| Section T | T index | 261-269 of 589 terms |
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thundersquall—Strictly, the combined occurrence of a thunderstorm and a squall, the squall usually being associated with the downrush phenomenon typical of a well-developed thunderstorm.
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thunderstorm charge separation—The process by which the large electric fields found within thunderclouds are generated; the process by which particles bearing opposite electrical charge are given those charges and transported to different regions of the active cloud. Accounting for the rapid and extensive separation of electric charge within thunderstorms is still one of the central problems in the study of thunderstorm electricity. Many theories have been proposed to explain charge separation, including the breaking-drop theory, the ion-capture theory, a theory involving the Workman–Reynolds effect, and a mechanism involving the bounce of ice crystals from growing graupel. None is entirely satisfactory in being able to account fully for the observed charge separation required to maintain a very active thunderstorm producing one discharge per second or so. Much evidence points toward particle-size difference and hence falling- speed difference as a necessary factor in the transportation of the oppositely charged particles in opposite directions in the updrafts of convective clouds, in regions where ice crystals are produced in the presence of graupel in regions between updraft and downdraft.
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thunderstorm charge—The existence of regions of net charge in a thunderstorm. During transient collisions of ice crystals with riming graupel pellets, charge is transferred. The separating particles then carry equal and opposite charges; the larger (often negative) particles fall while the smaller ones (often positively charged ice crystals) are carried up in the updraft to produce a vertical electric field that eventually produces lightning. The charge transfer process is not completely understood, but possible processes include charges on the surface layers of the particles, charges on dislocations in the ice lattice, temperature differences along surface features that may be broken off during collisions, and contact potential differences between the surfaces of the interacting particles. See breaking-drop theory, ion-capture theory.
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thunderstorm dipole—The simplest representation of the electrostatic structure of an electrified cloud with overall charge neutrality. Ordinary thunderstorms are characterized by upper positive charge and lower negative charge.
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thunderstorm electrification—The process by which regions of net positive and negative electric charges are produced in clouds.
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thunderstorm initiation mechanism—Rising motion associated with an atmospheric feature capable of releasing convective instability leading to thunderstorm development. Examples are rising motion associated with fronts, the dryline, gust fronts, upper-air disturbances, heated elevated terrain, terrain–airflow interaction, and sea breezes and other mesocale circulations resulting from horizontal gradients in radiative properties of the underlying surface.
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