| Section F | F index | 401-409 of 522 terms |
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freshet—1. The annual spring rise of streams in cold climates as a result of melting snow. 2. A flood resulting from either rain or melting snow. In this sense it is usually applied only to small streams and to floods of minor severity. 3. A small freshwater stream.
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freshwater—Water that contains less than 1000 mg l−1of dissolved solids.
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Fresnel reflection—In radar, a scattering mechanism proposed to explain certain kinds of clear- air echoes observed by UHF and VHF radars. Such echoes are observed by vertically pointing radars operating at wavelengths of about 1 m and longer. They are in the form of thin, horizontal layers that exhibit strong aspect sensitivity, in the sense that the reflectivity for a vertical beam is greater than that for off-vertical beams. They are thought to be explained by partial reflections from thin layers containing sharp vertical gradients of refractivity. The layers have vertical extents that are comparable to or less than a wavelength and horizontal extents that are as large as the width of the first Fresnel zone, namely, (zλ)½, where z is the altitude and λ is the radar wavelength. Echoes explained by Fresnel reflection have longer coherence times than those explained by Bragg scattering from beam-filling echoes and are more in the nature of specular reflections. Sometimes a distinction is made between Fresnel reflection and Fresnel scattering. The term scattering is used if there are several or many thin reflective layers in the pulse volume; reflection is reserved for the situation of only one layer in the pulse volume.
Röttger, J., and M. F. Larsen, 1990: UHF/VHF radar techniques for atmospheric research and wind profiler applications. Radar in Meteorology, D. Atlas, Ed., American Meteorological Society, 241–242.
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Fresnel zone—A circular zone centered about the direct path between a transmitter and a receiver (or between radar antenna and target), so defined that the distance along a path from transmitter to receiver through a point within the zone has a path length equal to some value between [L + nλ/2] and [L + (n + 1)λ/2], where L is the length of the direct path, λ is the wavelength, and n is a positive integer or zero. The first Fresnel zone is the zone defined by n = 0 and containing the minimum path length. Fresnel zones are a useful concept for analyzing the interference between the direct signal on a propagation path and signals reflected by objects that are displaced from the direct path. Thus, for a given path, reflected radio energy arriving at the receiver from any point will have a phase determined by the particular Fresnel zone in which the point is located. Reflected signals from zones defined by even values of n will interfere constructively with the direct signal; those from zones defined by odd values of n will interfere destructively.
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friagem—(Also spelled vriajem.) A period of cold weather in the middle and upper parts of the Amazon Valley and in eastern Bolivia. Such periods occur during the dry season in the Southern Hemisphere winter.
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friction head—The head that is lost by fluid flowing in a stream or conduit due to friction per unit weight of fluid.
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friction loss—The irrecoverable conversion of flow energy into thermal energy due to shear stress and the generation of turbulent vortices.
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