Beaufort Scale
Professional Wind Force Calculator, Wind Pressure & WMO Classification
Marine & Sailing
Ideal for docking and mooring. Sailing vessels motionless (engine propulsion only).
Cranes & Construction
All lifting and crane operations permitted without restrictions (up to 12-14 m/s).
UAV & Drone Flight
Perfect stability for all multicopters, including lightweight drones (<250g).
Public Safety
Completely calm and comfortable weather for all outdoor activities.
International Beaufort Wind Scale (WMO Standards)
| Force | Description | Wind Speed | Wave Height | Sea Conditions | Land Conditions |
|---|
Beaufort Wind Scale: Forces 0–12 Table, Wave Dynamics, Wind Pressure, and Online Calculator
A comprehensive professional guide to the International Beaufort Wind Force Scale. Historical origins by Sir Francis Beaufort, mathematical conversion formulas, aerodynamic wind load calculations, visual sea state criteria, and operational safety thresholds across commercial shipping, yachting, heavy crane operations, and drone piloting.
1. What Is the Beaufort Wind Scale and Its History?
The Beaufort Wind Force Scale is an empirical system for classifying wind speeds based on observed conditions at sea and on land. It enables navigators, deck watchkeepers, and meteorologists to determine wind velocity accurately without an anemometer or during electronic sensor failures.
The scale was devised in 1805 by the British hydrographer and Royal Navy Admiral Sir Francis Beaufort. During the age of wooden sail, logbook entries regarding weather conditions were subjective: one captain might describe a wind as a "moderate gale" while another termed it a "fresh breeze." Beaufort introduced an objective benchmark to standardize observations across the fleet.
• 1805: Beaufort's original scale contained no numeric speed values in knots or m/s. It gauged wind force by how much sail a typical British 74-gun ship of the line could carry—ranging from "just sufficient to give steerage way" to "that which no canvas could withstand."
• 1838: The British Admiralty made the Beaufort scale mandatory for all Royal Navy logbooks.
• 1906: Sir George Simpson, Director of the UK Meteorological Office, extended the scale to land by defining observable effects on tree branches, leaves, and roofing.
• 1946: The World Meteorological Organization (WMO) standardized the 13-point scale (Forces 0 to 12), linking each force number to a defined mean wind speed measured at a standard height of 10 meters above flat ground or sea level (WMO Guide to Instruments and Methods of Observation, No. 8).
2. Wind Physics: The Empirical Beaufort Formula and Aerodynamic Wind Load
The correlation between Beaufort force B (from 0 to 12) and mean wind speed v at a standard 10-meter elevation is governed by Simpson's empirical equation, adopted by the WMO:
To compute wind loads exerted against commercial vessel superstructures, deck container stacks, shipboard gantry cranes, and harbor terminals, structural engineers and navigators use the dynamic fluid pressure formula (Eurocode 1 / ISO 4354):
Unit conversion: 1 Pa = 1 N/m² ≈ 0.102 kg/m². Due to the quadratic relationship with velocity, a Force 9 severe gale (22 m/s) exerts ~300 Pa (30 kg/m²) of lateral pressure, while a Force 12 hurricane (35 m/s) exceeds 750 Pa (76 kg/m²).
3. Complete Beaufort Scale Reference Table (WMO Standards)
The official classification establishes wind velocity thresholds, average open-sea wave heights, and distinct visual observations on water and land:
| Force | Designation | m/s | Knots | km/h | Wave (m) | Visual Markers at Sea | Visual Markers on Land |
|---|---|---|---|---|---|---|---|
| 0 | Calm | 0.0 – 0.2 | < 1 | < 1 | 0 | Sea surface smooth and mirror-like. | Smoke rises vertically. Tree leaves remain stationary. |
| 1 | Light air | 0.3 – 1.5 | 1 – 3 | 1 – 5 | 0.1 | Scaly ripples form without foam crests. | Smoke drift indicates wind direction; wind vanes stay still. |
| 2 | Light breeze | 1.6 – 3.3 | 4 – 6 | 6 – 11 | 0.2 – 0.3 | Small wavelets with glassy crests that do not break. | Wind felt on face; leaves rustle; ordinary wind vanes move. |
| 3 | Gentle breeze | 3.4 – 5.4 | 7 – 10 | 12 – 19 | 0.6 – 1.0 | Large wavelets; crests break; scattered whitecaps (white horses). | Leaves and small twigs in constant motion; light flags unfurl. |
| 4 | Moderate breeze | 5.5 – 7.9 | 11 – 16 | 20 – 28 | 1.0 – 1.5 | Small waves becoming longer; fairly frequent white horses. | Raises dust and loose paper; small tree branches move. |
| 5 | Fresh breeze | 8.0 – 10.7 | 17 – 21 | 29 – 38 | 2.0 – 2.5 | Moderate waves of longer form; many white horses; sea spray. | Small trees in leaf sway; crested wavelets form on inland waters. |
| 6 | Strong breeze | 10.8 – 13.8 | 22 – 27 | 39 – 49 | 3.0 – 4.0 | Large waves begin to form; extensive white foam crests; spray. | Large branches in motion; whistling in telephone wires; umbrellas difficult. |
| 7 | Near gale | 13.9 – 17.1 | 28 – 33 | 50 – 61 | 4.0 – 5.5 | Sea heaps up; white foam blown in streaks along the wind direction. | Whole trees in motion; walking against the wind requires effort. |
| 8 | Gale | 17.2 – 20.7 | 34 – 40 | 62 – 74 | 5.5 – 7.5 | Moderately high waves of greater length; crests break into spindrift. | Twigs and small branches break off trees; walking impeded. |
| 9 | Severe gale | 20.8 – 24.4 | 41 – 47 | 75 – 88 | 7.0 – 10.0 | High waves; dense streaks of foam along wind; spray reduces visibility. | Slight structural damage occurs (chimney pots and roof slates removed). |
| 10 | Storm | 24.5 – 28.4 | 48 – 55 | 89 – 102 | 9.0 – 12.5 | Very high waves with overhanging crests; sea surface white with foam. | Trees uprooted; considerable structural damage to buildings. |
| 11 | Violent storm | 28.5 – 32.6 | 56 – 63 | 103 – 117 | 11.5 – 16.0 | Exceptionally high waves; small/medium ships lost to view behind seas. | Widespread infrastructure damage; trees snapped; power lines downed. |
| 12 | Hurricane | ≥ 32.7 | ≥ 64 | ≥ 118 | ≥ 14.0 | Air filled with foam and driving spray; sea completely white; visibility zero. | Devastating catastrophic damage to buildings and structures. |
4. Operational Safety Thresholds Across Marine and Industrial Sectors
Understanding wind force grades governs critical operations at sea, in port terminals, on construction sites, and in aerial drone missions:
• Forces 0–4 (0–8 m/s): Optimal conditions for vessel berthing, bunkering operations, and overboard hull painting;
• Forces 5–6 (8–14 m/s): Small craft advisories issued; yachts reef sails. On commercial ships, working aloft and over the side is prohibited, and deck cargo lashings are re-checked;
• Forces 7–8 (14–21 m/s): Formal Gale Warnings issued. Harbors suspend pilot transfers, small coastal craft seek shelter, and ocean vessels alter heading to heavy weather survival courses;
• Logbook Entries: Watchkeeping officers log Beaufort numbers alongside true wind direction. Compute real-time apparent-to-true wind vectors using our True Wind Calculator.
Under international safety and labor codes (e.g., ISO 4302 and OSHA crane standards):
• 10–12 m/s (Force 6): Suspended cradle operations and lifting large-surface containers or panels are halted;
• 14–15 m/s (Force 7): CRANE STOP! Tower cranes, container gantry cranes, and vessel cargo gear must cease lifting. Booms are parked in storm positions, rail clamps locked, and wind-vaning enabled. Review personnel lifting protocols in Transfer of Personnel by Crane Between Vessels.
• Up to 5–7 m/s (Forces 3–4): Smooth flight for ultralight sub-250g drones (e.g., DJI Mini);
• 8–10 m/s (Force 5): Operating ceiling for mid-size consumer drones. Heavy battery drain occurs as GPS flight controllers fight wind drift;
• Above 12 m/s (Forces 6+): Strict no-fly conditions for commercial survey and mapping drones due to risk of flyaways and motor stall.
5. The Extended Beaufort Scale (Forces 13–17) and Tropical Cyclones
While standard WMO marine broadcasts cap the scale at Force 12 (≥ 32.7 m/s or 64 knots), the US National Weather Service and meteorological agencies in China and Taiwan historically formulated the Extended Beaufort Scale to classify severe tropical cyclones:
- Force 13: 37.0 – 41.4 m/s (72 – 80 knots);
- Force 14: 41.5 – 46.1 m/s (81 – 89 knots);
- Force 15: 46.2 – 50.9 m/s (90 – 99 knots);
- Force 16: 51.0 – 56.0 m/s (100 – 108 knots);
- Force 17: 56.1 – 61.2 m/s (109 – 118 knots).
In contemporary oceanic forecasting, once sustained wind speeds cross 33 m/s (64 kn / Force 12), tropical storms are categorized under the Saffir-Simpson Hurricane Wind Scale—from Category 1 (33–42 m/s) to Category 5 (exceeding 70 m/s or 137 knots).
6. Advantages and Operational Limitations of the Beaufort Scale
| Key Operational Advantages | Sources of Observational Error |
|---|---|
| Complete Autonomy: Requires no electrical power, mechanical sensors, or sensor calibration. Reliable during shipboard blackouts. | Sea State Lag Effect: Wave heights lag several hours behind rapid shifts in wind velocity as wind-driven sea builds up or decays. |
| Global Standard: Provides a shared meteorological vocabulary across merchant fleets of all flags. | Fetch and Sheltered Waters: In enclosed fjords or lee shores, wave heights remain small even during severe gale-force winds. |
| Integrative Metric: Observes actual atmospheric resistance against tangible physical objects rather than point-source readings. | Subjectivity: Heavily reliant on observer experience, tidal current interactions, water depth, and nighttime visibility. |