Beaufort Follies

I cannot take credit for this article, it was sent to me by a friend two days ago and I am amazed that I did not realise what he explained in his article, with my qualifications and experience this is something that I should have known about. I welcome any comments on it.

Martin.

I was curious about the way our excellent Kiss wind generator works. At wind speeds less than 15 knots it generates barely one or two amps, at twenty knots it generates a steady five to ten amps, at twenty three knots it generates fifteen amps or more, and at wind speeds much above that it starts to overload and freewheel. A good friend explained that the wind exerts its pressure on an area, so its power does not increase linearly with wind speed but in proportion to the square of the wind speed.
This set me thinking about how our passion for quantification can lead to terrible over-simplification with catastrophic consequences. The Beaufort wind scale provides an example.

Beaufort Force Knots Description

1 .................. 1 – 3 knots Light Air
2 ................. 4 – 6 knots Light Breeze
3 ..................7 – 10 knots Gentle Breeze
4 ................11 – 16 knots Moderate Breeze
5 ................17 – 21 knots Fresh Breeze
6 ................22 – 27 knots Strong Breeze
7 ................28 – 33 knots Near Gale
8 ................34 – 40 knots Gale
9 ................41 – 47 knots Strong Gale
10 ..............48 – 55 knots Storm
11 ..............56 – 63 knots Violent Storm
12 ..............64 or more kts Hurricane

This classification system, though it is in use throughout the nautical world, suggests that the strength of the weather increases more or less linearly with wind speed. But wind presses on an area and its strength increase with the square of its speed. Thus a twenty knot wind is not twice as powerful as a ten knot wind but four times as strong, forty knot winds are not four but sixteen times as strong, and eighty knot winds not eight but sixty-four times more powerful than ten knot winds. Such misperceptions caught out some of the experienced sailors in the 1998 Sydney to Hobart race. Five yachts were lost, fifty-five people were rescued and six people died. The inquest (according to the book “Fatal Storm: The Inside Story of the Tragic Sydney-Hobart Race” by Rob Mundle) found that underestimating the difference in severity of a gale and a storm was a significant factor in the tragedy. Judging by Beaufort numbers alone, the forecast storm (Force 10) would seem to be only 25 per cent more powerful than a gale (Force 8). If judged by wind speed in knots, the storm (48 to 55 knots) would be only 40 per cent more powerful than a gale (34 to 40 knots). But by the correct measure – the square of the wind speed – the storm was twice as violent as a gale.