In 1805, Rear Admiral Francis Beaufort devised a thirteen-point scale for estimating wind speed at sea, based on observations of how a well-maintained man-of-war responded to different wind conditions. He never had an anemometer. The scale was about translating observed effects into a consistent number that different people, on different ships, could report in the same way. Two centuries later, you can still find Beaufort numbers in Met Office Shipping Forecast bulletins. The fact that a pre-industrial naval officer's observational system remains in operational use deserves some explanation.
What the scale actually measures
The Beaufort scale is, strictly speaking, a scale of wind effects rather than a scale of wind speed. Beaufort's original descriptions referred to what a Royal Navy frigate could carry in sail at each force. The later equivalents for land — smoke rising vertically at Force 0, twigs in motion at Force 3, difficulty walking at Force 8, structural damage at Force 10 and above — are approximations calibrated to match the marine scale, not independent measurements.
The relationship between Beaufort number and wind speed in metres per second is not linear. Force 12 (hurricane force) begins at around 32.7 metres per second; Force 6 (strong breeze) sits at around 10.8 to 13.8 metres per second. The scale compresses at the top end and expands at the bottom, which turns out to be a reasonable match for how wind damage and human experience of wind actually distribute across speed ranges. Small differences in low wind speeds are perceptible and meaningful. At high wind speeds, the difference between Force 11 and Force 12 is less important than the fact that both are dangerous.
Why pressure drives wind
Wind is air moving from high pressure to low pressure. That much is straightforward. Less obvious is why it does not move directly from the centre of a high to the centre of a low in a straight line — which is what you would expect if pressure gradient were the only force involved.
The complication is the Coriolis effect. Because the Earth is rotating, moving air deflects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is proportional to wind speed and latitude. The result is that, rather than flowing directly across isobars (lines of equal pressure), wind tends to flow roughly parallel to them — circulating clockwise around highs and anticlockwise around lows in the Northern Hemisphere.
This is called geostrophic flow, and it is a reasonable approximation of wind behaviour in the free atmosphere above the boundary layer — roughly above 1,000 metres. Near the surface, friction with the ground modifies this, causing the wind to cross the isobars at an angle of roughly 10 to 30 degrees toward the low-pressure centre.
The boundary layer and why surface wind is gusty
The planetary boundary layer is the lowest portion of the atmosphere where the surface has a direct influence on airflow. It extends from the ground up to roughly 1 to 2 kilometres depending on conditions. Within it, wind is slower on average than the free atmosphere above, because friction — mechanical turbulence generated by the roughness of the surface — removes energy from the flow.
This friction is also responsible for gusts. The boundary layer is not uniformly turbulent. Eddies of different sizes mix faster-moving air from above with slower-moving air near the surface. When a faster eddy reaches the surface, you experience a gust. When it moves on, the wind drops back. The variability between mean wind speed and gust speed depends on the stability of the atmosphere and the roughness of the terrain. Over a rough city, wind is highly variable. Over a flat sea, gusts are less pronounced relative to mean speed because there is less mechanical turbulence.
Orographic effects: wind and terrain
When wind meets a ridge or mountain range, several things can happen. The simplest is deflection: the wind flows around rather than over the barrier. This is common when the hill is steep and the flow is stable. More interesting is the wave effect: air forced up over a ridge can set up a standing wave pattern in the lee — alternating bands of rising and descending air that can extend hundreds of kilometres downwind. These waves are responsible for the large lenticular clouds sometimes seen over hills on otherwise clear days, and they are a significant hazard for aviation.
The foehn effect is another orographic consequence worth noting. When moist air is forced up one side of a mountain range, it cools and condenses, releasing latent heat. As it descends the other side, it warms again — but now as dry air rather than moist, at the dry adiabatic lapse rate rather than the saturated rate. The descending air arrives in the lee valley warmer and drier than when it departed. This is the mechanism responsible for the warm, dry wind in various regional forms across the Alps, the Rockies and the Cairngorms.
Back to Beaufort
What makes Beaufort's scale durable is that it describes physical effects, not instrument readings. At sea, a well-calibrated anemometer is reliable. On a yacht in a squall, a qualitative scale based on what you can observe is more practically useful. The scale also has the considerable advantage of not requiring translation when different observers in different places report the same number. Beaufort 7 means roughly the same thing in the Norwegian Sea and off the Lizard Peninsula, which was exactly the property that Beaufort was trying to create in 1805.
The shipping forecast still uses it partly for this reason: the people who rely on it include sailors who may not have electronics, who need consistent, communicable information about wind conditions. A thirteen-point verbal scale, mapped to observed effects, meets that need in a way that raw numerical wind speed does not always.