18 NC counties and 2 VA counties are under alert, including Johnston, Nash, Halifax, Edgecombe, and Northampton counties. Details
Published: 2007-10-13 11:59:25
Updated: 2007-10-13 11:59:25
Posted October 13, 2007
By Lainie Adams
MIKE MOSS SAYS: Lainie, A tornado "picks things up" because of strong horizontal winds at the surface that dislodge debris from the ground, and then some of that debris can be caught in upward moving air swirling in toward the tornado funnel. So, the tornado has to be on the ground in order to get that process started.
Most tornadoes last only a few minutes, but some of the strongest may remain intact for over an hour. It is unclear what the longest-lasting single tornado has been. There are historical accounts of very long-lived tornadoes that are now thought to have more likely been a series of separate tornadoes produced by especially long-lived supercell thunderstorms.
When the tornado dissipates, any debris being carried in its primary debris cloud (near the lower portion of the funnel) will simply fall back to earth at a speed that depends on the weight and shape of each individual piece, along with the speed of the wind that remains in the wake of the tornado. Some lighter weight debris may be carried much farther downwind if it had been lofted to a height where ambient winds are quite strong prior to the dissipation of the tornado below.