Fall wind-
A wind that accelerates as it moves downslope because of its low temperature and greater density.
False warm sector-
The sector, in a horizontal plane, between the occluded front and a secondary cold-front of an occluded cyclone.
Family of tornadoes-
A sequence of long-lived tornadoes produced by a “cyclic” supercell storm.
Tornadoes touch down at quasi-regular intervals (typically 45 min). Usually a new tornado develops in a new mesocyclone just after an old tornado has decayed in an old, occluded neighboring mesocyclone. Sometimes, two successive tornadoes may overlap in time for a few minutes. The two mesocyclones may rotate partially around each other. If the damage tracks of the tornadoes appear to form a wavy broken line, the family is classified as a series mode. In the more common parallel-mode family, the damage tracks are parallel arcs with each new tornado forming on the right side of its predecessor. The parallel mode is subcategorized into left turn and right turn, according to the direction in which the paths curve.
Fanning-
A pattern of smokestack plume dispersion in a statically stable atmosphere,
in which the plume spreads out in the horizontal like an oriental fan and meanders about at a fixed height with little vertical spread.
Fine line-
A narrow radar echo indicating a boundary (such as a frontal boundary, gust front, or dryline) across which a density or moisture discontinuity exists. The reflectivity is apparently explained by scattering from the refractive index gradients across the discontinuity and from insects and insect-eating birds that are concentrated along the line. See clear-air echo.
Flanking line-
An organized lifting zone of cumulus and towering cumulus clouds, connected to and extending outward from the mature updraft tower of a supercell or strong multicell convective storm.
Flare echo-
A weak radar echo in the form of a tapered radial extension beyond the far edge of a severe thunderstorm, associated with the presence of large hydrometeors, especially hail, in the storm.
Flux-
The rate of flow of some quantity, often used in reference to the flow of some form of energy.
Foehn-
A warm, dry, downslope wind descending the lee side of the Alps as a result of synoptic-scale, cross-barrier flow over the mountain range.
Foehn cloud-
Any cloudform associated with the foehn, usually referring to standing clouds of two types, orographic clouds and mountain wave clouds. Orographic clouds may include crest clouds and the foehn wall. Wave clouds may consist of lenticular (including altocumulus standing lenticular, or ACSL) clouds, lee-wave clouds and cloud bands, and rotors. See also Bishop wave, chinook arch, contessa di vento, Moazagotl.
Foehn wall-
The leeward edge of the orographic stratiform cap cloud as seen from the lee side of a mountain barrier, preceding or during a foehn or chinook event. The edge is generally abrupt and resembles a wall of cloud (“foehnwand” in German). This cloud often signifies the occurrence of orographic precipitation, especially snowfall in the cold season, over the peaks. See foehn cloud.
Foehn wave-
Mountain waves, lee waves, or trapped lee waves in the air stream flowing over the mountain barrier that occur in association with foehn conditions. The Moazagotl is one example of a foehn wave made visible by lee-wave clouds.
Fog-
Water droplets suspended in the atmosphere in the vicinity the earth's surface that affect visibility.
Fog wind-
The humid east wind that crosses the divide of the Andes east of Lake Titicaca and descends on the west in violent squalls; probably the same as puelche.
Forano-
A sea breeze of Naples in Italy.
Forced cloud-
Small cumulus clouds that are everywhere negatively buoyant compared to the surrounding environment, but that exist because the inertia of the rising thermals feeding them from underneath is sufficient to penetrate the lifting condensation level (LCL). Morphologically, these are often cumulus humilis clouds and are typically found at the top of the convective boundary layer during daytime over land, when a strong
temperature inversion aloft prevents the clouds from growing deeper. Compare active cloud, passive cloud.
Freezing-
The phase transition of a substance passing from the liquid to the solid state; solidification; the opposite of fusion.
Freezing fog-
A fog the droplets of which freeze upon contact with exposed objects and form a coating of rime and/or glaze.
Frequency-
The rate of recurrence of any periodic phenomenon, often associated with waves of all kinds. Without qualification frequency often means temporal frequency, the rate of recurrence of a time-varying function, but could mean spatial frequency, the rate of recurrence of a space-varying function. Spatial frequency is the reciprocal of the repeat distance (sometimes the wavelength). The dimensions of (temporal) frequency are inverse time. A common unit for frequency is cycle per second, formerly abbreviated cps, but superseded by hertz, abbreviated as Hz.
Fresh breeze-
In the Beaufort wind scale, a wind with a speed from 17 to 21 knots (19–24 mph) or Beaufort Number 5 (force 5).
Fresh gale-
In the Beaufort wind scale, a wind with a speed from 34 to 40 knots (39–46 mph) or Beaufort Number 8 (force 8).
Front-
1. In meteorology, generally, the interface or transition zone between two air masses of different density. Since the temperature distribution is the most important regulator of atmospheric density, a front almost invariably separates air masses of different temperature. Along with the basic density criterion and the common temperature criterion, many other features may distinguish a front, such as a pressure trough, a change in wind direction, a moisture discontinuity, and certain characteristic cloud and precipitation forms. The term front is used ambiguously for 1)
frontal zone, the three- dimensional zone or layer of large horizontal density gradient, bounded by 2) frontal surfaces across which the horizontal density gradient is discontinuous (frontal surface usually refers specifically to the warmer side of the frontal zone); and 3) surface front, the line of intersection of a frontal surface or frontal zone with the earth's surface or, less frequently, with a specified constant-pressure surface. Types of front include polar front, arctic front, cold front, warm front, and occluded front. See also anafront, katafront, intertropical front, secondary front, upper front. 2. See wave front.
Frontal fog-
Fog associated with frontal zones and frontal passages. It is usually divided into three types: warm-front prefrontal fog; cold-front post-frontal fog; and frontal-passage fog. The first two types are a result of rain falling into cold stable air and raising the dewpoint temperature. Frontal-passage fog can result from the mixing of warm and cold air masses in the frontal zone or by sudden cooling of air over moist ground.
Frontal inversion-
A temperature inversion (temperature rising with height) in the atmosphere, encountered upon vertical ascent through a sloping front (or frontal zone).
Frontal lifting-
The forced ascent of the warmer, less dense air at and near a front, occurring whenever the relative velocities of the two air masses are such that they converge at the front.
Frontal wave-
A horizontal wavelike deformation of a front in the lower levels, commonly associated with a maximum of cyclonic circulation in the adjacent flow. It may develop into a wave cyclone.
