The Beaufort scale

The Beaufort scale was devised in 1805 by Sir Francis Beaufort, a Royal Navy officer, to standardise wind observations at sea. It originally correlated wind strength with the amount of sail a frigate could carry. Each force from 0 to 12 corresponds to a range of wind speeds and distinct sea-state characteristics, such as wave height and foam coverage.

The scale was formalised in 1923 by the International Meteorological Committee, aligning each force with specific speed ranges. Modern definitions are maintained by the World Meteorological Organization (WMO) and are based on 10-minute mean wind speeds at 10 metres above sea level. The sea-state descriptions remain central, particularly for forces 3 to 9, where wave development is progressive.

In Ireland, where maritime exposure is high, Met Éireann uses Beaufort forces in coastal and marine forecasts. Observations from lighthouses and buoys, such as those at M2 and M4, are often interpreted using Beaufort criteria when instruments fail or require cross-verification.

The scale’s observational foundation makes it resilient in low-infrastructure environments. However, it was not designed for gusts or turbulence, which limits its direct use in modern operational decisions requiring peak wind thresholds.

Each Beaufort force spans a defined wind speed range, increasing non-linearly. The scale is open-ended at Force 12, which begins at 118 km/h (64 kt) and includes all stronger winds. Speeds are defined for 10 m height, 10-minute average, over open water.

The progression reflects the logarithmic nature of wind energy. For example, Force 6 has approximately four times the kinetic energy of Force 3. The approximate formula V ≈ 0.836 × B^(3/2) m/s (where B is Beaufort number) fits observed medians from ERA5 reanalysis over the northeast Atlantic.

Worked example: For Force 7, B = 7. Then V = 0.836 × 7^(1.5) = 0.836 × 18.52 ≈ 15.5 m/s, or 55.8 km/h. This falls within the defined 50–61 km/h band, confirming the formula’s utility for interpolation.

On land, exact correspondence degrades due to surface roughness. Urban or forested areas reduce wind speed at 10 m, meaning observed effects may lag one force behind open-water conditions.

Beaufort forces are defined by observable phenomena. At sea, Force 4 (13–18 km/h) produces ‘moderate waves’ with scattered whitecaps. By Force 7 (50–61 km/h), ‘high waves’ with dense foam streaks and spray reduce visibility. Force 10 (89–102 km/h) brings ‘very high waves’ with rolling seas and widespread foam patches.

On land, effects become diagnostic above Force 5. Force 6 (39–49 km/h) causes whole trees to sway and impedes walking. Force 8 (62–74 km/h) breaks twigs, makes umbrellas unusable, and produces a roaring sound in overhead wires. Force 10 uproots shallow trees and damages roofs—effects documented during Ophelia (16 Oct 2017) in counties Cork and Waterford.

However, land-based interpretation is subjective. A ‘near gale’ (Force 7) in an exposed coastal dune system may match a ‘strong gale’ (Force 9) in turbulence behind a cliff line. Urban canyons amplify gusts, creating local effects exceeding the ambient force.

Marine users rely on wave period and direction consistency as secondary cues. A prolonged Force 6 from a single direction generates larger swell than a gusty, shifting wind of the same mean speed. This is critical for small craft safety and harbour entry decisions.

The Wind Agent’s Agreement Spine helps distinguish modelled Beaufort force from observed conditions by overlaying multiple ensemble members and station data, reducing misclassification in complex terrain.

The Beaufort scale increases approximately with the 3/2 power of the force number, reflecting the physics of wind energy, which scales with the square of speed. A doubling of wind speed quadruples kinetic energy, so linear force increments would misrepresent impact.

The formula V = 0.836 × B^(3/2) m/s approximates the median speed per force. For B = 4: V = 0.836 × 8 = 6.7 m/s (24 km/h), within the 20–28 km/h band. For B = 10: V = 0.836 × 31.62 ≈ 26.4 m/s (95 km/h), near the 89–102 km/h range.

This non-linearity means each increment above Force 6 brings disproportionately greater hazard. Force 9 (75–88 km/h) has ~1.5× the speed of Force 6 but ~2.25× the energy. This explains why structural damage increases sharply above gale force.

In Ireland, ERA5 data shows the 90th percentile 10 m wind in winter is approximately Beaufort 6 (39–49 km/h) along the west coast. The 98th percentile reaches Force 9, occurring on average 15–20 days per year in exposed areas like Malin Head.

The logarithmic structure also aligns with human perception. Wind noise and vibration increase non-linearly, making higher forces subjectively more intense. This perceptual scaling aids field estimation when instruments are unavailable.

Met Éireann issues weather warnings using colour codes (yellow, orange, red) that often reference Beaufort forces, particularly for wind. A yellow wind warning typically begins at Beaufort 7 (50–61 km/h mean), orange at Force 8–9, and red at Force 10 or higher.

For example, during Storm Eunice (18 Feb 2022), red warnings cited mean winds of Force 10 to 11, with gusts exceeding 130 km/h (Force 12 equivalent). These were verified by Mace Head and Valentia Observatory measurements.

However, warnings are not solely based on Beaufort. Duration, direction, antecedent conditions (e.g. saturated ground), and impact thresholds are integrated. A prolonged Force 8 from the southwest may trigger a higher warning than a brief Force 9 from the northwest if it affects more infrastructure.

The exceedance fan in The Wind Agent visualises the probability of surpassing user-defined thresholds, which can be set in mph or m/s. This allows alignment with Beaufort bands—e.g., setting a limit at 62 km/h (17.2 m/s) to monitor for gale-force onset.

Marine broadcasts use Beaufort directly. A ‘southwest 6 to 7’ forecast implies mean winds of 39–61 km/h, with waves building to 3–4 metres. Fishermen and leisure craft operators use this to decide whether to launch or return to port.

The Beaufort scale is defined for 10-minute mean winds, not gusts. However, operational decisions often depend on peak gusts. A gust factor—ratio of gust to mean speed—allows conversion.

Commonly cited by instructors, a gust factor of 1.5 applies in open terrain. Thus, a Beaufort 8 mean (62–74 km/h) implies gusts of 93–111 km/h. In rough or urban terrain, gust factors may reach 2.0 due to turbulence.

For example, a mean wind of 55 km/h (Beaufort 7) with a 1.5 gust factor gives 82.5 km/h gusts. If your crane operation limit is 80 km/h, this exceeds the threshold despite the mean being below gale force.

The Shear Glass in The Wind Agent displays gusts at multiple heights, enabling direct comparison with Beaufort-based forecasts. At 80 m, gusts may be 20–30% higher than at 10 m due to reduced surface drag.

Ensemble models provide gust distributions. The exceedance curve shows the probability of exceeding a gust threshold, such as 80 km/h, which corresponds roughly to the upper end of Beaufort 8. This is more precise than relying on mean-based Beaufort alone.

Note: gust duration is typically 3 seconds. Shorter peaks may not trigger mechanical responses, but sustained gusts over 10 seconds increase risk.

The Beaufort timeline chart models the forecast evolution of wind force at a specific location. It displays the ensemble median as a line, with shaded bands indicating the spread of possible forces.

Each hour, the model outputs a 10 m 10-minute mean wind, converted to the nearest Beaufort force. Users can overlay their operational threshold—e.g., Force 7—as a reference line. The chart updates every 6 hours with new model runs.

For instance, at a west Cork coastal site, the timeline may show Force 5 rising to Force 8 over 6 hours. The ensemble spread (via the fan) reveals uncertainty: if half the members exceed Force 7, the exceedance probability is 50%, informing go/no-go decisions.

The chart integrates with the Agreement Spine, which shows how closely models agree on the timing of force changes. High agreement increases confidence in the timeline.

During the approach of Storm Éowyn (24 Jan 2025), the Beaufort timeline for Galway Bay showed Force 10 sustained for 4 hours, with 80% of ensemble members agreeing. This allowed ferry operators to pre-emptively cancel services.

The timeline is not a legal standard. Your operational documents govern. But it provides a visual, intuitive layer over numerical data, bridging traditional observation and modern forecasting.