Sprites or red sprites are large-scale electric discharges that occur high above thunderstorm clouds, or 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 appear as luminous reddish-orange flashes. They often occur in clusters above the troposphere at an altitude range of 50–90 km. Sporadic visual reports of sprites go back at least to 1886 but they were first photographed on July 4, 1994, by scientists from the University of Minnesota and have subsequently been captured in video recordings many thousands of times.
Sprites are sometimes inaccurately called upper-atmospheric lightning. However, sprites are cold plasma phenomena that lack the hot channel temperatures of tropospheric lightning, so they are more akin to fluorescent tube discharges than to lightning discharges. Sprites are associated with various other upper-atmospheric optical phenomena including blue jets.
Sprites have been observed over North America, Central America, South America, Europe, Central Africa, Australia, the Sea of Japan and Asia and are believed to occur during most large thunderstorm systems.