Another lightning phenomenon is called ball lightning. It looks like slowly rotating balls of light, a phenomenon that has been reported for centuries. The light balls are the size of grapefruits and appear to be floating above the ground for about 10 seconds each during electrical storms. In 2012, ball lightning was first captured on videotape, and scientists have hypothesized about its origin. Spheres of highly compressed air are thought to form during the storm. The spheres reflect white light in all directions, giving the appearance of a rolling ball. Ball lightning is very intense, and scientists believe that its light intensity is one billion times greater than straight lightning. Ball lightning is also very rare. Sprites and elves describe other mysterious lightning phenomena. Red bursts of electricity look like jellyfish and hang above a thunderstorm, then stream down. These are called sprites , and scientists think they form due to an imbalance of electrical charge above them in the cloud. Pilots had often talked privately about sprites but were afraid to widely discuss them so their mental state would not be questioned. Sprites were finally captured on video in the 1980s. Elves are similar to sprites and can be seen above storm clouds in broad daylight, but they appear as rings and are much larger, at up to 185 miles (298 km) wide. Another strange form of lightning is called dark lightning . It is called dark because it does not produce much visible light. Dark lightning is composed of very high-energy electrons that collide with air particles to produce gamma rays. The US National Aeronautics and Space Administration (NASA) discovered this intense electrical phenomenon called a terrestrial gamma-ray flash (TGF), which is a burst of gamma rays in the earth's atmosphere. Scientists think the TGFs are created inside or above thunderstorms. Red lightning bolts are weaker and quicker; they can’t be seen by the naked eye because they occur too quickly, but they can be detected by other instruments. Blue lightning happens in the stratosphere and can be seen by pilots in airplanes. TGFs are very powerful and can blind a satellite sensor that is hundreds of


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