The phrase "a bolt from the blue" originated from observations of a seemingly inexplicable phenomenon — a flash of lightning on a day without a storm cloud nearby. This event would be startling under any circumstances, but imagine the shock of seeing such a bolt strike the 363-foot-high Apollo 12/Saturn V rocket while it was more than a mile above KSC. Perhaps being in an airliner while it was "zapped" by lightning at 20,000 feet would be more of a scare, though. While not really bolts from the "blue," because they occur inside of clouds, they occur in clouds which otherwise do not contain lightning -- which are not "storm clouds."

Why are rockets and airplanes struck in these circumstances? It was first thought that they just "got in the way" of a lightning bolt jumping from a positive to a negative-charged area of a thundercloud. Later research provided evidence that the buildup of strong electric fields at certain points of the aircraft were the culprit.

Such concentrated fields of electrical energy can develop before lightning occurs. When an aircraft or a rocket enters such a high electric field, electrical charges are compressed, and they concentrate around the sharp edges and protuberances of the vehicle. If the electrical fields around the airplane’s sharp and protruding parts build up to where there is an electrical breakdown of the air, lightning leaders form at two or more locations on the airplane. The aircraft also contributes to the conducting path between a positive and a negative electrical field, triggering the resultant lightning bolt. In the case of Atlas/Centaur-67, a lightning strike caused the rocket’s computer to upset and issue an extreme yaw command that led to the vehicle’s breakup in flight.