Here, warm air __1____ from the Gulf of Mexico in the spring and summer and _2___ cold air_3____ from Canada .The meeting produces violent thunderstorms called _4___ . Scientists don't completely understand how or when tornados form, but they do know asupercell like this one can produce a twister if the conditions are right. As more moist air _5____ a storm, it gets _6___and _7____ by upper level winds. As this _8___ column of air gathers force, conditions are right for a _9____ below. When rain cooled _10____hit warm air near the ground, a low hanging revolving cloud forms beneath thecell. A tornado is imminent.