontaine Maury of the U.S. Navy was the first man to realise the scientific and commercial value of weather information collected from ships. Owing to his initiative, the first International Meteorological Conference was held in Brussels in 1853 to consider international cooperation and a uniform system of observation.

With the advent of radio communications early in the twentieth century it became possible for observations from ships to be transmitted to meteorological offices ashore, and warnings of dangerous conditions to be transmitted to ships.

At the 1929 meeting of the International Convention for the Safety of Life at Sea (SOLAS), provision was made for the international encouragement of meteorological work at sea. The current Solas Convention was adopted in 1974.

Meteorological services of most maritime countries made arrangements with ships regularly visiting their shores to take marine meteorological observations and transmit them to shore at no cost to the ship. The observations themselves are usually provided free of charge by shipping companies in return for the instrumentation and the forecasting and warning service. Hence the name of the scheme - Voluntary Observing Ship (VOS).


» View the VOS brochure and the MSC Circular 1293.

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» Rationale

Meteorological data are required from the seas and oceans for a number of purposes:

  • For the preparation of marine forecasts and warnings;
  • For the preparation of forecasts and warnings for offshore industries;
  • For marine consultancy;
  • For global computer models of the future state of the atmosphere;
  • To monitor the state of the oceans using delayed-mode data in weekly and monthly analyses;
  • For climatological data banks for many purposes, e.g. design of ships and structures at sea, determination of economic shipping routes; and
  • To build long-term records to monitor changes in the climate of the earth.

The requirements for VOS data in support of Numerical Weather Prediction (NWP) and climate applications are given below in Table 1 below. A full description of the requirements is contained in the VOS Framework Document.


Table 1. Observational Requirements.
variable Numerical Weather Prediction Ocean Obs System for Climate
spatial resolution temporal resolution accuracy spatial resolution temporal resolution accuracy
atmospheric pressure 100 km 0.5 hr 0.99 hPa 300 km 6 hr 0.65 hPa
wind 100 km 0.5 hr 1.5 ms-1 40 km 2 hr 0.6 ms-1
air temperature 50 km 0.5 hr 1 oK 50 km 36 hr 0.15 oK
sea surface temperature 15 km 24 hr 0.5 oK 8 km 5 hr 0.126 oK
wave height 50 km 0.5 hr 0.3 m 50 km 6 hr 0.3 m
sea ice extent 15 km 24 hr 10% (max) 25 km 3 day 6% (max)

"Breakthrough" values from the VOS Framework Document are shown in the table above.


The oceans cover about two-thirds of the surface of the earth, and for decades ships were the only means of obtaining meteorological data from them. Although there are now several other means - satellites, drifting buoys, floats and radar - ships still play a very important part. They provide ground truth for the calibration of satellite observations and make measurements not yet obtainable by other means, such as air temperature and dew point.

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» Existing System

There are eight classes of ships in the international VOS Scheme as follows:

A Selected ship is equipped with sufficient certified meteorological instruments for making observations, transmits regular weather reports and enters the observations in meteorological logbooks. It should have at least a barometer, a thermometer for sea-surface temperature, a psychrometer and a barograph. Most of the VOS are selected ships.

A Selected AWS ship is a mobile ship station equipped with an Automatic Weather Station (AWS) system comprising certified meteorological instruments to measure at least at least air pressure, pressure change, temperature and humidity. Optional sensors would include wind speed and direction and sea temperature measurement. The AWS may or may not have the facility for manual input of the visual elements, and transmit reports at least three hourly or more frequently. The AWS should have the facility to log the data.

A VOSClim ship is a mobile ship station equipped with sufficient certified meteorological instruments for making observations, transmits regular and timely weather reports, enters the observations in an International Maritime Meteorological Tape (IMMT) compliant electronic logbook including the extra VOSClim delayed-mode groups, and has a proven record of providing high quality observations. A VOSClim ship should have at least a barometer, a thermometer to measure SST, a psychrometer (for air temperature and humidity), a barograph and possibly an anemometer. The full range of metadata must be maintained in WMO No. 47, the full suite of digital images, sketches and drawings must be available, and the delayed-mode IMMT data must be submitted to the Global Collecting Centres (GCCs) according to recommended WMO procedures. It is highly desirable for a VOSClim ship to be inspected at less than six monthly intervals.

A VOSClim AWS ship is a mobile ship station equipped with an AWS system comprising certified meteorological instruments to measure at least air pressure, pressure change, temperature and humidity. Optional sensors would include wind speed and direction and sea temperature measurement. The AWS may have a facility for manual input of the visual elements, and transmit reports at least three hourly or more frequently. The AWS must have the facility to log the data including the additional IMMT delayed-mode VOSClim groups. The full range of metadata must be maintained in WMO No. 47, the full suite of digital images, sketches and drawings must be available, and the delayed-mode IMMT data must be submitted to the GCCs according to the recommended WMO procedures. It is highly desirable for a VOSClim AWS ship to be inspected at less than six monthly intervals.

A Supplementary ship is a mobile ship station equipped with a limited number of certified meteorological instruments for making observations. It transmits regular weather reports and enters the observations in a meteorological logbook.

A Supplementary AWS ship is a mobile ship station equipped with an AWS system comprising a limited number of certified meteorological instruments and reporting regularly. The AWS should at least measure air pressure.

An Auxiliary ship is a mobile ship station normally without certified meteorological instruments, which transmits in a reduced code form or in plain language, either on a routine basis or on request, in certain data sparse areas and under certain conditions.

An Auxiliary AWS ship is a mobile ship station equipped with an AWS system comprising non-certified meteorological instruments and reporting regularly. The AWS should at least measure air pressure.


Table 2. The elements observed by the various types of VOS
element Selected VOSClim Supplementary Auxiliary
present and past weather X X X X
wind direction and speed X X X X
cloud amount X X X X
cloud type and height of base X X X  
visibility X X X X
temperature X X X X
humidity (dew point) X X    
atmospheric pressure X X X X
pressure tendency X X    
ship's course and speed X X    
sea surface temperature X X    
direction, period and height of waves X X    
sea ice and/or icing X X X X
special phenomena X X    
max height of deck cargo from SLL   X    
height difference from the SLL to the water line   X    
course of ship over ground   X    
ship's ground speed   X    
ship's heading   X    

X = reported by non-AWS and AWS ships    ||    X = reported by non-AWS ships only.


Ships are recruited by Members of WMO, usually through Port Meteorological Officers (PMOs), who recruit if possible into their VOS fleets ships of all flags, not just those on their national register. Ships are recruited on the basis of the willingness of the ships' officers to perform the observations and the regular route followed by the ship. A Member will generally recruit ships which regularly visit ports in the country concerned. Recruited ships are usually on the national register of the Member, but may be on a foreign register, in which case the meteorological service of the country of registry is informed.

Ships' observations are generally made at the standard synoptic hours of 0000, 0600, 1200 and 1800 UTC and are sent to a meteorological service as by INMARSAT-C or email communication. In the case of INMARSAT C, the cost of transmission is paid by the meteorological service of the receiving country. Observations at the intermediate reporting times of 0300, 0900, 1500 and 2100 UTC are also welcomed, and observers are urged to report at any UTC hour if they missed the standard or intermediate reporting times or if weather conditions varied markedly from that forecast.

A list of all VOS and their instrumentation is maintained in WMO No. 47 on the basis of information supplied by Members. Metadata about the type of instrumentation, location and exposure are absolutely essential to a correct interpretation and use of the observations.


» Read about the VOS Ancillary Pilot Project.

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» Data Management

Marine meteorological observations have traditionally been recorded on board most ships in special meteorological registers (logbooks) provided by national Meteorological Services. The logbooks are collected by the PMO of the recruiting country and the observations are transferred from the logbooks to magnetic media, in a standard, internationally agreed, format. Increasing numbers of ships are now using Electronic Logbook Software, eg. TurboWin, OBSJMA, SEAS, to compile and record their observations in the internationally agreed format. These data are periodically collected by the PMO.

The data are then sent, at approximately three-month intervals, to global collecting centres in Germany and the United Kingdom in support of the Marine Climatological Summaries Scheme (MCSS). These centres ensure that minimum quality control has been applied to the data, and then, every three months, supply data to eight Members, each with a specific area of responsibility for the preparation of climatological summaries.


» View the JCOMM data management diagram.

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» Real-Time Data Quality

PMOs visit VOS calling at their ports to check the instrumentation, calibrate the barometer, supply stationery such as barograph charts or logbooks as required, and discuss any observational problems with the Master and officers.

The quality of VOS reports is monitored by several major meteorological centres, with the Met Office, United Kingdom, being the designated WMO centre for surface marine data. The results of this monitoring are compiled and distributed monthly to national VOS Program Managers and PMOs, who are expected to take follow-up actions to correct deficiencies.

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» Automation of Observations

Stena Adventurer AWSTo reduce the workload on ships' officers, several means of automation of the observations have been developed. Using Electronic Logbook Software, the observations are still taken manually but then entered into a software package which then calculates the true wind, the mean sea level pressure (correcting for the height of the bridge) and the dew point. This same software can also perform quality control, code the observation for transmission, and format the observation in logbook format for digitisation. If this software is run on a ship's personal computer, care must be taken, to ensure this process does not adversely affect the ship's network PC and/or its communication carriage requirement under SOLAS.

Increasingly, many National Meteorological Services (NMSs) are equipping ships with an AWS that may either operate totally in stand-alone mode, or accept manual input of the visual parameters (cloud, weather, sea and swell) via a computer. The accompanying photograph shows the AWS installed by the Met Office on the Stena Adventurer.

The best locations for sensors are not easy to find, particularly for wind and dew point. Most sites for an anemometer sensor will be affected by wind flow distortion over the superstructure.

An additional factor is that the changing nature of international shipping creates problems in selecting a vessel that is likely to stay on the same trade route for a predetermined period. NMS may be reluctant to invest in ship AWS installations when there is no guarantee that a ship will continue trading in their area of forecasting responsibility.

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» International Coordination

SOT logoThe international VOS Scheme is coordinated by the VOS Panel (VOSP), a sub-panel of the Ship Observations Team (SOT) in the Observations Programme Area of JCOMM.

The Terms of Reference of the VOSP are:

  1. Review, recommend and coordinate the implementation of new and improved specialized shipboard meteorological instrumentation, siting and observing practices, as well as of associated software;
  2. Support the development and maintenance of new pilot projects;
  3. Oversee the upgrade of ships to VOSClim class standard, and encourage new ships to be recruited to the VOSClim class;
  4. Develop and implement activities to enhance ship recruitment, including promotional brochures and training videos; and
  5. Prepare annually a report on the status of VOS operations, data availability and data quality.
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