Millimetres Per Hour

Millimetres per hour (mm/h) is the rate at which precipitation falls over the sea or a coastal area, expressed as the depth of water accumulated in one hour. In marine forecasts, it describes rainfall intensity rather than total rain over a day, helping crews judge how quickly conditions may deteriorate.

For a skipper, mm/h matters because heavy showers can reduce visibility, obscure navigation marks, increase cockpit workload and make decks hazardous. Comparing the rate with Rain, Rainfall and Rain Radar forecasts helps decide whether to delay departure, reef early, choose a safer harbour approach or brief crew for poor visibility.

PredictWind rain maps and model data display mm/h alongside wind and wave information, giving sailors and power boaters a clearer view of approaching showers, squalls and frontal rain. Offshore crews can also download relevant GRIB data to monitor precipitation intensity while at sea.