_Flashing Arcs._--Visible atmospheric sound waves, or explosion waves. _Flat._--Featureless; said of weather maps. _Foehn._--A dry fall wind warm for the season, characteristic of many mountainous regions. The air is cooled dynamically in ascending the mountains, but this leads to condensation, which checks the fall in temperature through the liberation of latent heat. The wind deposits its moisture as rain or snow. In descending the opposite slope it is strongly heated dynamically and arrives in the valleys beyond as a warm and very dry wind. Some writers apply this term to any wind that is dynamically heated by descent; e. g., the sinking air of an anticyclone. _Foehn-sickness._--Headache, lassitude, depression, etc., attributed, in the Alpine valleys, to the blowing of the foehn. _Foehn-wall_ (German: _Föhnmauer_).--A wall of cloud that forms along the crest of a mountain ridge over which the foehn is blowing. _Fog._--A cloud at or near the earth’s surface. A fog and a cloud are identical in structure, though the former is due to thermal conditions of the earth’s surface, while the latter is most frequently clue to the dynamic cooling of ascending air. In ordinary speech, the term “fog” generally implies an obscurity of the atmosphere sufficiently great to interfere with navigation or locomotion. (Cf. _dry fog_.) _Fogbow._--A rainbow, colorless or nearly so, formed in a fog. _Fog-drip._--Moisture that is deposited on terrestrial objects by fog, and drips from them to the ground. _Fracto-cumulus_; _Fracto-nimbus_; _Fracto-stratus_.--Forms of cloud. (See Chapter VI.) _Freeze._--1. Freezing of plants without deposit of hoarfrost. 2. Freezing temperatures prevailing generally over a region; not exclusively nocturnal and not confined to the air close to the earth’s surface. (Cf. _frost_.) _Frost._--1. The act or state of freezing. In America, a “frost” generally means the occurrence, near the beginning or end of the growing season, of nocturnal temperatures low enough to be injurious to vegetation; distinguished from a “freeze,” which is more general and severe. The Weather Bureau classifies frosts, according to their effects, as “light,” “heavy,” and “killing.” 2. Atmospheric moisture condensed upon terrestrial objects in the form of ice; sometimes frozen dew. Also called _hoarfrost_. _Frost-smoke._--Frozen fog rising from the water. _Fulgurite._--A glassy tube formed in sandy soil or in rock by the passage of lightning. _Garúa._--A wet fog of the west coast of South America. _Geocoronium._--The name applied by Dr. A. Wegener to a hypothetical atmospheric gas, supposed to be much lighter than any gas now known to chemists.