| Section F | F index | 491-499 of 522 terms |
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frostless zone—(Also called thermal belt, thermal zone, green belt, verdant zone.) That warmest part of a slope above a valley floor lying between the layer of cold air that forms over the valley floor on calm, clear nights and the cold hilltops or plateaus. The air flowing down the slopes is warmed by mixing with the air above ground level and to some extent also by adiabatic compression. The frostless zone is not a fixed belt but varies in level from night to night and season to season according to the initial temperature, the length of the night, and the clearness of the sky. Its lower limit is sometimes clearly marked by the upper limit of frost damage to crops, following the hillsides at a small angle to the horizontal. See thermal belt.
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Froude number—1. The nondimensional ratio of the inertial force to the force of gravity for a given fluid flow; the reciprocal of the Reech number. It may be given as  where V is a characteristic velocity, L a characteristic length, and g the acceleration of gravity; or as the square root of this number. 2. For atmospheric flows over hills or other obstacles, a more useful form of the Froude number is  where NBV is the Brunt–Väisälä frequency of the ambient upstream environment, V is the wind speed component across the mountain, and Lw is the width of the mountain. Fr can be interpreted as the ratio of natural wavelength of the air to wavelength of the mountain. Sometimes π will appear in the numerator, and other times the ratio will be squared. When Fr = 1, the natural wavelength of the air is in resonance with the size of the mountain and creates the most intense mountain waves, which can sometimes contain lenticular clouds and rotors of reverse flow at the surface. For Fr < 1, some of the low-altitude upstream air is blocked by the hill, short-wavelength waves separate from the top of the hill, and the remaining air at lower altitudes flows laterally around the hill. For Fr > 1, very long wavelengths form downwind of the hill, and can include a cavity of reverse flow just to the lee of the hill near the surface. Another form of the Froude number, using (zi − zhill) in place of Lw, is useful for diagnosing downslope windstorms and hydraulic jump, where zi is the depth of the mixed layer above the base of the mountain, and zhill is the height of the mountain.
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frozen ground—Soil within which the moisture has predominantly changed to ice, the unfrozen portion being in vapor phase. Ice within the soil bonds (adfreezes) adjacent soil particles and renders frozen ground very hard. “Permanently” frozen ground is called permafrost. “Dry” frozen ground is relatively loose and crumbly because of the lack of bonding ice. Frozen ground is sometimes inadvisedly called frost or ground frost.
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frozen-in-field—A magnetic field in a (zero resistivity) electrically conducting fluid (or plasma), so named because when the plasma moves, the magnetic lines of force may be said to move with it. The field lines are fixed within the plasma. Because a plasma is an electrical conductor, when it moves in a magnetic field an electric current results, which in turn produces a magnetic field that adds to the existing field. The net field (assuming negligible resistivity) is as if the existing field lines had moved with the plasma.
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frozen turbulence—Turbulence that advects with the mean wind and is not statistically changed during the advection process. In measurements, frozen turbulence is also referred to as Taylor's hypothesis, which allows time series measured at a single point to be interpreted as spatial variations.
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Fujita–Pearson scale—(Abbreviated FPP scale.) Characterizes a tornado's intensity (F) by its path length (PL) and path width (PW). The six-point scale for PL and PW is as follows. The six-point scale for damage intensity is presented under the Fujita scale.
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