104 IN^STKUCTIONS TO MARIXK METEOROLOGICAL OBSERVERS 



Normal. — The average value which in the coiirse of years any meteorological 

 element is found to have on a specified date or during a specified month or 

 other portion of the year, or during the year as a whole. Also used as an 

 adjective in such expressions as "normal temperature", etc. Thus, for any 

 station at which records have been maintained for years, we may compute 

 the normal temperature of January 1. the normal pressure of February, the 

 normal rainfall of the year. etc. The normal serves as a standard with which 

 values occurring in a particular year may be compared in order to determine 

 the departure from normal. 



Niiclrid'i. — A particle upon which condensation of water vapor occurs in the 

 free atmosphere in the form of a water drop or an ice crystal. 



Oblique arcs of the authclioii. — A rare form of halo, consisting of intersect- 

 ing arcs, usually white, passing through the anthelion or the place where the 

 anthelion would occur if visible. 



OccJudvd front. — The front that is formed when and where the cold front 

 overtakes the warm front of a cyclone. This front marks the iiosition of an 

 upper trough of warm air. originally from the warm sector, which has been 

 forced aloft by the action of the converging cold and warm fronts. 



Occlusion. — The term used to denote the process whereby the air in the 

 warm sector of a cyclone is forced from the surface to higher levels. The 

 process is accompanied by an increase in the intensity of the cyclone. 



Ozone. — An allotropic "form of oxygen which occurs transiently in small 

 quantities in the lower atmosphere and is supposed to be permanently present 

 and relatively abundant at high atmospheric levels. 



J'araiithcli'on. — A halo phenonu'uoii similar to a parhelion, but occurring at 

 a distance of IX >° or more in azimuth from the sun. The solar distance of the 

 ordinary i)arantlielia is 120°. (Analogous phenomena produced by the moon 

 as source of light are called parantiselenjie.) 



Paraselene (plural paraseleufe.) — (See Parhelion.) 



Paraselenie circle. — (See Parhelic circle.) 



Parhclic circle. — A halo consisting of a white circle passing through the 

 sun and parallel to the horizon. A similar phenomenon in cijnnection with the 

 moon is called a paraselenie circle. 



Parhelion (plural parhelia). — A mock sun, or sun dog; a form of halo con- 

 sisting of a more or less distinctly colored image of the sun at the same 

 altitude as the latter above the horizon, and hence lying on the parhelic circle, 

 if present. The ordinary parhelia are 22° from the sun in azimuth, or a 

 little more, according to the altitude of the luminary. Parhelia have occa- 

 sionally been seen about 46° from the sun. Analogous phenomena seen in 

 connection with the moon are called paraselenie. mock moons, or moon dogs. 



Pilot balloon. — A small free balloon the drift of which, as observed from 

 the ground, indicates the movements of the air aloft. 



Polar continental air. — The term used to describe any air mass that orig- 

 inates over land or frozen ocean areas in the polar regions. Polar continental 

 air is characterized by low temperatures, low specific humidity and a high 

 degree of vertical stability. 



Polar front. — The surface of discontinuity separating an air mass of polar 

 origin from one of tropical origin. 



Polar maritime air. — The term used to describe any air mass that originally 

 came from the polar regions but has since been modified by reason of its 

 passage over a relatively warm ocean surface. Polar maritime air is char- 

 acterized by moderately low surface temperatures, moderately high surface 

 specific hiunidity, and a considerable degree of vertical instability. 



Potential tcniijerature. — The temperature that a specimen of air or other 

 gas would assume if brought adiabatically to a standard pressure, now usually 

 selected as 1,000 millibars. 



Precipitation. — The collective name for deposits of atmospheric moisture in 

 liquid and solid form, including rain, snow, hail, dew, hoarfrost, etc. 



Pressure. — An elliptical expression, current in meteorological literature, for 

 atmospheric pressure, or barometric pressure. 



Pressure f/radicnt. — The decrease in barometric pressure per unit horizontal 

 distance in the direction in which the ]u-essure decreases most rapidly. 



Prerailinii iresterlics. — The belts of winds lying on the poleward sides of the 

 subtropical high-pressure belts. 



Pst/chromcter. — An instrument for measuring atmospheric humidity, consist- 

 ing usually of a dry-bulb thermometer and a wet-bulb thermometer. The former 



