Cumulonimbus and aviation at this altitude, updraughts with speeds that can exceed . Such an updraught speed corresponds to the wind speed of a small hurricane. The speed can even exceed . The maximum number in the Beaufort scale is 12 ("hurricane force" wind) and is assigned to wind speeds of 64 knots or greater. If the Beaufort scale were extended, these updraughts would have a Beaufort number of 14 "in the vertical direction". The turbulence is then "extreme" at this altitude. Moreover, the diameters of the updraught columns vary between 2 km (air mass thunderstorm) and 10 km (supercell thunderstorm). The height
Burns' Day Storm the Netherlands and Belgium. The strongest sustained winds recorded were between 70 and 75 mph (110–120 km/h), comparable to a weak Category 1 hurricane or Hurricane-force 12 on the Beaufort Scale. Strong gusts of up to 104 mph (170 km/h;) were reported, and it was these which caused the most extensive damage. The Burns' Day Storm of 1990 has been given as an example of when the Met Office "got the prediction right". The model forecast hinged on observations from two ships in the Atlantic near the developing storm the day before it reached the UK. During the day of