How the Oklahoma tornado was formed
21 May 2013 Last updated at 11:37 BST
At least 91 people, including 20 children, are feared killed by a huge tornado which tore through Oklahoma City suburbs, officials in the US state say.
Worst hit was Moore, south of the city, where neighbourhoods were flattened and schools destroyed by winds of up to 200mph (320km/h).
BBC weather presenter Ben Rich explains how the tornado was formed.