| Section T | T index | 131-139 of 589 terms |
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tethersonde—A radiosonde attached to a fixed or tethered balloon. The balloon is usually larger than a balloon used for upper-air soundings, and the tether usually limits the sounding to the boundary layer. The radiosonde is typically moved up and down the tether to get multiple, high-resolution profiles of the boundary layer.
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tetroon—A balloon of tetrahedron shape made from a cylinder of plastic film. The cylinder is sealed orthogonally at the two ends, producing a low-cost balloon that simulates a sphere.
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texture—In a photo image, the frequency of change and arrangement of tones, often due to highlights and shadows created by the irregular surface being viewed.
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thaw—1. To melt a substance, ice for example, by warming it to a temperature greater than the melting point of the substance, or to have frozen contents melted. 2. To free something from the binding action of ice by warming it to a temperature above the melting point of ice. 3. A warm spell when ice and snow melt, for example, “January thaw.”
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thawing index—As used by the U.S. Army Corps of Engineers, the number of Fahrenheit degree- days (above and below 32°F) between the lowest and highest points on the cumulative degree- days time curve for one thawing season. The thawing index determined from air temperatures at 4.5 ft above the ground is commonly designated the air thawing index, while that determined from temperatures immediately below a surface is called the surface thawing index. Compare freezing index; see also growing season.
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thawing season—As used by the U.S. Army Corps of Engineers, the period of time between the lowest point and the succeeding highest point on the time curve of cumulative Fahrenheit degree- days above and below 32°F; the opposite of freezing season. See also growing season.
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thematic mapper—A seven-channel instrument on Landsat series satellites used to make maps of infrared emission and reflection from the earth. Most thematic mapper images are used to study vegetation, geology, and other surface features.
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theodolite—An optical instrument, similar to a surveyors's transit telescope, used to visually track a radiosonde balloon and determine its azimuth and elevation angles while in flight.
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