Sprites are large-scale electrical discharges that occur high ( 50–90 km) above cumulonimbus, giving rise to a quite varied range of visual shapes flickering in the night sky. They are usually triggered by the discharges of positive lightning between an underlying thundercloud and the ground. Sprites have been blamed for otherwise unexplained accidents involving high altitude vehicular operations above thunderstorms. Since their 1989 discovery, sprites have been imaged from the ground, from aircraft and from space, and have become the subject of intensive investigations.