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, GPRS, Satellite  phones or HF radio to Internet link. The alternative, sometimes prohibitively costly, sometimes impossible approach is to access weather charts in analogue form.

Where do the data come from and what are they?

The major source is the US NWS Numerical Weather Prediction model, the GFS (Global Forecasting System) which calculates how the weather is expected to develop for several days ahead (up to around 16).,  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. 

From the forecast wind at this level and using algorithms embodying surface roughness and stability, the NWS derives a forecast at 10 metres.  The forecast winds are also used to drive a wave model known as the MMAB Operational Wave Model (aka Wwave3). 

NWS wind data in GRIB format are available, free, from several sources. several, such as Saildocs provides the wind at the 10 metre  level directly from the GFS. One, GMN provides wind data from the output of Wwave3.  These are  derived using a slightly different algorithm.   The situation is somewhat  confused because the GFS output is available on a 1 deg x 1 deg grid while the

wwave3 output is on a 1 deg x 1.5 deg grid which involves some spatial interpolation.

The differences between the 10 m (GFS) winds and the winds output from MMAB  will not be great and probably well within the noise level of the actual wind on many occasions.  Remember that the atmosphere does not know itself to better than one Beaufort force. Basic US NWS output can be obtained free of charge from Saildocs and GMN.  Other, more extensive  services on repayment  are available from a variety of sources see my page Which GRIB Service?.

A summary of what data are available at no cost can be found on another page of this site.,

Meso Scale Output

Because of the interest in small scale weather features and their better prediction near land, there are now data available from meso-scale models. To read about these - and their limitations go to the NWP page of this site. Some meso-scale output is free from SailDocs .

Cautionary NOTE -  

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. However,  when preparing forecast charts at 4 and 5 days ahead. forecasters at the UK Met Office do look at other forecasts from the US, ECMWF etc and try to produce a "best" forecast from a consensus point of view.
For a very sensible appraisal of the use of ANY GRIB based forecast, send a blank email to GribWarning@saildocs.com.

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 chart format with coastlines and latitude/longitude grids. The isobars also have to be constructed. 

The most basic, generic, GRUB service comes from the free SailDocs system.  They provide a free, simple GRIB viewer called  Viewfax, that can be used with most other services. More information is available from the "gribnews" document. Send a blank email to info@saildocs.com. 

To get Viewfax use the  http://www.siriuscyber.net/sailmail/amdisk1.exe floppy disk link.. This is a 1.1 mb download that creates a folder called Airmail. Run this program. Then download the Viewfax viewer by going to the  http://www.siriuscyber.net/wxfax/ "Fax companion" page and looking for the paragraph headed Grib/Fax Viewer. There will be a "Click here to download Viewfax" Download and run this program.

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. Although the display is "static" a movie can be simulated by holding or tapping the ê arrow

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  or the MaxSea site..  JVcomm32  and Mscan Météo  are other programs which have been reported to work well. Two other free viewers are ExpeditionLT that comes with ProGRIB and the UGrib viewer.

Windplot has very clear coastlines. Viewfax has a nice facility letting you read the actual forecast speed. That helps distinguish between a 15 knot wind vector which could be anything between 12.5 and 17.5 knots.

File Sizes

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 info@saildocs.com, this will return the how-to document (about 5K).