AROME, from the French coast to the islands

Close to land, the wind is rarely the wind of the open sea. The sea breeze fills in along a coast in the afternoon, the Mistral accelerates out of the Rhone valley, the trade wind bends round a headland and dies in the lee of a high island. A global model sees none of it: its route sails through a wind shadow as if it were open sea, or misses the gust at the end of a channel.

Météo-France's fine model, AROME, computes these effects: at 1.3 km around France, with a new run every three hours, and at 2.5 km over five overseas areas, hour by hour for about two days. A passage whose departure and destination both lie within one of its areas gets an AROME route beside the global models', and close to land it is often the route to watch.

Around France, its area runs from the southern North Sea and the Channel to the Balearics, Sardinia and the west of Italy, with the Bay of Biscay and the Atlantic out beyond Ireland. Overseas, it covers five cruising grounds: the West Indies, from Hispaniola to well east of Barbados; French Guiana; the south-west Indian Ocean, from the Mozambique Channel to Rodrigues, with Réunion, Mauritius and Mayotte; New Caledonia and nearly all of Vanuatu; and French Polynesia, from the Society Islands to the western Tuamotu and the Austral Islands but Rapa.

Take Guadeloupe to Martinique, past Dominica. The global models' routes, blind to the island's lee, sail further out; AROME's keeps close under Dominica and motors where it sees the wind die. Before you leave, you know a calm is waiting under the island and how many hours under engine it will cost: enough to check the fuel, or to choose another hour to sail.

Its two days suit a day sail, a coastal passage or an island hop, not a crossing: beyond them, the global models see further, and the large weather systems come into AROME from a global model at its edges, early or late as they are there. Both ends must lie within one area: an Atlantic crossing to the West Indies gets no AROME route, nor does a landfall in the Marquesas or the Gambier, which no area covers. And under a squall, the gusts can blow far harder than its mean wind.