Once a dangerous lightning threshold is reached, horns blast at a local golf course, directing golfers from the course. More than 8,000 Americans have been killed by lightning over a 45 year period. Your chances of being struck by lightning in the United States are 1 in 260,000. Your chances increase if you are golfing. In the United States, between 75 and 150 people are killed by lightning each year with 5 to 30 times that number suffering injuries. The deadliest month for lightning fatalities and injuries in the United States is July. Golfers Lee Trevino and Jerry Heard were hit by lightning during the 1975 Western Open. In Minneapolis on June 13, 1991 a spectator and five others were injured while taking shelter under a tree during the U.S. Open. On that same date, a 37-year-old man was killed by a bolt while golfing near Louisville and two others were injured, while standing under a cluster of trees. There are different classifications of lightning. Ribbon lightning appears infrequently and is created when strong winds enhance a cloud to ground lightning channel over a long distance. Between the multiple strokes, each stroke in the flash is visibly separated. Another rare form of lightning is bead lightning. It occurs when the lightning channel to the ground encounters atmospheric obstacles and breaks up or disperses. When this occurs, glowing segments appear as beads or sausage links and persist longer than a normal flash. Positive or top-of-cloud lightning happens when a single stroke shoots out from the top of a cloud and strikes the ground either ahead or behind the main thundercloud.