How to read a sailing window

A sailing window is the stretch of hours when conditions favour your passage. MARLOG compresses the next 24 hours into one strip: each bar is an hour, its height the combined comfort of wind and sea state, its colour the verdict — lime for go, amber for caution, teal for calm-but-late.

The forecast behind the bars blends two kinds of model. Atmosphere — wind, gusts, visibility, precipitation — comes from Apple Weather. The sea itself — wave height, period, surface currents, sea temperature — comes from the EU's Copernicus Marine models at the sharpest resolution available for where you float: 4.2 km on the Mediterranean and Aegean, 2.5 km on the Black Sea, 500 m on the Marmara. MET Norway's sea-state alerts run over the top as a separate safety check.

Height tells you less than period. A metre of long-period swell is a comfortable ride; a metre of short, steep chop on the nose is a wet, slow slog. When two hours score the same, open the hour and compare the wave period — the longer one wins.

On the Aegean in summer, respect the afternoon. Thermal winds build through midday and the meltemi does not read forecasts politely. If the strip shows a green morning and an amber afternoon, that is not two options — it is one option and a warning.

The rule of thumb the app encodes: leave on a rising window, not a peaking one. If the best bar is 16:00, the good decision usually departs at 15:00 — you take the improvement with you instead of arriving at the peak as it turns.

Before you trust any window, check the alert line. A sea-state alert overrides a pretty strip: the window says comfortable, the alert says the sea disagrees somewhere on your track. The alert wins.

And the standing caveat, always: the sailing window is an advisory instrument, not an authority. It exists to help you choose your hour — the decision to sail, and the official forecast behind it, remain yours. Carry official charts, listen to official broadcasts.