Back to studiesKnots, km/h and the Beaufort scale
Two agencies can quote the same storm 20 per cent apart. They are not disagreeing; they are measuring different things.
Concept 1 of 3: One wind speed, different units
One wind speed, different units
One concept at a time. Open the full article below for context, limitations and sources.
Read the full articleText, original figures, limitations and sources
Unit conversion is the easy part. The hard part is the averaging period. US agencies quote a one-minute mean; other agencies quote a ten-minute mean. For the same storm the one-minute figure runs roughly 14 per cent higher; the WMO puts the factor at 0.88.
Read the averaging period alongside the number, or two agencies will look like they are disagreeing when they are not. A ten-minute mean of 100 knots can correspond to a one-minute mean of 114 knots for the same storm. Both are correct.
| Hong Kong signal | Sustained km/h | Knots | Beaufort |
|---|---|---|---|
| No. 3 Strong Wind | 41–62 | 22–33 | 6–7 |
| No. 8 Gale or Storm | 63–117 | 34–63 | 8–11 |
| No. 10 Hurricane | 118 and above | 64 and above | 12 |
Height, and what Beaufort really is
Height is the other condition people miss. The international standard is ten metres above open, level ground. A hilltop or a rooftop reads higher by nature, so those numbers cannot be compared with a signal threshold directly.
Beaufort is not a wind speed at all. It describes the state of the sea — wave height, whitecaps, spray — and the wind speeds attached to each force were fitted afterwards. That is why the same force spans slightly different ranges in different tables.
Storm sizes on the map are drawn from one-minute winds: the ten-minute figures issued by the agency are divided by 0.82 — raising them about 22 per cent — before they enter the map, while figures from the US centres are already one-minute and are left alone. That 0.82 is not the pure averaging-period factor of 0.88 quoted above, which would raise them about 14 per cent. It is a calibration value inverted from Hong Kong station peaks across five cases, and it also absorbs the agency’s systematic under-reading of intense western North Pacific storms. Those converted numbers exist only inside the map. Readings and tables use converted and rounded values, with names translated by Aethopia. The intensity classes used on this site follow the agency’s ten-minute scale.