Tornadoes are usually produced from thunderstorms. Most tornadoes have wind speeds less than 180 kilometers per hour and are less than 100 meters in diameter. Gale force winds are usually gusty, while some tornadoes can last for longer than one hour. Professor Meng Zhiyong, from the Department of Atmospheric and Oceanic Sciences at Peking University, explains the causes of tornadoes and gives advice on what to do during this weather event.