Beaufort is a scale of effects
The Beaufort scale is usually presented as a set of speed bands, which gets it backwards. It is a scale of observations, and the speeds came later.
Francis Beaufort set it out in 1805 for the Royal Navy in terms of how much sail a ship of the line could carry — from “just sufficient to give steerage” up to “that which no canvas could withstand”. It was restated in the nineteenth century in terms of what the sea does, and later given land equivalents. The equivalent wind speeds were fitted to it in 1946, well over a century after the scale was in use, by the relation v = 0.836 × B1.5 metres per second.
Which means the descriptions are the definition and the numbers are the approximation. “Whole trees in motion; inconvenient to walk against” is what force 7 is; 32 to 38 mph is a later estimate of what usually produces that.
That is not a historical curiosity. It is why the scale remains useful without an anemometer, why it works at sea where the observable state of the water integrates the wind over time far better than an instantaneous reading, and why a shipping forecast still says “gale force 8” rather than a number in knots.
Double the force, eight times the load
Two power laws stack here, and the result is worth carrying around.
Dynamic pressure goes as the square of wind speed: q = ½ρv². So doubling the wind quadruples the push. And Beaufort speed goes as force to the power one and a half. Put those together and pressure goes as the cube of the Beaufort number — exactly.
What doubling the Beaufort number does| From → to | Speed | Pressure |
|---|
| Force 3 → 6 | ×2.83 | ×8 |
|---|
| Force 4 → 8 | ×2.83 | ×8 |
|---|
| Force 5 → 10 | ×2.83 | ×8 |
|---|
| Force 6 → 12 | ×2.83 | ×8 |
|---|
Eight, every time, because the ratio only depends on the two force numbers. A “moderate breeze” at force 4 and a “gale” at force 8 sound like neighbours on a thirteen-point scale; one pushes eight times as hard as the other.
In absolute terms: at 20 m/s — force 8 — the pressure is about 245 pascals, or 25 kilograms of force on every square metre facing the wind. At 40 m/s it is nearly 100 kilograms per square metre. This is why a modest-sounding increase in a forecast matters so much for anything with a large flat surface: a fence, a trampoline, a marquee, a lorry side.
Over what period, at what height?
A wind speed on its own is incomplete, because wind is not steady. Any figure is an average over some interval, measured at some height, and both are usually left unstated.
The height convention is settled: 10 metres above open ground is the standard for a surface wind. Wind slows near the ground through friction, so wind at roof height is measurably less than the 10-metre figure, and wind in a built-up area less again for the same synoptic conditions.
The averaging period is not settled, and this is where headline numbers go wrong. The United States uses a 1-minute sustained wind for tropical cyclones; most of the rest of the world, following WMO practice, uses 10 minutes. The longer average smooths out more of the peaks, so it reads lower — by about 12% for the same storm.
A cyclone reported at 150 mph by an American agency and 132 mph by another is very often the same storm measured two ways, not a disagreement about its strength. Comparing storm intensities across basins without checking the averaging period is comparing two different quantities.
Gusts are not the same wind
A gust is a brief peak, conventionally the highest three-second average within the observing period. It is a different quantity again from either sustained figure, and typically 1.3 to 1.5 times the 10-minute mean over open terrain — more over rough ground, which breaks the flow up more.
This matters because damage is usually done by gusts rather than by the mean. A forecast of “40 mph with gusts to 60” is describing a wind whose peak pressure is more than twice its average — 60 against 40 is a factor of 1.5 in speed and 2.25 in force.
Which also explains a common confusion about warnings. A named storm reported as a “category 1” on sustained wind can produce gusts well into the next category, and the damage reports afterwards describe the gusts. The two numbers are both correct and they are not the same measurement.
For anything practical — securing property, deciding whether to travel — the gust figure is the one that matters, because it is the peak load that decides whether something moves.
Why knots, of all things
Marine and aviation forecasts use knots, which looks like stubbornness and is not. A knot is one nautical mile per hour, and a nautical mile is one minute of latitude — 1,852 metres exactly, by definition since 1929.
That makes navigation arithmetic trivial. A vessel making 10 knots for six hours has covered 60 nautical miles, which is one degree of latitude — a distance you can read straight off a chart’s vertical scale without converting anything. No other unit has that property, which is why the ones that measure position on a globe kept it.
The name is literal. Speed was measured by paying out a line with knots tied at regular intervals behind a floating log and counting how many ran out in a timed interval — hence both “knots” and the ship’s “log”.
A useful approximation for reading forecasts: knots are roughly mph minus 13%, or km/h divided by 1.85. Ten knots is about 11.5 mph or 18.5 km/h. And the Beaufort scale is defined against knots rather than any other unit, which is why its bands look tidier there than in mph.
Sources and methodology
The unit conversions are exact definitions — a statute mile is 1609.344 metres and a nautical mile exactly 1852. Everything interesting here is convention rather than arithmetic: what the Beaufort scale actually defines, at what height and over what period a surface wind is measured, and the factor different agencies use to compare storms they measure differently. Those are the points where a confident number can be describing something other than what the reader thinks.