Lightning striking on 17th street with a car light trail. Longmont, Colorado. Boulder County. Fact: Thunder is a shock wave that results from the quick heating and cooling of air along the lightning channel. Most forest fires in the United States are caused by lightning. There are over 1.4 billion lightning flashes worldwide every year; 8,640,000 strikes a day; and 100 strikes per second. An average of 20 million cloud-to-ground lightning strikes occur annually Black and white Print. Buy Prints, Posters and Greeting Cards available all sizes. Great for your home, office or makes a GREAT gift AND supports a Storm Chaser TOO ;-)!.