Chapter 36: Weather Observations and Forecasts

Shipboard observation

A synoptic observation reports position, wind direction and speed, pressure and pressure tendency over three hours, air and sea temperature, dew point or wet bulb, visibility, present and past weather, cloud amount and type, and wave and swell height, period and direction. Voluntary Observing Ship reports are transmitted at the main synoptic hours 0000, 0600, 1200 and 1800 UTC. Consistency and honesty matter more than precision: an estimated sea state clearly marked as estimated is more useful than an invented figure.

Instrument corrections

An aneroid barometer must be corrected for instrument error from its calibration certificate and reduced to sea level by adding a height correction for the barometer's elevation above the waterline; it requires no temperature or gravity correction unlike a mercury barometer, which needs both plus a reduction to standard conditions. Wind measured aboard is apparent wind and must be resolved with the ship's course and speed to recover the true wind. Sea temperature by bucket or intake, and psychrometer readings shielded from spray and radiation, complete the set.

• Reduce barometer readings to sea level before comparing or reporting
• Apparent wind + ship vector = true wind
• Pressure tendency over three hours is as informative as the pressure itself

Products and dissemination

Forecasts and warnings reach ships by NAVTEX, SafetyNET/EGC, HF radiofax, and internet or satellite data services carrying GRIB files of forecast wind, pressure and wave fields. Warnings escalate as gale, storm, hurricane-force and tropical cyclone. GRIB data is model output without a forecaster's judgement — always compare it with the text forecast and with your own barometer and observation.