Ball lightning is an unexplained atmospheric electrical phenomenon. The term refers to reports of luminous, usually spherical objects, which vary from pea-sized to several meters in diameter. It is usually associated with thunderstorms, but lasts considerably longer than the split-second flash of a lightning bolt. Many of the early reports say that the ball eventually explodes, sometimes with fatal consequences, leaving behind the odor of sulfur.
Laboratory experiments have produced effects that are visually similar to reports of ball lightning, but it is unknown whether these are actually related to any naturally occurring phenomenon. Scientific data on natural ball lightning is scarce owing to its infrequency and unpredictability. The presumption of its
existence is based on reported public sightings, and has therefore produced somewhat inconsistent findings.
Given inconsistencies and the lack of reliable data, the true nature of ball lightning is still unknown. The first optical and spectral characteristics of ball lightning were published in January 2014.
Characteristics
Descriptions of ball lightning vary widely. It has been described as moving up and down, sideways or in unpredictable trajectories, hovering and moving with or against the wind; attracted to, unaffected by, or repelled from buildings, people, cars and other objects. Some accounts describe it as moving through solid masses of wood or metal without effect, while others describe it as destructive and melting or burning those substances. Its appearance has also been linked to power lines as well as during thunderstorms and also calm weather. Ball lightning has been described as transparent, translucent, multicolored, evenly lit, radiating flames, filaments or sparks, with shapes that vary between spheres, ovals, tear-drops, rods, or disks.
Ball lightning is often erroneously identified as St. Elmo’s fire. They are separate and distinct phenomena.
The balls have been reported to disperse in many different ways, such as suddenly vanishing, gradually dissipating, absorption into an object, "popping," exploding loudly, or even exploding with force, which is sometimes reported as damaging. Accounts also vary on their alleged danger to humans, from lethal to harmless.
A review of the available literature published in 1972 identified the properties of a "typical" ball lightning, whilst cautioning against over-reliance on eye-witness accounts:
- They frequently appear almost simultaneously with cloud-to-ground lightning discharge
- They are generally spherical or pear-shaped with fuzzy edges
- Their diameters range from 1–100 cm, most commonly 10–20 cm
- Their brightness corresponds to roughly that of a domestic lamp, so they can be seen clearly in daylight
- A wide range of colours has been observed, red, orange, and yellow being the most common.
- The lifetime of each event is from 1 second to over a minute with the brightness remaining fairly constant during that time
- They tend to move, most often in a horizontal direction at a few metres per second, but may also move vertically, remain stationary or wander erratically.
- Many are described as having rotational motion
- It is rare that observers report the sensation of heat, although in some cases the disappearance of the ball is accompanied by the liberation of heat
- Some display an affinity for metal objects and may move along conductors such as wires or metal fences
- Some appear within buildings passing through closed doors and windows
- Some have appeared within metal aircraft and have entered and left without causing damage
- The disappearance of a ball is generally rapid and may be either silent or explosive
- Odors resembling ozone, burning sulfur, or nitrogen oxides are often reported
Direct Measurements of Natural Ball Lightning
In January 2014, scientists from Northwest Normal University in Lanzhou, China, published the results of recordings made in July 2012 of the optical spectrum of what was thought to be natural ball lightning made during the study of ordinary cloud–ground lightning. At a distance of 900 m (3,000 ft), a total of 1.64 seconds of high-speed video of the ball lightning and its spectrum was made, from the formation of the ball lightning after the ordinary lightning struck the ground, up to the optical decay of the phenomenon. The emission lines of neutral atomic silicon, calcium, iron, nitrogen and oxygen were detected in contrast with mainly ionized nitrogen emission lines in the spectrum of the parent lightning. Oscillations in the light intensity and in the oxygen and nitrogen emission at a frequency of 100 Hx, caused by the electric field of the 50 Hz high-voltage power transmission line in vicinity, were observed. From the spectrum, the temperature of the ball lightning was assessed as being lower than the temperature of the parent lightning (<15,000–30,000 K). The observed data are consistent with vaporization of soil as well as with ball lightning's sensitivity to electrical fields.
http://en.wikipedia.org/wiki/Ball_lightning
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