It's getting to be that time of year when tornadoes become more frequent. The ingredients for making twisters typically start to brew in late February over the Gulf States, as building warmth and moisture surges northward, battling against relatively cold and dry air that doesn't want to let go. Where these contrasting air masses meet, strong thunderstorms often erupt, some with tornadoes.

Over the last decade, the United States averaged 1200 tornadoes per year. The peak months are May and June, with each averaging more than 200. The peak in strong tornadoes - the ones that cause the most damage - occurs a little earlier in the year, when temperature contrasts tend to be largest. So injuries and fatalities from tornadoes actually peak in April.

Tomorrow night we'll take a look at tornado trends over the last 50 years, and answer the question: "Are there really more tornadoes now than there used to be?"

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