How the wind is computed, in VR and in the application

Expert

The essentials

A GRIB file gives the wind at precise forecast times (every 3 hours for example), not every minute. To know the wind at 13:52, the application interpolates: it mixes the two forecast times around that moment, in proportion to the time. It does the same from one grid point to the next to get the wind at the exact place of the boat.

This file does not come from a single run: it assembles the forecast times of several successive runs of the model (00Z, 06Z…). According to the FAQ, VR does the same, which is why the game and the application agree on the boat’s wind.

Going further

An example. A file dated December 9, run 06Z, contains among others:

  • 20241209.00.012: the wind forecast at hour 12 of the 00Z run, so for 12:00 UTC;
  • 20241209.06.009: the wind forecast at hour 9 of the 06Z run, so for 15:00 UTC.

At 13:52 UTC, it is 112 minutes after 12:00 and 68 minutes before 15:00, a gap of 180 minutes. The wind used is therefore 38% of the 12:00 forecast time (68 out of 180) and 62% of the 15:00 one (112 out of 180). The same operation is applied to each component of the wind.

In VR. The game’s two gauges show this interpolation: the blue part of the left gauge is the weight of the 00Z run (38% here), the green part that of the 06Z run (62%). The right gauge is entirely green: from 15:00 UTC on, all forecast times come from the 06Z run. VR does not display the percentages, only the gauges.

In the application. The routing computes the wind the same way: a linear weight between the forecast time before and the one after, then interpolation between the grid points.

  • For the GFS 0.25° grid, the interpolation between grid points is bilinear.
  • For the other grids (GFS 1.00°), the application uses VR’s interpolation.
  • In both cases, wind speed is converted to knots and direction to true degrees.

Practical consequence. The wind changes gently between two forecast times, and more abruptly when a new run replaces the old one: a routing redone after a new forecast arrives may give a different route. See Update times and delays.