Sprites are a type of transient luminous event (TLE) that occur during lightning storms and are rarely observed with the human eye. They are large-scale electrical discharges that occur high above thunderstorm clouds, or cumulonimbus, giving rise to a varied range of visual shapes flickering in the sky. Sprites are sometimes inaccurately called upper-atmospheric lightning, but they 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 and ELVES.
Sprites are poorly understood, but atmospheric electrodynamicists have figured out the basics behind their formation. They are often triggered by a strong, positive bolt of ordinary lightning near the ground. They’re thought to be a balancing mechanism that the atmosphere uses to dispense charges vertically. To photograph a sprite, you need a dark sky and a clear view toward a distant thunderstorm. The sky needs to be dark because you’ll be taking long exposures; too much stray light in your sky will wash out the sprite.
