What is a GRIB File?

Use of numerical weather model forecasts by individual mariners is an ubiquitous component of modern marine weather. Almost any navigation program these days connected to a wireless source on land or at sea can press a button to generate the latest wind, waves, and currents forecast across the chart with forecasts extending out a week or more.

The data come to us in a digitized format called GRIB, standing for gridded binary. It is a vector product, meaning it is all numbers and symbols, but when rendered in an appropriate software program (“GRIB viewer app”) it can appear as a graphic map of the isobars, wind vectors, rain distribution, and other parameters, laid out on a Lat-Lon grid. 

The GRIB standard was developed by the World Meteorological Organization (WMO) specifically as a way to transfer weather data in an efficient, standardized manner amongst meteorologists and mariners. The grid is a Lat-Lon lattice of points, with digital values (binary data) presented only at those specific points. The distance between points on the grid is the resolution of the file. This can vary from as large as 1º (60 nmi)  intervals to as small as 1.3 km (0.7 nmi), which is about the finest step size available outside of the laboratory.

A GRIB weather map at 0.5º resolution over a 10º x 10º area including wind and pressure will be about 50 kb in size. File size increases very roughly proportional to the number of parameters, but increases as the square of the resolution and area covered.

Several apps let the viewer request the latest grib forecast from a specific model from within the app, and these generally estimate the file size as you define what you wish to request.

Terminology: We often hear or say something like, “Have you downloaded the Grib?,” or “What does the Grib tell us,” and so on. Usually this is not ambiguous, but we should keep in mind that “Grib” is not a thing; it is a format.  It is like saying “What does the PDF tell us? The term Grib alone does not tell us at all what we are looking at. This could be a wind forecast from the GFS model or a wave forecast from the WW3 model. Use of the word Grib in such discussions requires a very clear understanding of the context.

The following links contain information related to use of GRIB files

Continue reading the full post on David Burch Navigation Blog.