Regular thunderstorms begin when warm air near the ground mix with moist air, which makes an updraft (a wind that goes upward). A severe thunderstorm needs a strong "up"draft and a strong "down"draft. A strong updraft is made when, firstly, the ground is very hot; secondly, when the air is very moist; and thirdly, the air above is very cold. When the updraft gets stronger, so does the thunderstorm.