How a forecast is built
The Met Office's shipping forecast covers the sea areas around the British Isles. It opens with any gale warnings and a general synopsis of the pressure systems and how they're moving. Then each sea area gets the same four items, always in the same order: wind direction and force, sea state, weather and visibility.
Gale warnings
- Gale: force 8 (34 to 40 knots), or gusts reaching 43 to 51 knots.
- Severe gale: force 9 (41 to 47 knots), or gusts reaching 52 to 60 knots.
- Storm: force 10 (48 to 55 knots), or gusts reaching 61 to 68 knots.
- Violent storm: force 11 (56 to 63 knots), or gusts of 69 knots or more.
- Hurricane force: force 12 (64 knots or more).
Timing: imminent, soon and later
| Term | Meaning |
|---|---|
| Imminent | Within 6 hours of the time of issue |
| Soon | Within 6 to 12 hours of the time of issue |
| Later | More than 12 hours from the time of issue |
All three are measured from when the forecast was issued, not from when you hear it.
Visibility
| Term | Visibility |
|---|---|
| Good | More than 5 nautical miles |
| Moderate | 2 to 5 nautical miles |
| Poor | 1,000 metres to 2 nautical miles |
| Very poor | Less than 1,000 metres |
Sea state
| Term | Wave height |
|---|---|
| Smooth | Less than 0.5m |
| Slight | 0.5 to 1.25m |
| Moderate | 1.25 to 2.5m |
| Rough | 2.5 to 4m |
| Very rough | 4 to 6m |
| High | 6 to 9m |
| Very high | 9 to 14m |
| Phenomenal | More than 14m |
Pressure systems on the move
How fast a low or high is moving, in the general synopsis:
| Term | Speed |
|---|---|
| Slowly | Less than 15 knots |
| Steadily | 15 to 25 knots |
| Rather quickly | 25 to 35 knots |
| Rapidly | 35 to 45 knots |
| Very rapidly | More than 45 knots |
Pressure tendency
In the reports from coastal stations, the change in pressure over the previous three hours. “Now rising” or “now falling” means the pressure has changed direction at the time of the observation.
| Term | Change |
|---|---|
| Rising (or falling) slowly | 0.1 to 1.5 hPa in the past 3 hours |
| Rising (or falling) | 1.6 to 3.5 hPa in the past 3 hours |
| Rising (or falling) quickly | 3.6 to 6 hPa in the past 3 hours |
| Rising (or falling) very rapidly | More than 6 hPa in the past 3 hours |
The Beaufort scale
| Force | Description | Knots |
|---|---|---|
| 0 | Calm | Less than 1 |
| 1 | Light air | 1 to 3 |
| 2 | Light breeze | 4 to 6 |
| 3 | Gentle breeze | 7 to 10 |
| 4 | Moderate breeze | 11 to 16 |
| 5 | Fresh breeze | 17 to 21 |
| 6 | Strong breeze | 22 to 27 |
| 7 | Near gale | 28 to 33 |
| 8 | Gale | 34 to 40 |
| 9 | Severe gale | 41 to 47 |
| 10 | Storm | 48 to 55 |
| 11 | Violent storm | 56 to 63 |
| 12 | Hurricane force | 64 or more |
Wind shifting clockwise is veering; anticlockwise is backing. For these and other weather terms, see the Yachtmaster glossary.
Where to get it at sea
The shipping forecast is broadcast on BBC Radio 4. Afloat, you'll also get forecasts and warnings from the Coastguard's maritime safety information broadcasts on VHF (announced on channel 16) and from NAVTEX.