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
Storm warning is used to indicate a storm warning (the use of two such flags denotes a hurricane force wind warning or a hurricane warning). The same flag as a storm warning is used to indicate a tropical storm warning. On land, the National Weather Service issues a 'high wind warning' (Specific Area Message Encoding code: HWW) for storm-force winds, which also encompasses the lesser gale-force and greater hurricane force winds. In most cases, the warning applies to winds of 40-114 MPH for at least 1 hour; or any gusts of 58-114 miles per hour on land unless a tropical storm warning,