INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS IQl 



Frontoffcnesis. — The term used to describe the process which creates a front 

 i. e., produces a discontinuity in a continuous field of the meteorological ele- 

 ments; also applied to the process which increases the intensity of a pre- 

 existing front. Frontogenesis is generally set up by the horizontal conver- 

 gence of air currents possessing widely different properties. 



Frontoli/sis. — The term used to describe the process which tends to destroy 

 a pre-existing front. Frontolysis is generally brought about by horizontal 

 mixing and divergence of the air within the frontal zone. 



Frost. — Atmospheric moisture deposited upon terrestrial objects in the form 

 of ice crystals. Also called hoarfrost. 



Frost smoJce. — A fog produced by apparent steaming of the sea in the pres- 

 ence of air having a temperature much below freezing. Also called "Arctic 

 sea smoke." 



Gale. — Wind with an hourly velocity exceeding some specified value. In 

 American practice a wind of or exceeding force 8 on the Beaufort scale is 

 counted a gale. 



Glaze. — Term applied by the United States Weather Bureau to a smooth 

 coating of ice on terrestrial objects due to the freezing of rain ; often popu- 

 larly called sleet. In Great Britain such a deposit is called glazed frost. 

 A deposit of glaze on an extensive scale constitutes an "ice storm." 



Glory. — A series of concentric colored rings seen around the shadow of the 

 observer, or of his head only, cast upon a cloud or fog bank. It is due to 

 the diffraction of reflected light. 



Gradient. — Change of value of a meteorological element per unit of distance. 

 The gradients commonly discussed in meteorology are the horizontal gradient 

 of pressure, the vertical gradient of temperature, and the vertical gradient 

 of electric potential. Meteorologists now prefer the term "lapse-rate" to "ver- 

 tical gradient." 



Gradient wind. — A wind of the velocity which is necessary to balance the 

 pressure gradient. The direction of the gradient wind is along the isobars, 

 and the velocity is so adjusted that there is equilibrium between the force 

 pressing the air toward the region of low pressure, and the centrifugal action 

 to which the moving air is subject in consequence of its motion. 



Granular snow. — A form of precipitation consisting of small nontranspar^nt 

 grains of snow. 



Green flash. — A bright green coloration of the upper edge of the sun's disk, 

 sometimes seen when the rest of the disk is below the horizon at sunrise or 

 sunset. 



Gust. — A sudden brief increase in the force of the wind. Most winds near 

 the earth's surface display alternate gusts and lulls. 



Hail. — Balls or irregular lumps of ice, often of considerable size, having a 

 complex structure ; large hailstones generally have a snowlike center, sur- 

 rounded by layers of ice, which may be alternately clear and cloudy. Hail 

 falls almost exclusively in connection with thunderstorms. (Cf. Sleet.) 



Halo. — A generic name for a large group of optical phenomena caused by 

 ice crystals in the atmosphere. The commonest of these phenomena is the 

 halo of 22° (i. e., of 22° radius), surrounding the sun or moon. The halo of 

 46° and the rare halo of 90°, or halos of Hevelius, also surround the luminary. 

 Other forms of halo are the tangent arcs, parhelia (or paraselenae), parhelic 

 (or paraselenic) circle, anthelion, etc. 



Haze. — A lack of transparency in the atmosphere caused by the presence 

 of dust or of salt particles left by evaporated ocean spray. At a certain 

 distance, depending on the density of the haze, all details of landscape and of 

 color disappear. (See also part V.) 



High. — An area of high barometric pressure; an anticyclone. 



Hoarfrost.— (See Frost.) 



Hot wave. — A period of abnormally high temperatures. It has sometimes 

 been defined, in the United States, as a period of three or more consecutive 

 days during each of which the maximum temperature is 90° F. or over. 



Humidity. — The degree to which the air is charged with water vapor. This 

 may be expressed in several ways. Absolute liumidity expresses the weight 

 of water vapor per unit volume of air; relative htiniiditi/ is the ratio of the 

 actual vapor pressure to the vapor pressure corresponding to saturation at the 

 prevailing temperature, or simply the percentage of saturation; speciflc liu- 

 midity expresses the mass of water vapor contained in a unit mass of moist 

 air. Specific humidity is the only truly conservative air mass property of the 

 three. 



