Educational - Ball lightning
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Ball Lightning (from Usenet)
Ball lightning (boules de feu or foudre spherique; Kugeblitz) is the name given to the mobile luminous spheres which have been observed during thunderstorms. A typical ball lightning is about the size of an orange or a grapefruit and has a lifetime of a few seconds. Compilations of eye-witness reports of ball lightning have been published by Brand(1923), Rodewald(1954), Dewan(1964), Silberg(1965), McNally(1966) and Rayle(1967) among others. Visual sightings are often accompanied by sound, odor, and permanent material damage, and hence it would appear difficult to deny the reality of the phenomenon [as Humphreys(1936) has done]. In a letter to the editor of the London Daily Mail, Morris(1936)described an unusual incident in which a ball lightning caused a tub of water to boil:
"During a thunderstorm I saw a large, red hot ball come down from the sky. It struck our house, cut the telephone wire, burnt the window frame,and then buried itself in a tub of water which was underneath.The water boiled for some minutes afterwards, but when it was cool enough for me to search I could find nothing in it."
Photographs purported to be of ball lightning have been published by Jensen(1933), Kuhn(1951), Wolf(1956), Davidov(1958), Jennings(1962), and Muller-Hillebrand(1963). A phenomenon very similar to, if not identical with, ball lightning has been reported to occur in submarines due to discharge of a current about 150,000 amp direct current from a 260-volt source across a circuit breaker(Silberg, 1962). In addition a number of reports were received of ball-lightning-like phenomena being initiated accidentally in high-power electrical equipment.
Ball lightning and St. Elmo's fire are sometimes confused. St. Elmo's fire is a corona discharge from a pointed conducting object in a strong electric field. Like ball lightning, St. Elmo's fire may assume a spherical shape. Unlike ball lightning, St. Elmo's fire must remain attached to a conductor, although it may exhibit some motion along the conductor. Further, St. Elmo's fire can have a lifetime much greater than the lifetime of the usual ball lightning.
From the many published ball lightning observations, it is possible to compile a list of:
Ball Lightning characteristics
Most observations of ball lightning are made during thunderstorm activity. Most, but not all, of thunderstorm-related ball lightning appear almost simultaneously with a cloud-to-ground lightning discharge. These ball lightnings appear within a few meters of the ground. Sometimes ball lightnings are reported to occur near the ground in the absence of a lightning discharge. Ball lightnings have also been observed to hang in mid-air far above the ground and have been observed falling from a cloud towards the ground.
Ball lightnings are generally spherical, although other shapes have been reported they are usually 0.1-0.2m in diameter, with reported diameters ranging from 0.01-1.00m. Ball lightnings come in various colors, the most common colors being red, orange, and yellow. Ball lightning are generally not exceptionally bright, but can be seen clearly in daylight. They are usually reported to maintain a relatively constant brightness and size during their lifetimes, although ball lightnings which change in brightness and size are not uncommon.
Ball lightnings generally have a lifetime of less than 5 seconds. A small fraction of reports indicate a lifetime of over a minute.
Ball lightning usually move horizontally at a velocity of a few meters per second. They may also remain motionless in mid-air or may descend from a cloud towards the ground. They do not often rise, as would be the case if they were spheres of hot air at atmospheric pressure in the presence of only a gravitational force. Many reports describe ball lightning which appear to spin or rotate as they move. Ball lightnings are sometimes reported to bounce off solid objects, typically the ground.
5. Heat, sound and odor
Rarely do observers of ball lightning report the sensation of heat. However, accounts of ball lightning which burned barns and melted wires do exist. One report found in McNally (1966) described a ball lightning which hit a pond of water with a sound "as if putting a red hot piece of iron into the water." Sometimes ball lightnings are reported to emit a hissing sound. Many observers report a distinctive odor accompanying ball lightning. The odor is usually described as sharp and repugnant, resembling ozone, burning sulphur, or nitric oxide.
6. Attraction to objects and enclosures
Ball lightnings are often reported to be attracted to metallic objects such as wire fences or telephone lines. When attached to metallic objects, they generally move along those objects. Some or all these observations may refer to a type of St. Elmo's fire. Ball lightnings often enter houses through screens or chimneys. Sometimes they are reported to enter houses through glass window panes. They are also reported to originate within buildings, on occasion from telephones. Ball lightnings can exist in an all-metal enclosure such as the interior of an airplane (Uman, 1968).
Ball lightnings decay in one of two modes, either silently or explosively. The explosive decay takes place rapidly and is accompanied by a loud noise. The silent decay can take place either rapidly or slowly. After the ball has decayed, it is sometimes reported that a mist or residue remains. Occasionally a ball lightning has been observed to break up into two or more smaller ball lightnings.
There may be more than one type of ball lightning. For example, the ball lightning that attaches to conductors may be different from the free-floating ball lightning; and the ball lightning that appears near ground may be different from the ball lightning that hangs high in the air or the ball lightning that falls out of a cloud.