Upper-atmospheric lightning or upper-atmospheric discharge are terms sometimes used by researchers to refer to a family of electrical-breakdown phenomena that occur well above the altitudes of normal lightning. The preferred current usage is transient luminous events (TLEs) to refer to the various types of electrical-discharge phenomena in the upper atmosphere, because they lack several characteristics of the more familiar tropospheric lightning. TLEs include red sprites, sprite halos, blue jets, gigantic jets, and elves.
Sprites are large-scale electrical discharges which occur high above a thunderstorm cloud, or cumulonimbus, giving rise to a quite varied range of visual shapes. They are triggered by the discharges of positive lightning between the thundercloud and the ground.The phenomena were named after the mischievous sprite (air spirit) Puck in William Shakespeare's A Midsummer Night's Dream. They normally are colored reddish-orange or greenish-blue, with hanging tendrils below and arcing branches above. They can also be preceded by a reddish halo.They often occur in clusters, lying 50 miles (80 km) to 90 miles (145 km) above the Earth's surface. Sprites were first photographed on July 6, 1989 by scientists from the University of Minnesota and have since been witnessed tens of thousands of times. Sprites have been held responsible for otherwise unexplained accidents involving high altitude vehicular operations above thunderstorms.
Blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere 40 to 50 km (25 to 30 miles) above the earth. In addition, whereas red sprites tend to be associated with significant lightning strikes, blue jets do not appear to be directly triggered by lightning (they do, however, appear to relate to strong hail activity in thunderstorms). They are also brighter than sprites and, as implied by their name, are blue in color. The color is believed to be due to a set of blue and near-ultraviolet emission lines from neutral and ionized molecular nitrogen. They were first recorded on October 21, 1989, on a monochrome video of a thunderstorm on the horizon taken from the Space Shuttle as it passed over Australia. Blue jets occur much less frequently than sprites. By 2007, fewer than a hundred images had been obtained. The majority of these images, which include the first colour imagery, are associated with a single thunderstorm studied by researchers from the University of Alaska. These were taken in a series of 1994 aircraft flights to study sprites.
St. Elmo's fire (also St. Elmo's light) is an electrical weather phenomenon in which luminous plasma is created by a coronal discharge originating from a grounded object in an atmospheric electric field (such as those generated by thunderstorms created by a volcanic explosion).
St. Elmo's fire is named after St. Erasmus of Formiae (also called St. Elmo, the Italian name for St. Erasmus), the patron saint of sailors. The phenomenon sometimes appeared on ships at sea during thunderstorms and was regarded by sailors with religious awe for its glowing ball of light, accounting for the name.
Physically, St. Elmo's fire is a bright blue or violet glow, appearing like fire in some circumstances, from tall, sharply pointed structures such as lightning rods, masts, spires and chimneys, and on aircraft wings. St. Elmo's fire can also appear on leaves, grass, and even at the tips of cattle horns. Often accompanying the glow is a distinct hissing or buzzing sound.
St. Elmo's fire is, in fact, a mixture of gas and plasma (as are flames in general, and also stars). The electric field around the object in question causes ionization of the air molecules, producing a faint glow easily visible in low-light conditions. Approximately 1000 volts a centimeter induces St. Elmo's fire; however, this number is greatly dependent on the geometry of the object in question. Sharp points tend to require lower voltage levels to produce the same result because electric fields are more concentrated in areas of high curvature, thus discharges are more intense at the end of pointed objects.
Conditions that can generate St.Elmo's fire are present during thunderstorms, when high voltage is present between clouds and the ground underneath, electrically charged. Air molecules glow due to the effects of such voltage, producing St. Elmo's fire.
The nitrogen and oxygen in the Earth's atmosphere causes St. Elmo's fire to fluoresce with blue or violet light; this is similar to the mechanism that causes neon lights to glow.
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