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Thunder: A Child of Lightning
Part 2. The Modern Theory
Today, the causes for thunder and its various voices are well known by atmospheric scientists. In this section we will detail those processes. Because thunder is so intimately associated with lightning, much of what was learned about the location, shape and orientation of lightning has been actually deduced using the sound waves of thunder.
We start the process with the two main ingredients: air and lightning. Having electrical resistance, air becomes heated when an electrical current, such as lightning, passes through it. Thus, each lightning flash, which has a temperature hotter than the surface of the sun, superheats the air surrounding its path, resulting in a channel of gas at very high temperature and pressure. As the superheated gas rapidly expands into the surrounding air, first a shock wave and then a sound wave radiate from the lightning channel.
When the energy that the shock wave received from the lightning stroke is expended, the wave "relaxes" and the pressure in the vicinity of the channel returns to normal levels. The shock wave in relaxing produces an acoustic, or sound, wave, which travels through the air perpendicular to the lightning segment that produced it.
Although less than one percent of the total energy in the initial shock wave is transformed into the acoustic wave (the remaining 99 percent is dissipated into heating the surrounding air), the total energy available for that sound wave is still extremely large. Therefore, thunder is one of nature's loudest sounds. A nearby thunderclap may reach a sound level of around 120 decibels, equivalent to being within 60 metres (200 feet) of a jet aircraft during take-off or about 1 metre (3.3 feet) from an auto horn. A chain saw is rated around 100 decibels. A sound of 140 decibels is painfully loud. [Note each 10 decibel increase in noise level seems twice as loud.]
The radius of the shock wave at the time of relaxation determines the characteristic frequency or pitch of the thunder from that stroke: the more powerful the lightning stroke, the wider the channel and the lower the pitch of the resulting thunder. Thunder generally has a pitch either between 15 to 40 hertz or between 75 and 120 hertz. (For a reference mark, the lowest note on a standard 61-key piano has a tuned pitch of 66 hertz.)
Thunder, however, is more than a simple, loud explosive sound following a lightning bolt. Thunder peals. Thunder rolls and rumbles through the stormy sky. Thunder cracks and claps. Thunder varies in duration and the distance over which it may be heard. What then accounts for this wide variety of sounds that we classify under the label thunder?
Thunder travels through the lower atmosphere as an acoustic or sound wave moving at a speed of roughly 1230 kilometres per hour (770 miles per hour) -- the speed of sound. The character of the sound of thunder -- its pitch, loudness and form (crack, rumble, etc.) -- depends upon the lightning flash that produced it. And the order in which the various sound waves from a lightning stroke reach the observer are all primarily determined by the lightning flash's shape and location. The sound waves are also modified by the atmosphere through which they travel.
Thunder sound waves originating from the lightning flash do not radiate with equal strength in all directions from the lightning channel. More than 80 percent of the acoustic energy is radiated into zones 30 degrees above and below the surface of the plane which perpendicularly bisects the spark. Since the average change in the direction or orientation between adjacent lightning segments of the lightning stroke is only about 16 degrees (and is thus smaller than the zone into which most of the acoustic energy is radiated), the largest segments of the thunderbolt will emit their loudest sound in roughly the same direction. However, it is the degree of that orientation change between segments that determines whether thunder is heard as a sudden clap or a prolonged rumble.
Sound waves from all segments of the lightning stroke are produced almost simultaneously, typically over a time interval much less than a second in length. What variations we hear in a thunder peal result from the time required for the sound from different segments of the lightning bolt to reach our ears, the nearest segments being heard before the more distant. This time differential, coupled with the length and orientation of the larger segments of the lightning flash, determines the unique character of each thunder peal we hear.
For example, if the main channels of the lightning bolt are end-on to the listener, the thunder will be relatively quiet since most of the sound is being radiated perpendicular to the channel, away from the listener. Since the sound is generated from portions of the bolt progressively further from the listener, those sound waves which do reach the ear combine to produce a prolonged soft roll or rumble.
Since each lightning flash is composed of a number of large segments oriented in any number of ways relative to the listener, the thunder that one generally hears is a combination of claps and rumbles. Listeners separated by some distance will each perceive the thunder in a unique way. Scientists have taken advantage of this structure by using sensitive microphones and recording devices placed in a listening array to probe the structure of the lightning bolt.
Atmospheric Effects on Thunder
The above discussion assumes that the thunder sound wave has moved through an ideal atmosphere and flat terrain. Thunder does not, however, travel from the lighting channel to the receiver through a uniform atmosphere or always over an ideal, featureless terrain. Since the real atmosphere varies in its density (both vertically and horizontally) and has winds blowing through it at various speeds and directions, the thunder wave may be scattered, attenuated, refracted, or reflected on its way to the observer. The total effect of these modifications of the sound wave is to further alter the volume, pitch and character of the sound heard.
The scattering and attenuation processes in the atmosphere alter the total sound package reaching the listener, mostly by weakening the higher pitched frequencies in the sound packet. Thus, by the time the thunder wave reaches a listener several miles from the lightning stroke, the predominant sound will be a low-pitched rumble. If the lightning flash happens to be of low energy -- a situation which produces mostly higher pitched sounds -- no audible thunder will be heard except close to the lightning channel.
Air temperature and wind -- and the vertical variation of both through the atmosphere -- refract, or bend, the thunder wave from a straight line path toward the listener. Because the temperature of the lower atmosphere usually decreases with height and sound travels faster in warm than cold air, sound waves moving through the lower atmosphere curve upward. Thunder propagating from the lowest portions of the lightning bolt, therefore, may not be heard as refraction bends the sound wave upward, causing it to pass over the head of the listener on the surface.
Wind can have two different effects on thunder. First, it may increase or decrease the speed at which the sound wave moves through the air. Sound moves faster downwind that it does upwind. Second, the variation of wind with height may refract sound waves similar to the effects of vertical temperature gradients. If wind increases with height as is usually the case near the earth's surface, the sound wave will refract upward, bending away from the surface-based listener.
The combined effects of scattering, attenuation, refraction and, in most cases, reflection, limit the distances at which thunder may be heard by a ground-based observer. Although this distance varies with the temperature gradient, wind speed and the height of the lightning flash, thunder generally will not be heard further than 10 to 25 kilometres (6 to 15 miles) from the lightning bolt. This effect is best observed in the phenomenon known as heat lightning, where lightning from distant thunderstorm cells is visible (usually as in-cloud and between-cloud flashes) but from which no thunder is heard by the observer. Such events are common during hot humid weather when thunderstorms are widely scattered across an a region.
Part 1. Thunder: Voice of the Heavens
Part 3. "Watching" Thunder
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