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AMS Glossary
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Section FF index1-9 of 522 terms
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  		•  f-plane approximation—Used in atmospheric dynamics; an approximation in which the Coriolis parameter f is assumed to be invariant with latitude.
  		• f-plane—A local approximation of the spherical earth as a plane normal to the zenithal component of the earth's rotation.
    The rotation rate f is assumed to be constant on the plane. This approximation is valid in describing atmospheric or oceanic motions with time scales smaller than or comparable to 1/f.
  		• F-ratio—(Also called variance ratio.) Ordinarily, the ratio of two independent estimates of a common variance.
    It is used as a significance test in analysis of variance and is known in this regard as the F- test.
  		• F-region—The general region of the ionosphere containing the F1-layer and F2-layer.
  		• F-scale—Abbreviation for Fujita scale.
  		• F-test—See F-ratio.
  		• F1-layer—The layer of the ionosphere that exists as an appendage on the lower part of the F2-layer during the day.
    It exhibits a distinct maximum of free-electron density, except at high latitudes in winter, when it is not detectable. Its virtual height ranges from 200 to 300 km, being lowest at around noon. The F1-layer is formed by absorption of solar radiation in the extreme ultraviolet (EUV) range of wavelengths and is often roughly similar in shape to an idealized Chapman layer.
  		• F2-layer—The highest permanently observable layer of the ionosphere.
    It exhibits a distinct maximum of free-electron density occurring at a height that ranges from about 225 km in the polar winter to over 400 km in daytime near the magnetic equator. Like the other ionospheric layers, the F2-layer is formed by absorption of short-wavelength solar radiation, but its behavior and properties are more complex. Unlike the other ionospheric layers, the F2-layer tends to rise during the middle of the day, except at middle to high latitudes in winter. Its maximum electron density occurs during the day, its minimum usually just before sunrise. It is the layer that is most useful for long-range radio transmission. See F1-layer.
  		• 4D chart—A chart showing the field of D-values (deviations of the actual altitudes along a constant- pressure surface from the standard atmosphere altitude of that surface) in terms of the three dimensions of space and one of time.
    It is a form of a four-dimensional display of pressure altitude. The space dimensions are represented by D-value contours, and the time dimension is provided by tau-value lines. 4D charts are used primarily for relatively long-distance preflight planning over ocean areas. See also S-values.
  		• 46° lateral arcs—A halo in the form of arcs in the vicinity of the halo of 46°, the form of which changes markedly with solar elevation.
    Sometimes given the names supralateral and infralateral, depending on whether the arcs are mainly above or below the sun. The supralateral arc appears to the sides and above the halo of 46° when the sun is low; it is concave toward the sun but vanishes for solar elevations above 32°. The infralateral arc can be either convex or concave. For some elevations, either arc may or may not be tangent to the halo of 46°. These arcs are explained by refraction through the 90° prism ends of columnar ice crystals oriented with their long axis horizontal.
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