Lightning

From Futurepedia, the Back to the Future Wiki

Lightning was responsible for several instances in which the DeLorean time machine traveled.

It was first the inspiration of Marty McFly after reading the front of the flyer he was given by the Hill Valley Historical Society, announcing that the Courthouse would be struck by lightning at 10:04 pm that next Saturday night. Doc Brown of 1955, knowing that it would be enough power to trigger time travel, devised a plan to channel a bolt of lightning directly into the flux capacitor, sending Marty back to 1985. They succeeded when the clock tower was hit at precisely the exact time given and 1.21 jigowatts of electricity was funneled into the time machine.

Lightning later came into play while the time machine was malfunctioning. While back in 1955 to destroy the Grays Sports Almanac, the time circuits switched to January 1, 1885 and the car was hit in midair from a bolt, sending Doc back to that date. This time, the lightning bolt created a short circuit, frying the flying circuits and damaging the time circuits. Because replacement parts for the time circuits would not be available until 1947, Doc hid the car in the Delgado Mine.