TODAY   |  May 21, 2013

How ‘perfect storm’ of conditions spawns tornadoes

Tornadoes like the one that flattened Moore, Okla., on Monday typically only take minutes to leave miles of devastation. TODAY’s Al Roker explains the weather conditions that combine to form tornadoes.

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This content comes from Closed Captioning that was broadcast along with this program.

>> matt we sometimes take these storms for granted but it's a complex system that develops when you find out how a tornado works. we thought you might be interested to know this is how they form. the tornado that flattened this oklahoma suburb estimated to have been about two miles wide and reportedly on the ground for about 40 minutes. tornadoes like this one typically last only minutes. generally leaving miles of devastation in their wake. a tornado begins as a strong thunderstorm in order for it to become a tornado its winds at different altitudes need to increase and change direction. this is called wind shear , which creates a horizontal tunnel of air. if that air gets caught up in the upward energy of the storm it picks up speed and strengthens and the energy of the air goes from horizontal to vertical, causing a funnel cloud . when the rain from the thunderstorm hits the ground a tornado is formed. the national weather service classifies this tornado as an ef4, the second most severe classification on a scale of zero to five although that rating could be increased to an ef5 once more damage assessment teams come out to look at the damage. and matt, the death toll in this, if it stands, it will be the second deadliest single day tornado , single tornado since 1953 .

>> you just said that teams will come out here and assess the damage to decide whether it was an ef4 or ef5. how exactly do they do that?

>> they look at the amount of damage, they look at the damage field, the debris fields, they look at the amount of the scope of it and look at the rotation, like the wrapping of that girder around that telephone pole , and that's how they'll assess it.