INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 97 



Air mass. — A term applied by meteorologists to an extensive body of air 

 within which the conditions of temperatnre and moisture in a horizontal iilane 

 are essentially uniform. 



Air tnass property. — Any quality or quantity the nature or value of which 

 can be used in a characterization of the physical state or condition of an air 

 mass. 



Altocmnulus. — A form of cloud. (See part V.) 



Altostratiis. — A form of cloud. (See part V.) 



Anemograin. — The record traced by a self-registering anemometer. 



Anemometer. — An instrument for measuring the force or speed of the wind. 



Anemoscope. — An instrument for indicating the existence of wind and 

 showing its direction. 



Aneroid l>aro)iieter. (See part II.) 



Anomaly. — The difterence between the mean of any meteorological element, 

 or phase of that element, over a given time at a particular place, and the 

 mean of the same element or phase over the same time for all other points on 

 the same parallel of latitude. 



Anthelion. — A rare species of halo, consisting of a brilliant, usually white, 

 image of the sun opposite the latter in azimuth. (This term has also been 

 applied to the glory, q. v.). 



AnticrepiisciUar rays. — The continuation of the crepuscular rays, converg- 

 ing toward a point in the sky opposite to the sun. 



Anticycloyenesis. — The term applied to the process which creates or de- 

 velops a new anticyclone. The word is applied also to the process which 

 produces an intensification of a pre-existing anticyclone. 



Anticyclone. — ^An area of high barometric pressure and its attendant sys- 

 tem of winds. (Cf. Cyclone.) 



Antitrades. — The term applied to the westerly winds which are observed 

 at high elevations above the trade winds. 



Antitivilight arch. — The pinlc or purplish zone of illumination bordering the 

 shadow of the earth (dark segment), in the part of the sky opposite the sun 

 after sunset and before sunrise. 



Aqueous vapor. — Water vapor. (Cf. Humidity.) 



Arcs of Loivitz. — A pair of rare halo phenomena. These arcs are directed 

 obliquely downward from the parhelia of 22° on either side of the sun toward 

 the halo of 22°. 



Arctic front. — The line of discontinuity between very cold air flowing directly 

 from the Arctic regions and polar maritime air that has moved away from its 

 source region in a more or less circuitous path and been warmed through 

 contact with the ocean surface. 



Atmosphere. — The whole mass of air surrounding the earth. 



Aurora. — A luminous phenomenon due to electrical discharges in the atmos- 

 phere ; probably confined to the tenuous air of high altitudes. It is most 

 commonly seen in sub-Arctic and sub-Antarctic latitudes. Called aurora bore- 

 alis or aurora australis, according to the hemisphere in which it occurs. Ob- 

 servations with the spectroscope seem to indicate that a faint "permanent 

 aurora" is a normal feature of the sky in all parts of the world. 



Back. — Of the wind, to shift in a counterclockwise direction ; opposite of 

 veer. In scientific practice this definition now applies to both hemispheres. 



Bar. — ^A unit of pressure equal to 1,000,000 dynes per square centimeter. 

 A bar = 100 centibars = 1,000 millibars. A barometric pressure of one bar is 

 sometimes called a "C. G. S. atmosphere," and is equivalent to a pressure of 

 29.531 inches of mercury at 32° F. and in latitude 45°. 



Barocyclonometer. — One of the several instruments that have been devised 

 for locating tropical hurricanes without the aid of a weather map. 



Barogram. — The continuous record made by a self-registering barometer. 



Barograph. — A self-registering barometer. 



Barometer. — An instrument for measuring the pressure of the atmosphere. 

 The two principal types are the mercurial and the aneroid. The microba- 

 rometer is used to show very small changes of pressure. 



Barometric tendency. — The change of barometric pressure within a specified 

 time (usually three hours), before the observation. 



Beaufort scale. — The scale of wind force devised by Admiral Sir Francis 

 Beaufort in 1805. 



Bishop's ring. — A large corona due to fine dust in the atmosphere. It has 

 been seen after certain great volcanic eruptions, especially that of Krakatoa, 

 in 1883. 



