| Section T | T index | 251-259 of 589 terms |
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threshold of hearing—The smallest root-mean-square sound pressure that can be perceived by an average person without special aids. Usually specified as 0 dB, it corresponds to 2 x 10−5 N m−2 or 0.0002 microbar.
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threshold of pain—(Or threshold of discomfort, threshold of feeling.) The root-mean-square sound pressure at which an average person will begin to experience physical pain from ambient sound. It corresponds to a sound pressure level of 134 dB, that is, 100 N m−2 or 1 millibar.
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throughfall—Part of rainfall that reaches the ground directly through the vegetative canopy, through intershrub spaces in the canopy, and as drip from the leaves, twigs, and stems.
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thunder—The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Some three-fourths of the electrical energy of a lightning discharge is expended, via ion–molecule collisions, in heating the atmospheric gases in and immediately around the luminous channel. In a few tens of microseconds, the channel rises to a local temperature of the order of 10 000°C, with the result that a violent quasi-cylindrical pressure wave is sent out, followed by a succession of rarefactions and compressions induced by the inherent elasticity of the air. These compressions are heard as thunder. Most of the sonic energy results from the return streamers of each individual lightning stroke, but an initial tearing sound is produced by the stepped leader; and the sharp click or crack heard at very close range, just prior to the main crash of thunder, is caused by the ground streamer ascending to meet the stepped leader of the first stroke. Thunder is seldom heard at points farther than 15 miles from the lightning discharge, with 25 miles an approximate upper limit, and 10 miles a fairly typical value of the range of audibility. At such distances, thunder has the characteristic rumbling sound of very low pitch. The pitch is low when heard at large distances only because of the strong attenuation of the high-frequency components of the original sound. The rumbling results chiefly from the varying arrival times of the sound waves emitted by the portions of the sinuous lightning channel that are located at varying distances from the observer, and secondarily from echoing and from the multiplicity of the strokes of a composite flash. See electrometeor.
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thunderbolt—In mythology, a lightning flash accompanied by a material “bolt” or dart; this is the legendary cause of the damage done by lightning. It is still used as a popular term for a lightning discharge accompanied by thunder.
Schonland, B. F. J., 1950: The Flight of Thunderbolts, 1–8.
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thunderhead—A popular term for the incus (“anvil”) of a cumulonimbus cloud; or, less appropriately, the upper portion of a swelling cumulus, or the entire cumulonimbus.
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