ARPEGE, Météo-France's global model

Off the European coasts, a global model draws in broad strokes: the Mistral fanning out over the Gulf of Lion, a front turning round Finisterre, the trade wind accelerating between the Canaries. The fine models of the coast see them better, but they cover only so far out into the Atlantic, and the islands of the open ocean lie beyond most of them.

ARPEGE, Météo-France's global model, built together with the European Centre, has a grid stretched over France, finest over French waters. Saily WR reads it twice: the global product, on a quarter-degree grid, for a route anywhere, and ARPEGE-EU, its finer grid over Europe and the North-East Atlantic, which Saily WR reads at about 11 km, hour by hour.

ARPEGE-EU reaches from the Canaries and the coast of Morocco to Iceland and northern Norway, from the Azores to the Black Sea, with the Channel, the North Sea, the Baltic and the whole Mediterranean. Both products look a little over four days ahead: enough for a coastal passage or an offshore leg, not for an ocean crossing.

From Madeira to Lanzarote, two days at sea, the finer models of the European coasts are left behind, and ARPEGE-EU still gives a route beside the global models', on a grid finer than GFS's. Here the trade wind is the whole story: if their routes agree, the crossing holds no surprise; if ARPEGE-EU's swings closer to an island or further from it, it has seen a detail of the wind around the islands that the coarser grids average away.

Its four days make it a model for passages of a few days: on an ocean crossing it falls silent long before the arrival. Outside ARPEGE-EU's area, in the Caribbean or the Pacific, the global ARPEGE, read on its quarter-degree grid, is no finer than GFS. And it does not compute squalls: under an unstable sky, the wind can blow far harder than the one its route sails in.