A reddish flash high above a thunderstorm is called a red sprite.
Sprites are large electrical discharges in the upper atmosphere, usually occurring above powerful thunderstorms. They can appear as red, orange, or sometimes bluish structures, often with branching shapes or dangling tendrils. Unlike ordinary lightning, they occur far above the main cloud and extend into the mesosphere.
The first documented scientific images of sprites were captured in 1989 by researchers at the University of Minnesota. Earlier reports and photographs had hinted at brief flashes above storms, but the phenomenon was not widely confirmed until sensitive low-light cameras recorded it.
Sprites belong to a broader family of transient luminous events. Blue jets shoot upward from thundercloud tops, while elves are rapidly expanding rings of light caused by electromagnetic pulses. These phenomena are brief and difficult to observe from the ground.