1973 was the first year in which more than 1,000 tornadoes were verified in the United States .
Owing to increases in storm spotters , the number of tornadoes in the U.S. reached new heights in the early 1970s .
1973 was the first year in which more than 1,000 tornadoes were verified in the U.S.   .
Owing to increases in storm spotters , the number of tornadoes in the U.S. reached new heights in the early 1970s .