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
Great Lakes Storm of 1913 indicating a storm warning with northwesterly winds. By late afternoon, the storm signal flags were replaced with a vertical sequence of red, white, and red lanterns, indicating that a hurricane with winds over 74 mph (119 km/h) was coming. (The word "hurricane" here does not refer to a tropical cyclone, but to Force 12 winds on the Beaufort scale.) The winds on Lake Superior had already reached 50 mph (80 km/h), and an accompanying blizzard was moving toward Lake Huron. By Saturday, the storm's status had been upgraded to "severe". The storm was centered over eastern Lake Superior, covering the