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A/D converter—Abbreviation for analog-to-digital converter, an electronic device for quantifying a (typically) continuous-time voltage signal to a numeric discrete-sample sequence. A/D converters are commonly used in imaging systems to convert pixel values to numeric equivalents, in radar systems to convert signal intensity to digital values for subsequent signal processing, and in many other areas of in situ and remote sensing. Important A/D converter characteristics include the number of unique quantization levels (expressed by the number of binary bits in the resulting numeric values) and the speed of quantization.
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A-display—A radar display on which targets appear as vertical deflections from a horizontal line that represents elapsed time from the most recent transmitted pulse. Distance to the target is indicated by the horizontal position of the deflection from the origin of the time axis; the amplitude of the vertical deflection is a function of the received signal strength. The A-display was the first type of radar display in common use. It may be produced by amplitude modulating the horizontal sweep of an oscilloscope with the received signal. An oscilloscope displaying this is called an A-scope. On an A-scope the difference between the coherent echoes produced by aircraft or ships and the rapidly fluctuating incoherent echoes from precipitation is readily apparent.
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ab-polar current—(Obsolete.) An air current moving away from either of the earth's poles.
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abatement—The lessening or reduction of an atmospheric state that is considered detrimental to humans, animals, plants, or structures. In air pollution meteorology, refers to a reduction in peak intensity, duration, average concentration, or exposure to those chemicals in the air that are considered pollutants.
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abduction—Development of a hypothesis to explain observations; frequently used in diagnostic expert systems; can lead to false conclusions. For example, a particular instance of wind destruction by a microburst might be initially ascribed to a tornado. Compare deduction, induction.
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