GRIB files

What a GRIB file is

GRIB stands for GRIdded Binary. It is the format in which the world’s forecasting centres publish model output, and the format sailing apps and chartplotter software read. A file is a series of records. Each record holds the values of one parameter at one time on a grid, for example “10 m east-west wind, 24 hours after the run”, with the grid and time described inside the record. The values are packed to take little space.

• Two components for wind. Wind is stored as U (towards the east) and V (towards the north), in metres per second. Viewers compute speed (the square root of U² + V²) and the direction the wind blows from. A single file may also hold pressure, gusts, rain, waves and more.
• Time. Each record carries the run time and the forecast step, in UTC. Check them: they tell you which run you are looking at.
• Versions. GRIB2 is the current version; the older GRIB1 is still used. Most software reads both.
• Grid spacing in degrees. Files describe the grid in degrees, such as 0.25°, and east-west boxes are narrower the farther you are from the equator: What km and NM mean on a model.

How big will it be?

Keeping it small offshore

1. Limit the area to your route plus a margin, and move the box as you go.
2. Use a coarser grid for open ocean: 0.5° to 1° (30 to 60 NM) is usually plenty. Use a fine grid only for the coastal legs.
3. Choose only the parameters you use: wind and pressure first; add gusts and waves when needed.
4. Use a longer time step (6 or 12 hours) for the later days and a short one for the first day or two.
5. Request it by email from a service such as Saildocs, which sends a small subset by email over satellite or HF.

Reading a GRIB file

Software such as XyGrib, LuckGrib, SailGrib, qtVlm and Expedition draw GRIB files on a chart and let you step through time. OpenCPN and other chartplotter programs can do the same.