Nobody knows what causes lightning. Nevertheless, lightning bolts are common in clouds during rainstorms, and on average 6000 lightning bolts occur between clouds and the Earth every minute. Above , several lightning strokes were photographed behind Kitt Peak National Observatory in Arizona . Lightning has also been found on the planets Venus , Jupiter , Saturn , and Uranus . NASA launched the TRMM mission in 1997 that continues to measure rainfall and lightning on planet Earth .
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