| Section T | T index | 391-399 of 589 terms |
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tracking radar—A radar that is primarily used to automatically track the position of nonmeteorological targets that are usually small relative to the radar pulse volume, for example, an aircraft or balloon.
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tracking system—General name for apparatus, such as a tracking radar, used in following and recording the position of objects in the sky. A theodolite and an observer form an optical tracking system used in pilot balloon runs.
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tractive force—Force, parallel to the streambed, exerted by flowing water on a sediment particle at rest.
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trade inversion—A characteristic temperature inversion usually present in the trade-wind streams over the eastern portions of the tropical oceans. It is found in large-scale subsiding flows constituting the descent branches of the Hadley cell and Walker circulation. The subsidence warming in the inversion layer is balanced by radiative cooling and evaporation from the tops of trade cumuli. The height of the base of this inversion varies from about 500 m at the eastern extremities of the subtropical highs to about 2000 m at the western and equatorial extremities. In the equatorial trough zone and over the western portions of the trade-wind belt, the inversion does not exist as a mean condition, although it appears in certain weather patterns. The strength of the inversion varies enormously, occasionally being more than 10°C over 1 km, but sometimes being absent altogether, especially in the Northern Hemisphere. The inversion is generally strongest when the height of its base is lowest, and vice versa. The thickness of the inversion layer varies from only a few meters to more than 1000 m. On the average its thickness is about 400 m. The airflow below the inversion is very moist and filled with cumulus clouds (trade cumuli). Above it, the air is warm and exceedingly dry; this structure is so characteristic of the trade current that tropical analysts think of the tropical troposphere as consisting of a lower moist and an upper dry layer.
Riehl, H., 1954: Tropical Meteorology, ch. II.
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trade-wind belt—The latitudes occupied by the trade winds, generally between about 30° in the winter hemisphere and 35° in the summer hemisphere.
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trade-wind cumulus—(Or trade cumulus.) The characteristic cumulus cloud of the trade winds over the oceans in average, undisturbed weather conditions. These clouds are generally 5000–7000 ft thick at peak development and are based at about 2000–2500 ft altitude. The individual cloud usually exhibits a blocklike appearance since its vertical growth ends abruptly in the lower stratum of the trade-wind inversion. A group of fully grown clouds show considerable uniformity in size and shape.
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trade-wind desert—An area of very little rainfall and high temperature that occurs where the trade winds or their equivalent (such as the harmattan) blow over land. The best examples are the Sahara and Kalahari deserts. The trade winds, blowing from higher latitudes, are very drying, and cloudiness is almost absent in these desert regions.
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