10.3 Beaufort Wind Scale & Maritime Forecast Interpretation
Key Takeaways
The Beaufort Wind Scale standardizes surface wind measured at 10 metres elevation: Force 6 (22–27 knots) is a Strong Breeze, the threshold for UK strong-coastal-wind information, while Force 8 (34–40 knots) represents a Gale with spindrift foam streaks.
In official UK Met Office Shipping Forecasts, gale imminence is strictly defined: 'Imminent' means expected within 6 hours of issue, 'Soon' means between 6 and 12 hours, and 'Later' means beyond 12 hours.
Official Shipping Forecast visibility categories are strictly standardized: 'Good' (> 5 NM), 'Moderate' (2 to 5 NM), 'Poor' (1,000 m to 2 NM), and 'Very poor' (<1,000 m /0.54 NM); fog is a weather description.
Wind direction in forecasts always indicates the true direction the wind is blowing FROM; 'veering' signifies a clockwise shift, whereas 'backing' indicates an anti-clockwise shift.
Pressure falls of 3.6–6.0 hPa in 3 hours are described as falling quickly, and falls over 6.0 hPa as very rapidly. Check forecasts and warnings; neither category alone establishes gale force or arrival time.
10.3 Beaufort Wind Scale & Maritime Forecast Interpretation
Note
Converting weather forecasts, barometric tendencies, and visual sea states into sound operational seamanship decisions is fundamental to coastal navigation. Understanding official UK Met Office Shipping Forecast terminology, observing the Beaufort Scale, and interpreting the 3-hour barometric tendency provide vital advance warning of developing gales.
The Beaufort Wind Scale
Formulated in 1805 by Admiral Sir Francis Beaufort, the Beaufort Wind Scale provides an empirical system relating wind velocity to observable sea state.
Calibration & Practical Reference (Forces 4 to 9)
The Beaufort Scale standardizes wind speed measured at 10 metres above sea level over open flat water. While Forces 0 to 3 describe light conditions (up to 10 kn), Forces 4 through 9 govern passage making and heavy weather management:
| Force | Description | Wind Speed (knots) | Mean | Wave Height | Sea State Criteria | Small Craft Sail Plan & Action |
|---|---|---|---|---|---|---|
| 4 | Moderate breeze | 11–16 | 13 kn | 1.0 m (3.5 ft) | Small waves becoming longer; frequent white horses. | Assess sail area for the boat, gusts and sea state. |
| 5 | Fresh breeze | 17–21 | 19 kn | 2.0 m (6.5 ft) | Moderate waves of long form; many white horses; spray. | Reef early as needed for the boat and conditions. |
| 6 | Strong breeze | 22–27 | 24 kn | 3.0 m (9.5 ft) | Large waves form; white foam crests; blown spray. | UK strong-coastal-wind threshold; reef for the boat and conditions; use harnesses. |
| 7 | Near gale | 28–33 | 30 kn | 4.0 m (13.5 ft) | Sea heaps up; foam blown in streaks along wind; spindrift. | Prepare reduced or storm sails and assess safe shelter. |
| 8 | Gale | 34–40 | 37 kn | 5.5 m (18 ft) | High waves; crests break into spindrift; foam streaks. | Check Gale Warnings; prepare suitable storm sails and tactics. |
| 9 | Strong gale | 41–47 | 44 kn | 7.0 m (23 ft) | High waves; dense foam streaks; crests topple and roll over. | Severe broaching danger; use tactics suited to the yacht and sea room. |
Wave heights above refer to well-developed wind waves in the open sea; actual seas depend on fetch, duration, tide and depth.
Force 10 is Storm (48–55 kn), Force 11 Violent Storm (56–63 kn), and Force 12 Hurricane Force (64 kn or more). These represent extreme survival conditions.
Maritime Forecast Sources
Coastal Skippers rely on multiple official broadcast channels:
1. The UK Met Office Shipping Forecast
The Met Office forecast covers 31 sea areas. BBC Radio 4 carries it on FM and digital services, normally in two early daily broadcasts with an additional weekend evening bulletin. BBC schedules use UK local time; check the current schedule. The forecast begins with gale warnings, then the general synopsis and sea-area wind, weather and visibility; fuller bulletins add coastal reports and inshore forecasts.
2. Inshore Waters Forecast
Covers UK coastal waters out to 12 nautical miles offshore plus estuaries. Issued four times daily for 24 hours with a following 24-hour outlook, detailing wind direction/force, weather, visibility, and sea state.
3. Navtex (Navigational Telex)
Automated direct-printing radio telex within GMDSS on 518 kHz (International English) and 490 kHz (National/inshore), receiving scheduled station transmissions and urgent warnings. Select the stations and message categories appropriate to the passage.
4. VHF Marine Coastguard MSI
Announced on VHF Channel 16, then broadcast on 62,63 or 64, with Channel 10 used at some sites. Short MSI broadcasts recur every 3 hours and full broadcasts every 12 hours; gale, storm and navigation warnings are also broadcast on receipt where appropriate. Use the current MCA site schedule.
5. Numerical GRIB Files
Gridded Binary files provide wind/wave model predictions (GFS, ECMWF) for chartplotters. GRIBs must be validated against official synoptic analyses.
Official Shipping Forecast Terminology
The Met Office employs strict, standardized vocabulary:
Gale Classifications
- Gale: Mean winds of Force 8 (34–40 kn) or gusts to 43–51 kn.
- Severe Gale: Mean winds of Force 9 (41–47 kn) or gusts to 52–60 kn.
- Storm: Mean winds of Force 10 (48–55 kn) or gusts to 61–68 kn.
- Violent Storm: Mean winds of Force 11 (56–63 kn) or gusts .
- Hurricane Force: Mean winds of Force 12 ().
Timing (Imminence)
- Imminent: Expected within 6 hours of issue.
- Soon: Expected between 6 and 12 hours of issue.
- Later: Expected more than 12 hours after issue.
Wind Direction & Changes
- Wind Direction: True direction from which the wind blows.
- Veering: Wind direction turning clockwise (e.g. SW to W).
- Backing: Wind direction turning anticlockwise (e.g. SW to S).
Visibility Definitions
- Good: Visibility greater than 5 nautical miles.
- Moderate: Visibility between 2 and 5 nautical miles.
- Poor: Visibility between 1,000 metres and 2 nautical miles.
- Very Poor: Visibility less than 1,000 metres (0.54 NM). Fog is suspended water droplets causing visibility below 1,000 m.
Movement of Pressure Systems
- Slowly: Moving at less than 15 knots.
- Steadily: Moving at 15 to 25 knots.
- Rather Quickly: Moving at 25 to 35 knots.
- Rapidly: Moving at 35 to 45 knots.
- Very Rapidly: Moving at greater than 45 knots.
Barometric Pressure Warning Signs
The marine aneroid barometer provides real-time verification of synoptic trends:
The 3-Hour Tendency Rule
Log pressure and compare its three-hour change with Met Office terminology:
- Falling Slowly:0.1–1.5 hPa.
- Falling:1.6–3.5 hPa.
- Falling Quickly:3.6–6.0 hPa.
- Falling Very Rapidly:More than 6.0 hPa.
Warning
A quick or very rapid fall warrants prompt weather and passage reassessment. Check current forecasts, gale warnings, wind, sea state and available shelter. No local pressure-fall threshold automatically declares a gale or determines when it will arrive.
Reading Weather Satellite Images
Use a sequence, with its observation times, to follow cloud bands and developing systems towards the route; compare their movement with the synoptic chart and forecast. Visible imagery relies on sunlight. Thermal infrared imagery works by day and night; on the usual greyscale display, colder high cloud appears brighter than warmer low cloud or the surface. Check the product legend.
Cloud pictures do not directly establish visibility at sea level. High cloud can hide lower cloud, and low cloud may be difficult to distinguish from fog. Use the marine forecast, coastal observations and conditions aboard to judge the navigation risk.
Sources: Met Office satellite pictures, cloud-observation limitations.
Shipping Forecast Glossary
| Term | Category | Official Definition | Tactical Skipper Action |
|---|---|---|---|
| Imminent | Timing | Expected within 6 hours of issue | Immediate port refuge or heavy weather prep. |
| Soon | Timing | Expected between 6 and 12 hours | Evaluate remaining passage time vs refuge distance. |
| Later | Timing | Expected after 12 hours | Plan safe passage stopover before onset. |
| Veering | Direction | Shifting clockwise | Anticipate tack change and shifting sea state. |
| Backing | Direction | Shifting anticlockwise | Compare the shift with the forecast and synoptic situation. |
| Poor | Visibility | 1,000 m to 2 NM | Activate radar, post bow lookout, prepare foghorns. |
| Rapidly | System Speed | Moving at 35 to 45 knots | Expect fast front transit and rapid barometric changes. |
The UK Met Office Shipping Forecast broadcasts: 'Dover, Wight, Portland: Gale warning in force. South-westerly severe gale Force 9 imminent, veering north-westerly Force 7 soon.' What do the terms 'imminent' and 'soon' officially signify in maritime forecasts?
'Imminent' means within 12 hours of issue; 'soon' means within 24 hours of issue
'Imminent' means expected within 6 hours of time of issue; 'soon' means expected between 6 and 12 hours of time of issue
'Imminent' means already occurring at coastal stations; 'soon' means expected within 1 hour
'Imminent' means within 3 hours; 'soon' means between 3 and 6 hours
A skipper logging hourly barometer readings on a passage toward the Isles of Scilly observes atmospheric pressure drop from 1014 hPa at 0800 to 1009 hPa at 1100. How should the skipper interpret this 5 hPa barometric fall in 3 hours?
As normal diurnal barometric oscillation typical of summer anticyclones, requiring no adjustment to sail plan
As a sign of a passing thermal col that will bring calm winds and clearing skies within the hour
As falling quickly in Met Office terminology: check the latest forecast and warnings and reassess the passage, without inferring a gale from pressure alone
As an indication that a stationary high-pressure ridge is firmly establishing itself over the vessel
According to the Beaufort Wind Scale, which of the following sea state and wind conditions corresponds to Beaufort Force 6 (Strong Breeze)?
Wind 11–16 knots; small waves becoming longer; frequent white horses; ideal cruising conditions
Wind 34–40 knots; moderately high waves; edges of crests break into spindrift; foam blown in well-marked streaks
Wind 22–27 knots; large waves beginning to form; extensive white foam crests everywhere with some blown spray; UK strong-coastal-wind threshold
Wind 41–47 knots; high waves with dense foam streaks; sea begins to roll; spray severely affects visibility
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