Sprites and blue jets: electrical flashes above storms

A thunderstorm seems to end at the top of its cloud. Above that boundary, the sky may look empty. Yet sensitive cameras sometimes reveal another set of electrical flashes. A red figure branches far above the storm. A blue cone rises from the cloud top, then almost immediately disappears.

These are called transient luminous events, or TLEs. “Transient” is literal. Many last for only milliseconds, thousandths of a second, which helps explain why people reported them long before instruments supplied convincing evidence. They also occur above the brilliant lightning and obscuring cloud that produced them. The familiar bolt is therefore only one part of a taller electrical system.

A discovery made by accident

Pilots had described strange lights over storms, and written reports reach back centuries. But a report that cannot be measured or repeated is difficult to place within science. That changed on 6 July 1989, when researchers at the University of Minnesota were testing a low-light camera for a rocket experiment. The camera happened to be aimed toward a distant storm and recorded two faint columns above it.

Space Shuttle video from 1989 to 1991 supplied more examples, turning the accidental recording into a new research field. NASA Earthdata reconstructs how the ground camera and Shuttle tapes established the phenomenon.

The name sprite was chosen partly because the event was so elusive. Other playful names followed, including elves and trolls. The names distinguish different shapes, heights and electrical processes.

A sprite is a response above the storm

A sprite does not usually grow continuously out of a cloud like an ordinary bolt. It forms much higher, commonly around 80 kilometres above the surface, after a powerful lightning discharge below. NASA's Spritacular project describes sprites as brief upper-atmosphere flashes that can combine diffuse glows with bright tendrils.

Many sprites follow strong positive cloud-to-ground lightning, in which positive charge is transferred toward the ground. The lightning rapidly rearranges a large amount of charge inside the storm. That change creates an electric field above the cloud. In the thin upper atmosphere, the field can accelerate electrons and begin an electrical breakdown: normally insulating air becomes temporarily conductive.

The accelerated electrons transfer energy to nitrogen molecules in collisions. As those molecules release some of that energy as light, much of the red glow appears. The branching structures seen in detailed images are built from narrow, rapidly developing channels called streamers. A photograph may make a sprite look like a stationary jellyfish, but it records a process developing too quickly for an unaided eye to follow.

Association is not inevitability. Not every positive flash makes a sprite, and important questions remain about why one storm produces a particular shape while another does not. NASA lists sprite frequency, initiation conditions and effects on the global electrical circuit among the questions still being studied. The Great Sprites Chase explains both the established mechanism and those uncertainties.

A blue jet begins in a different place

A blue jet starts near the top of a thundercloud and travels upward, usually into the stratosphere, the atmospheric layer above the main weather layer. Its shape can resemble a narrow cone or fan. The colour comes from emissions in nitrogen that favour blue and near-ultraviolet wavelengths (just beyond the violet end of visible light) under the conditions lower in the atmosphere.

That geometry matters. A sprite is an upper-atmosphere response to a major charge change below; a blue jet is an upward discharge emerging from the storm region itself. Lumping both together as “upward lightning” hides the distinction.

Scientists also observe gigantic jets, which can bridge a thundercloud toward the lower ionosphere, a region containing many electrically charged particles, and extend much farther than an ordinary blue jet. The categories describe observed patterns. How jets begin, and how they relate to ordinary lightning and storm charge, remain active research questions. A 2025 observation from the International Space Station, initially mistaken for a sprite, was later identified as a gigantic jet, showing why position and development matter more than a dramatic still image. NASA compares that event directly with a sprite.

One storm, several atmospheric layers

Most weather occupies the troposphere. NASA places its average top near 12 kilometres, although towering thunderclouds can push into the lowest stratosphere. The stratosphere extends to roughly 50 kilometres, the mesosphere to roughly 80 kilometres, and the thermosphere begins above that. NASA's atmosphere guide gives these approximate boundaries.

Blue jets inhabit mainly the gap between cloud tops and the upper stratosphere. Sprites occupy much of the mesosphere and may extend across a broad altitude range. The separation explains a useful visual clue: a dark gap can lie between the bright storm and the red sprite above it.

The boundaries in textbooks are not hard lids. Atmospheric layers are defined largely by how temperature changes with height, and their transition altitudes vary. Likewise, a TLE is not assigned a name by altitude alone. Shape, colour, development and its relationship to the parent lightning all contribute.

Seeing what the cloud conceals

Ground observers need a distant storm, a clear view above it and a sensitive camera. Even then, the flash can fit within a single frame. Looking from above changes the geometry. Astronauts and instruments on the International Space Station can see storm tops and the atmosphere above them without the lower cloud blocking the view.

ESA experiments have used that vantage point to examine sprites, jets and other energetic events. The agency notes that sprites can reach about 90 kilometres and may last no more than a few tens of milliseconds. ESA's account of observing thunderstorms from orbit also explains why fast cameras and careful timing are essential.

There is a safety lesson in the distance required for observation. TLE research is not a reason to stand beneath a thunderstorm with a camera. If thunder is audible, the storm is close enough to threaten you with ordinary lightning; move to a substantial building or an enclosed metal-topped vehicle and follow official local guidance. The spectacular event is high above the cloud, but the immediate hazard remains at ground level.

A cloud that ends near the tropopause, the boundary above the main weather layer, can disturb air tens of kilometres higher. A sensitive camera pointed at a familiar storm revealed that the apparently empty darkness above it was active too.