Modern weather prediction was driven by concerns about safety at sea, and the Shipping Forecast has become a national treasure. But for sailors who still rely on wind power, there are now even more specialised weather forecasts available.
Yacht racing is as competitive as any other sport, and more expensive than most, creating a lively market in specialist forecasts. These may be prepared up to three days before a race.
They include detailed maps of the wind patterns showing how the speed and direction are expected to change hour by hour, as well as expected wave height and the interval between waves.
The strength and direction of any currents is shown, and the forecast might include sea surface altimetry giving the height of the sea – not the waves, but the variation of just a few centimetres in the average level of the surface.
Along with the sea surface temperature, this indicates the pattern of upwellings and indicates how currents are likely to develop.
Racing forecasts can even include bathymetry maps of the depth of the ocean. Deep sea swells get higher when they reach shallow water, and a navigator might aim for the slight benefit of deeper water when the swell is high.
The forecast might even warn about its own reliability. When the weather is settled a prediction might carry a high confidence rating, but in changeable conditions it will be hedged with caveats.
Weather forecasts once saved sailors’ lives; now they are about that vital fraction of a knot that divides winners and losers.
