| Section A | A index | 471-479 of 917 terms |
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amorphous frost—Hoarfrost that possesses no apparent simple crystalline structure; opposite of crystalline frost. The lack of distinct crystal structure in forms of amorphous frost, however, is only a matter of scale. Such frost is built up of a multitude of units each of which has its own crystal structure, although no unit fits compatibly with its neighboring unit.
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amorphous sky—A state of the sky, characterized by an abundance of fractus clouds, usually accompanied by precipitation falling from a higher, overcast cloud layer. It is termed “amorphous” because the clouds lack any distinctive form (bands, undulations, clear-cut base, etc.).
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amount of precipitation—Depth to which precipitation in liquid form would cover a horizontal projection of the earth's surface, in the absence of abstractions. It is typically expressed in millimeters, equivalent to liters per square meter. Snowfall is also measured by the depth of fresh snow covering a horizontal surface.
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amplitude-modulated indicator—(Also called deflection-modulated indicator.) One of two general classes of radar indicator in which the sweep of the electron beam is deflected vertically or horizontally from a base line to indicate the existence of an echo from a target. The amount of deflection is usually a function of the echo signal strength. Common types of amplitude-modulated indicators give only the slant range between radar and target. However, since the characteristics of the target signal can be easily observed with this type of indicator, it is very useful in radar meteorology.
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amplitude modulation—Changing the amplitude of a carrier wave by some means so that the resultant waveform transmits a signal conveying information. For example, if the carrier wave with angular frequency ω is cos(ωt), the corresponding amplitude-modulated signal is  where g(t) is a function that varies more slowly with time than does the carrier wave. Compare frequency modulation.
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amplitude—Often the greatest magnitude at a given point of any spatially and temporally varying physical quantity governed by a wave equation; can also mean the spatial part of a time-harmonic wave function. For example, in the time-harmonic (or sinusoidal) scalar wave function with circular frequency ω,  where φ(x) is the (complex) amplitude of the wave, although the modulus of φ also may be called its amplitude. The (complex) amplitude of the scalar plane harmonic wave  with wavenumber k and initial phase θ is A exp(ikx − iθ), the modulus of which,  , is also called the amplitude of the wave. In its most general sense, amplitude means extent or size. Thus the amplitude of a wave is some measure of its size.
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