What is GRIB Code?
(Apart from being GriddedInformation in Binary form)
Downloading big files of weather charts is no problem when at home using a conventional telephone link. Using a HF radio link, a GSM or satellite phone download speeds become important and acquiring charts may be costly. Even using web pages composed mainly of text can be frustrating because of delays in linking to a website.
Connections can all too easily be lost in the middle of a large file.
An attractive, low cost solution is to use GRIB coded products which are received as text/numeric attachments to emails. Forecasts of pressure patterns and wind vectors for several days ahead can be contained in a file of less than 15 kB compared to one chart downloaded from a website taking 80 kB. Processing into chart form is effected using standard software off line. This page briefly describes GRIB and its use with output from the US National Weather Service. Other providers, eg the Météo France Navimail and Weather Wizard will work in some similar way except that most will charge for their services.
What is GRIB?and
A warning about its use / Saildocs and GMN
GRIB Services / Viewers for GRIB Files / Requesting Files
Saildocs / Loading and Viewing Saildocs / Repeat Requests Saildocs
Requesting, and Viewing
GMN / Does it Cost? / File sizes / Saildocs or GMN? / Detailed Notes on Airmail/Saildocs
Software / List of GMN Areas
What is GRIB and why does it provide so much for so little?
GRIB is a WMO code form used by operational meteorological centres for storing and exchanging meteorological charts and other patterns of wind, sea state, temperature etc. The information is put into a compressed digital, binary format that enables high speed transmission and reception. It is useful to sailors for the efficient reception of charted information using GSM, Satellite phones or HF radio to Internet link. The alternative, sometimes prohibitively costly, sometimes impossible approach is to access weather charts in analogue form.
GRIB data are available fro various sources. This page deals mainly with Saildocs and Global MarineNet.
Where do the data come from?
The US NWS Numerical Weather Prediction model, the GFS (Global Forecasting System) calculates how the weather is expected to develop, For this purpose it defines the atmosphere on a three dimensional grid of points. The lowest level of this model is at about 20 metres above ground (or sea level). This is at a height where the pressure is about 3 hPa (mb) lower than at the surface of the earth.
NWS wind data in GRIB format are available from the model from two, related, sources. One, used by Saildocs, is the wind at a nominal 10 metres above ground or sea level. These winds are calculated (or estimated) taking into account topography, surface roughness, including ocean waves. The second wind is that used to drive the NWSOcean wave model known as the MMAB Operational Wave Model (wwave3). This is the wind at the bottom of the GFS model. These winds are available from GMN.
The differences between the 10 m (GFS) winds and the winds output from MMAB will not be great on many occasions. At low wind speeds they will be most marked, at higher speeds the difference is probably academic due to the great variation in wind speed that normally occurs.
The US NWS output can be obtained free of charge from Saildocs and GMN (see below). Other, more extensive services on repayment are from Météo France Navimail , TRANSAS Ltd and others.
NOTE - From whatever source, it is important to remember that Grib files are computer generated forecast files from a National Weather Service computer. These are sent without review, and are offered on an as-is basis. There is no assurance that the data are available, accurate or correct. Systems providing information and the computer models are automated systems, subject to a variety of failures and errors. By using the data, you acknowledge and agree to these limitations.
It should be remembered that human forecasters can still add value to computer model output. Consequently, the prudent Sailor will only use GRIB products in the short term (say up to 24 or 36 hours) in the light of other sources of information eg text forecasts, forecast charts generated by man-machine-mix, from the UK Met Office for example, or satellite pictures. For longer term predictions - say over 48 hours, the basic computer output is unlikely to be improved upon by a human to any great extent.
Saildocs GRIB code service
A basic GRIB data service is now available from Saildocs. Files are extracted on a per request basis from the US NOAA/NCEP global GFS computer model and can cover any requested area, worldwide. The resulting file size depends upon the amount of data requested but can, in principle, be anywhere from 1 kB to megabytes. For most normal cruising requirements, file sizes of a few kB may be sufficient and need not exceed 10 - 15 kB. Radio email services are available using Airmail software on one's laptop in conjunction with either the Winlink or Sailmail systems, or even both. (Currently, Sailmail will handle GRIB files up to 10K).
Output is available up to 10 days ahead although I would only recommend using up to 5 days which I regard as a reasonable limit for useful (rarely accurate) forecasting although this limit is slowly being pushed out beyond that time.
Global Marine Networks GRIB service
An alternative service is available from Global Marine Networks (GMN) which is now offering 5 day wind forecasts of the world as a public service via it's GRIB Mail Robot. The forecasts are generated daily for every region of the world. The forecasts are taken from the National Weather Service Wave Watch III model and give forecasts every 6 hours for up to 5 days into the future. All GRIB forecasts are delivered via e-mail and optimized specifically for low bandwidth wireless connections. Send a blank e-mail to GMN for information on how to receive these free GRIB forecasts or see their Grib Weather File page. A summary of their instructions is set out below.
Viewers for GRIB files
Much of the gain in transmission speed is because only the meteorological data are transmitted. Software is required to convert the information into a conventional map format with coastlines and latitude/longitude grids. The isobars also have to be constructed. A simple GRIB viewer is now available as part of a new version of Viewfax, Airmail's fax and image viewer. More information is available from the "gribnews" document (send a blank email to ). Or,go to Airmail Version 3.1 for SailMail. From here, download Airmail 3.1. This will create a folder called Airmail but is a rather long download and not necessary if you are not intending using the email via HF radio facility. The Viewer download is available from the Airmail home page , follow the link to the "Mad Science Projects" page.
Quicker is to choose the download to floppy disks link.This is a 1.1 mb download that creates a folder called Airmail.For those only wishing to access GRIB via GSM or satellite phones then the Airmail disk-1 and ViewFax are all that is required and can be kept in zipped form on floppy disk as backups.
For more complete notes for use with Airmail see the Appendix.
For some reason that I do understand it is not sufficient simply to create a new folder called Airmail and install the Viewer. It seems to be necessary to let the Sailmail software create the folder Airmail even though the only program to be used is the ViewFax program.
GRIB files can also be viewed with MAXSEA and Raytech charting programs. A MAXSEA viewer can be obtained at no charge from the GMN site . This seemed to be an easy site to access and a little less involved than Saildocs. I find that the Saildocs viewer will display isobars and winds at the same time. MAXSEA will not but that be my own finger trouble.
Requesting a GRIB file - Saildocs
The procedure described here uses Saildocs and is somewhat cumbersome to use but extremely flexible. The GMN service may be found easier to use. and details can be found at their site given above.
The format for a Saildocs grib request is a little cryptic. Follow the instructions below carefully. The format, which MUST be exactly followed, for a basic GRIB file request is: send grib:lat0,lat1,lon0,lon1|dlat,dlon|VTs|Params
lat0,lat1,lon0,lon1 are the latitude longitude limits (whole degrees followed by N/S or E/W
dlat,dlon is the grid increment in degrees (e.g. "1,1" for a 1-deg by 1-deg grid), if omitted the default is "2,2", The minimum increment is 0.5 degree For greater increments, whole degrees should be used (i.e. avoid "2.5,2.5").
VTs is a comma separated list of valid times (e.g. "24,48,72"), if omitted the default is "24,48,72". Available valid times are 00, 12, 24, 36, 48, 60, 72, 96, 120, 144, 168, 192, 216, 240 hours. Note that anything over 4 or 5 days should be viewed with additional scepticism.
Params is a comma separated list of parameters ("PRMSL" for mean sea level pressure, "U_GRD,V_GRD" for the surface wind (actually at 10 metres, the standard height for "surface" wind) and TMP for temperature, The default is "PRMSL,U_GRD,V_GRD".
For example, to request a grib file which covers 20N to 60N and 0E to 20W, on a 2-degree grid for valid times of 24 to 72 hours, containing surface pressure and wind, send the following email to
Subject: anything
send grib:40N,60N,0W,20W
Note that this is identical to:
send grib:40N,60N,0W,20W|2,2|24,48,72|PRMSL,U_GRD,V_GRD
IMPORTANT: Do not insert any spaces, Saildocs uses spaces to delimit parameters. The vertical bar character is the shift backslash key. There is not much format checking done, and it is not user friendly. At some stage the system may be revised so that requests are generated with some sort of fill-in-the-blanks window, similar to that used by Navimail. At this stage it is probably best to copy the above formats using Edit - Copy and then amend details of location and times as required.
Loading and Viewing - Saildocs
Having received the email reply, save the attachment to a folder. Open Viewfax by double clicking on the icon, then use File - Open. It is then possible to view the charts in sequence or at random.
Using MAXSEA to show just the wind vectors for one of the three charts, part of this request would look like this -
NOTE. One short feather = 5 knots, one long feather = 10 knots. The arrows "fly" with the wind.
Repeat Requests and Cancellations - Saildocs
For a daily repeat, use the same format but change the "send" to "sub".For example: send the following email to
Subject: anything
sub grib:40N,60N,10E,20W
This will enter a request which will be replied to daily, shortly after 12 UTC.
By default you will receive this information daily for 14 days. To change this add a space and "days=5" for a 5-day subscription, or "days=0" for an indefinite subscription, for example:
sub grib:20N,60N,10E,20W days=5
To cancel a request, send the same message except change "sub" to "cancel". The GRIB code must match exactly, even if it is incorrect.
Requesting and Viewing a GRIB file - GMN
This is simply done by sending a blank email to with various subjects as follows.
- By specifying an area eg Atlantic will return an e-mail with a grib file containing a 24 hour forecast. A list of areas available is at Appendix 2
- Alternately, specify the time of the forecast by entering the time after the area. Valid times are 6 to 120 hours in increments of 6 hours. i.e. 12, 24, 36, 48, 72 are all valid times. For example, entering Atlantic 24 48 72 will result in 3 e-mails each containing a separate forecast for the Atlantic region. Note that on average each e-mail will be between 3 and 5 Kb in size.
- Or, entering Atlantic 3day will return one compressed grib file between 30-40K that contains 3 days worth of forecasts on 6 hour intervals. Similarly for 4 or 5 days.
- Specific areas can be requested by by placing the centre Lat:Lon co-ordinates in the subject line. The syntax for custom regions is LAT[NS]:LON[EW]:RANGE [24 48 .. | 1day | 2day ...]. The optional RANGE is in nautical miles. A range of 1200 miles is assumed if none is specified. So entering the following in the subject line 18N:67W:600 3day would return a 3day forecast grib that was 600x600 miles centred at 18N and 67W. Entering 55S:70W 24 48 72 yields 3 GRIB s 1200 miles on the side centred at 55S 70W i.e. the Drake passage.
- For repeat requests, entering a number followed by the word "times" tells the grib robot to send you the request every day at 5 AM EST (!) UTC) for the number of days entered. For example entering 7times Caribbean 3day will cause the grib robot to send you the Caribbean 3day forecast every day for 7 days. "7times" must be entered as the first command on the Subject line.
- To cancel a request simply send an e-mail to the grib robot with the word cancel on the subject line.
NOTE: There are no spaces between the 7 and times. Also, times and cancel are in lower case.
To view GMN GRIB files with MAXSEA, simply click on the file name or open from the email attachment. Using the Saildocs viewer, the save the file to a folder with extension .grb (ie not.grb.bz2). Open the Saildocs viewer and use File - Open.
How much does it cost?
Saildocs is provided without charge, thanks to the support of Sailmail, a membership owned radio email service for cruising sailors which operates a seamless network of 13 stations world-wide (including five covering the Caribbean and Atlantic). For more information see the Sailmail web page or send a query to the office at
The GMN basic GRIB files are also provided free of charge although they do offer more extensive and tailor made services
File Sizes
This Saildocs example given above is around 2 kB. The size is directly proportional to the number of points: i.e. the size and resolution of the grid, the number of valid times, and the number or parameters. This sample is 900 points (10 latitude points x 10 longitude points x 3 valid times x 3 parameters).. These are uncompressed binary files which get compressed when sent via Sailmail or Winlink radio links, so there is no advantage to pre compressed files.
GMN file sizes are indicated in the GMN section above. With GMN, The more data you request the more efficient the compression will be. For example, entering Caribbean 24 48 72 will result in 3 grib files about 1.5-2K each for a total of around 6K. Entering Caribbean 3day results in 12 forecasts in one file that is 11K.
A file of GRIB data at a 1 degree latitude/longitude spacing covering the area 30N to 45N, 10W to 15E, giving wind arrows for eight times ie 12, 24, 36, 48, 60, 72, 96, 120 hours ahead (12 hourly to 3 days and 24 hourly to 5 days) would be 15x25x3x8 ie 9000 values. The file containing these data is less than 15 kB. Output for half the area would be half as much ie less than 8 kB. If only winds had been requested, then file sizes would be 2/3 of these values. An example of one of the eight charts received in this way is shown here using the Viewfax software -
The isobars shown here are at a rather fierce 2 hPa (mb) spacing. There are options to change colours, scale and isobar spacing.
More information on Saildocs is available by sending an email to , this will return the how-to document (about 5K).
Saildocs or GMN?
- Saildocs is the more cumbersome to request forecasts but gains by having slightly greater flexibility.
- Specific areas can be specified for both but GMN has a fixed area facility also - eg the Mediterranean although this will probably result in bigger file sizes the necessary...
- Saildocs can provide data at any or all of the times 00, 12, 24, 36, 48, 60, 72, 96, 120, 144, 168, 192, 216, 240 hours ahead. (But, see my caveat above.)
- GMN provides 3, 4 or 5 day forecasts. in the form of output every 6 hours thus giving better time resolution. (It is not clear to me just how useful is a 6 hourly interval at 5 days ahead compered to a 24 hour interval.
- GMN reduces file sizes by only giving wind vectors over the oceans. Saildocs includes output over land and this might help to see the overall pattern.
- Saildocs can provide temperature information
The two are very similar with some slight pluses and minuses for each.
Appendix 1 - Notes on using Saildocs
The following notes have been provided by Allan Riches of Brunei Bay Radio, Small Craft HF radio services. they were intended for people trying to get the software operational on their yachts. They are a more complete set of instructions than I have provided above and there will be some duplication as they are copied here unedited by me.
Download the (free) Viewfax software (or complete fax software - which is Viewfax and Getfax) from Jim Corenman's Mad Science Projects link on the downloads section of the SailMail website. If you want to receive weather faxes, the full (Viewfax & Getfax) version is designed to integrate with the AirMail software to control the radio frequencies from the software, if you have that feature working in AirMail.
Viewfax is the software used to read the (free) grib files you can request from Jim Corenman's (free) SailDocs service. These grib files display as a map of the requested region with isobars and wind speed/directions. It is created from the NOAA (the USA's National Oceanic and Atmospheric Administration) database and within the one transmitted file you can request a series of "pictures" up to 5 days ahead. In effect you get weather faxes via SailMail without needing to worry about transmission times, propagation, activities on board, changing modem set up, urgent sail changes etc.
SailMail allows grib file attachments up to 10 kB; sufficient for most uses. The more future "pictures" you request and the larger the area, the bigger the file gets. If the file is larger than 10 kB the SailMail server removes it.