Ariel in The Tempest, you may recall, was a spirit, or a "sprite". It was his pleasure to do the bidding of his master, Prospero, "be't to fly, to swim, to dive into the fire, or ride upon the curl'd clouds"; or as he himself described his antics:
"Sometimes I'd divide,
And then in many places would I flame distinctly,
To meet and join Jove's lightnings, the precursors
O' th' dreadful thunder-claps."
It was allegedly because of this colourful behaviour on the part of Ariel and his friends that meteorologists, romantics as they are at heart, gave the name of "sprites" to a phenomenon that had been a mystery for decades.
For over a century, reports existed of night-time flashes dancing in the atmosphere high above a thunderstorm; they were described lyrically by one observer as "a faint, almost curtain-like gauze of salmoncoloured orange-red". They seemed a little like aurora bore- alis, but much brighter, and similar to lightning, except that each flash was much shorter. But these "high-altitude flashes" occurred in the wrong places and at the wrong times to be associated directly with either of these homely happenings.
Since they had never seen these phenomena themselves, meteorologists for years were sceptical. But in 1989 the flashes were captured on a video-tape, and since then many good photographs have been obtained, leaving no doubt of their existence. And so we began to call them "sprites".
Soon the sprites had company. In the early 1990s researchers flying in the high atmosphere over Arkansas near an active hailstorm were startled to see beams of blue light shooting upward directly out of the cloud; such occurrences have been authenticated, although "blue jets" seem to be very rare, and have been captured only once on video-tape.
And after the red sprites and the blue jets came "elves". "Emissions of light and very low frequency perturbations from electromagnetic pulse sources", a phrase which bears a little resemblance to its acronym, were discovered in 1995. The elves appear as giant, expanding and instantaneous discs of light in the high atmosphere above a thunderstorm, each caused, it seems, by a very strong electromagnetic pulse.
Scientists suspect that the sprites, blue jets and elves result from powerful electric fields associated with thunderstorms, which may impart an electric charge to particles of stratospheric air. These may be caused to glow by a mechanism similar to that which produces the ghostly light of the aurora.