Recent satellite data suggests that there are more than 3 million lightning flashes worldwide per day, or more than 30 flashes per second on average. This includes flashes within or between clouds as well as flashes extending from cloud to ground. The amount of lightning found by satellites is considerably less than scientists once thought existed across the planet.The most accepted global measure of lightning frequency is the thunderstorm day--a day on which thunder is heard at a reporting site. By this standard, the tropics are the earth's lightning capital. From 100 to 200 thunderstorm days are reported each year across the equatorial belt from South America to Africa, southeast Asia, and northern Australia. However, thunderstorm days are not the ideal index of lightning, since this measure does not distinguish between a single clap of thunder and a prolonged severe storm.