Why Marine Forecasts Are Different
Marine weather forecasts are not the same as your phone's weather app. They are designed specifically for people on the water and use standardised terminology to convey critical information about wind, sea state, and visibility in a compact, unambiguous format. Misreading a forecast — or ignoring one — is one of the most common factors in sea emergencies.
The Key Elements of a Marine Forecast
Wind
Wind information is the most important element in any marine forecast. It is expressed in:
- Direction: The direction wind is coming from. A "northerly" wind blows from north to south.
- Speed: Given in knots (kt) or the Beaufort Scale (Force 0–12). Force 4 is moderate breeze; Force 7 is near gale; Force 10 is storm.
- Variations: "Backing" means wind shifting anticlockwise (e.g. SW to SE). "Veering" means shifting clockwise. "Gusting" indicates brief but significant speed increases above the mean.
The Beaufort Scale at a Glance
| Force | Description | Wind Speed (knots) | Sea State |
|---|---|---|---|
| 0–1 | Calm / Light air | 0–3 | Glassy or rippled |
| 3–4 | Gentle–Moderate breeze | 7–16 | Small waves, few whitecaps |
| 5–6 | Fresh–Strong breeze | 17–27 | Moderate waves, many whitecaps |
| 7–8 | Near gale–Gale | 28–40 | High waves, spray, reduced visibility |
| 9–10 | Severe gale–Storm | 41–55 | Very high waves, heavy sea |
| 11–12 | Violent storm–Hurricane | 56+ | Exceptionally high waves |
Sea State (Wave Height)
Wave height is given as significant wave height — the average of the highest one-third of waves. The actual highest waves can be considerably larger. Terms like "slight," "moderate," and "rough" correspond to approximate height ranges and affect vessel handling significantly.
Visibility
Marine forecasts describe visibility using standardised terms:
- Good: More than 5 nautical miles
- Moderate: 2–5 nautical miles
- Poor: 1,000 metres – 2 nautical miles
- Fog: Less than 1,000 metres
Where to Get Marine Forecasts
Reliable sources vary by region, but key ones include:
- UK/Ireland: Met Office Shipping Forecast (BBC Radio 4), Met Éireann
- US: NOAA National Weather Service — Marine Forecasts
- International: Navtex (automated radio broadcasts), PredictWind, Windy.com, Passage Weather
Always cross-reference at least two sources. No forecast is infallible, and models can disagree — especially beyond 48–72 hours.
The Golden Rules of Forecast Reading
- Check before you leave, and again if you can. Conditions evolve faster than forecasts sometimes anticipate.
- Understand the local geography. Headlands accelerate wind. Channels funnel it. Bays create confused sea states. Local knowledge often trumps the official forecast.
- Plan for worse than forecast. If the forecast says Force 5, be ready for Force 6. A good passage plan includes a bail-out harbour.
- Pay attention to pressure trends. A falling barometer accelerating its descent is a strong warning sign.
Developing Forecast Intuition
The best sailors develop an almost instinctive feel for weather patterns over time. This comes from consistently logging what the forecast said, what actually happened, and observing cloud types, wind shifts, and barometric pressure on passage. Keep a simple weather log aboard — over a season, it becomes an invaluable reference.