Large-scale electric discharges that occur high above thunderstorm clouds, or cumulonimbus, giving rise to a 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 red-orange flashes, with bluish hanging tendrils below, and can be preceded by a reddish halo. They last longer than normal lower stratospheric discharges, which last typically a few milliseconds, and are usually triggered by the discharges of positive lightning between the thundercloud and the ground, although sprites generated by negative ground flashes have also been observed. They often occur in clusters of two or more, and typically span the altitude range 50 to 90 kilometres (31 to 56 mi), with what appear to be tendrils hanging below, and branches reaching above.
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. Since then, sprites have been observed over North America, Central America, South America, Europe, Central Africa (Zaire), Australia, the Sea of Japan and Asia and are believed to occur during most large thunderstorm systems.
Sprites can be categorized in three types, based on their visual appearance:
Jellyfish sprite – very large, up to 50 by 50 km (31 by 31 mi).
Column sprite (C-sprite) – large scale electrical discharges above the earth that are still not totally understood.
Carrot sprite – a column sprite with long tendrils.