GRIB Files — Understanding Weather Data

What GRIB Files Are

GRIB stands for GRIdded Binary — a compact file format designed to store weather forecast data on a regular latitude/longitude grid. Every weather routing calculation starts with a GRIB file. Think of it as a spreadsheet laid over the ocean, where each cell contains predicted wind speed, wind direction, pressure, and potentially wave and current data for a specific time in the future.

A GRIB file contains multiple time steps — snapshots of the atmosphere at regular intervals (typically every 3, 6, or 12 hours) extending days into the future. When you step through a GRIB file in qtVlm, you are watching the forecast evolve: a high-pressure system building, a front sweeping through, trade winds filling in. The routing algorithm reads every time step to calculate how the wind will change along each possible route.

The format was developed by the World Meteorological Organization (WMO) and is used universally by weather services worldwide. There are two versions in common use: GRIB1 (older, still used by some services) and GRIB2 (more efficient compression, more data types). qtVlm reads both versions seamlessly — you do not need to worry about which version you are downloading.

Data Sources: GFS, ECMWF, and Others

Multiple weather services around the world run numerical weather prediction models, and each produces GRIB data you can use for routing. The two dominant global models are GFS and ECMWF, and understanding the differences matters for route quality.

GFS (Global Forecast System) is run by NOAA (the US National Oceanic and Atmospheric Administration). It is completely free, covers the entire globe, and is available at 0.25° resolution (about 28 km at the equator) or 0.5° resolution. GFS runs four times daily and produces forecasts out to 16 days. For most recreational weather routing, GFS is the workhorse — free, readily available, and good enough for routing decisions.

ECMWF (European Centre for Medium-Range Weather Forecasts) is widely considered the most accurate global model, particularly beyond 5 days. However, full-resolution ECMWF data is not freely available — it requires a subscription or access through commercial weather services. Some limited ECMWF data is available through services like Saildocs or commercial routing packages. When accuracy matters most (ocean crossings, departure timing for multi-week passages), ECMWF data is worth the investment.

Regional models offer higher resolution over smaller areas: NAM (North American Mesoscale) covers the US at 3 km resolution, ICON from the German DWD covers Europe and the Atlantic, and ARPEGE from Météo-France covers Europe. These regional models capture local effects — sea breezes, coastal convergence zones, island effects — that global models miss. For coastal routing, a regional model layered over a global model gives the best picture.

Resolution, Coverage, and Trade-offs

Every GRIB download involves a three-way trade-off between spatial resolution (how detailed the grid is), geographic coverage (how large an area), and file size (how long it takes to download). Understanding this trade-off is essential, especially when you are downloading data at sea over limited bandwidth.

Spatial resolution is expressed in degrees of latitude/longitude. GFS at 0.25° means a data point every ~28 km. At 0.5°, it is every ~56 km. At 1.0°, every ~111 km. Higher resolution captures more detail — wind acceleration through channels, island wind shadows, coastal convergence — but generates proportionally larger files. For open-ocean routing, 0.5° or even 1.0° is fine. For coastal passages, 0.25° or a regional model is much better.

Temporal resolution is how often you get a time step. GFS provides data every 3 hours for the first 10 days, then every 12 hours beyond that. More frequent time steps let the routing algorithm react to rapidly changing conditions (a frontal passage, a diurnal wind cycle), but they also increase file size. For most routing, 6-hour time steps are a reasonable compromise — 3-hour steps add precision for short coastal passages.

File size matters when you are downloading GRIB data via satellite phone (Iridium, InReach) or SSB radio (SailMail, Winlink). A full GFS 0.25° file covering the North Atlantic with all parameters can be 15-20 MB — fine on Wi-Fi, brutal on satellite bandwidth. Reduce file size by limiting the geographic area, choosing 0.5° or 1.0° resolution, selecting only wind and pressure (skip waves and precipitation), and using fewer time steps. A targeted request for your specific passage area at 1.0° with wind only might be under 100 KB — manageable even on Iridium.

Data Types: What Matters for Routing

GRIB files can contain many data types, but not all of them matter equally for weather routing. Understanding which parameters drive routing decisions helps you request the right data and keep file sizes manageable.

Wind at 10 meters (speed and direction) is the essential parameter — it is what the routing algorithm uses with your polar diagram to calculate boat speed at every grid point and time step. Without wind data, there is no routing. This is the one parameter you must always include in your GRIB request.

Mean sea level pressure (MSLP) does not directly affect routing calculations, but it is invaluable for understanding the weather pattern. Pressure contours (isobars) show you where the highs and lows are, where fronts are located, and how the pattern is evolving. Pressure data helps you make qualitative judgments that supplement the routing algorithm's output — for example, recognizing that the router is sending you too close to a developing low.

Wave data (significant wave height, period, direction) is a valuable supplement. Some routing software can factor wave height into the calculation — reducing polar performance in steep seas. Even without automated wave routing, seeing the wave forecast alongside your route helps you assess comfort and safety. Ocean currents (from models like RTOFS or OSCAR) directly affect boat speed over ground and can significantly alter optimal routes, especially near the Gulf Stream or in tidal waters. Precipitation and cloud cover are nice to have for passage planning but do not affect routing calculations.