A blue jet is an upward lightning discharge that shoots from the top of a thunderstorm into the lower stratosphere. It appears as a narrow, blue, cone-shaped flash rising above the cloud rather than traveling toward the ground.
Blue jets are a form of transient luminous event, a family of short-lived electrical phenomena above thunderstorms. They are thought to begin inside the cloud near the upper positive charge region and the negatively charged screening layer above it. A positive leader can then break through the cloud top and propagate upward through the surrounding atmosphere.
The blue color comes from emissions associated with nitrogen molecules and ions. Blue jets typically last only a few hundred milliseconds and can reach roughly 40–50 kilometers above the cloud, well beyond ordinary cloud-to-ground lightning. They were first recorded in 1989 in video taken from the Space Shuttle over Australia.
Sprites are different: they are usually red flashes much higher above storms. Gigantic jets also travel upward, but they can extend much farther toward the ionosphere. A blue jet is therefore the standard answer when the question focuses on a blue discharge emerging from a storm top into the stratosphere.