DICTIONARY OF AVIATION 229
than the lightning en account of the velocity of the sounds waves being much less than the velocity of the lightwaves.
thunder-cloud '0And3z,kLaod n. a cloud that produces light- ning and thunder, usually dense, dark, towering masses of cumulus or strato-cumulus, with a cirro?status overflow; a cumulo-nimbus cloud; a turreted cloud; a shower-cloud.
kick of the thundercloud, the kick-like, or sudden out- ward and downward thrust or posh of a thunder-cloud, due to the expansion of the rising air within the cloud, and producing a jump in barometric pressure and giving rise to squall-winds.
thunder-gust '0And9z,gASt n. [also spelt thunder gust] a gust accompanied by thunder; a thunder-squall; a thunder- sterm.
thunder-head '6And9z,hed n. one of the round, compact, stuelling cumulus clouds which frequently develop into thunder-clouds.
thunder-headed '9And9z,hedid adj. [also spelt thunder* heded] pertaining to, or like, a thunder-hed : as, a *thunder- heded cloud.
thunder-shower '0And9<r,$au-9z: n. a shower of rain accom- panied by thunder and lightning.
thunder-squall 'Andg^sktuc:} n. a squall of wind accom- panied by thunder; a thunder-gust.
thunder-storm '0And9z,ste:zm n. [also spelt thunderstorm] a sterm accompanied by lightning and thunder, due to a state of unstable atmosferic equilibrium and high relativ humidity.
cyclonic thunderstorm, a thunder-sterm associated with a cyclone or region of low barometric pressure, and having a progressiv movement which may, however, be quite dif- ferent from the cyclenic movement ef which it is a part.
heat thunder-storm, a thunder-sterm having little or no progressiv motion, characteristic of the het equatorial re- gions of the earth, and occurring in summer in temperate latitudes.
line thunder-storm, a thunder-sterm accompanying V- shaped isobanc depressions.
thunder-storm belt, a belt, zone, or long narrow area within which thunder-sterms occur simultaneously or con- tinuously, and tuhich moves stedily in an easterly direction.
thysanoptera ,0is9'nept9-z9 n. pi. an erder of insects in- cluding only the family Thripidae or Thripsidae, with leng, narrow, eften veinless wings furnisht with a leng fringe.
tie tai n. something which fastens or binds; a bend which withstands stress of poll ; a wire or other tension-member
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