A 



AAAS . American Association for the Advancement of 

 Science. 



AAG . Association of American Geographers. 



ABAF . In a direction nearer dead astern than dead 

 ahead. The opposite is forward. (17) 



ABAF THE BEAM . Bearing more than 090° relative 

 but less than 270° relative. The opposite is for- 

 ward of the beam. (17) 



ABDUCTOR. A muscle that draws a part away from the 

 axis of the body or a limb, or separates two parts 

 (such as that which opens the valves of clams). (19) 



ABEAM . Bearing approximately 090° relative (abeam 

 to starboard) or 270° relative (abeam to port). 

 The term is often used loosely for abroad on the 

 beam, or bearing exactly 090° or 270° relative. 

 Also called on the beam. (17) 



ABLATION . 1. The combined processes (such as 

 sublimation, melting, evaporation) which remove 

 snow or ice from the surface of a glacier, snow- 

 field, etc.; in this sense, the opposite of 

 ALIMENTATION. Particularly in glaciology, the 

 term may be applied to reduction of the entire 

 snow- ice mass, and may also include losses by wind 

 action and by CALVING. 



Air temperature is the dominant factor in con- 

 trolling ablation, precipitation amounts exercise 

 only secondary control. During the ablation season, 

 an ablation rate of about two millimeters per hour 

 Is typical of most glaciers. 



2. The amount of snow or ice removed by the 

 above described processes; in this sense, the op- 

 posite of ACCUMULATION. (24) 



ABROLHOS SQUALLS . Rain or thunder squalls of the 

 frontal type experienced near the Abrolhos Islands 

 (18°S.) off Brazil, mainly from May through August. 

 (24) 



ABSOLUTE ISOHYPSE . A line that has the properties 

 of both constant pressure and constant height above 

 mean sea level. Therefore, it can be any contour 

 line on a constant-pressure chart, or any ISOBAR 

 on a constant-height chart. (24) 



ABSOLUTE ZERO . The zero point of the Kelvin tem- 

 perature scale, of fundamental significance in 

 thermodynamics and statistical mechanics. It may 

 be interpreted as the temperature at which the 

 volume of a perfect gas vanishes, or more generally 

 the temperature of the cold source which would ren- 

 der a Carnot cycle 100 per cent efficient. The 

 value of absolute zero on the centigrade scale is 

 now estimated to be -273.16° +.01. (24) 



ABSORPTION . See SOUND ABSORPTION. 



ABSORPTION HYGROMETER . A type of HYGROMETER with 

 which the water vapor content of the atmosphere is 

 measured by means of the absorption of vapor by a 

 hygroscopic chemical. The amount of vapor absorbed 

 may be determined in an absolute manner by weighing 

 the hygroscopic material, or in a non-absolute man- 

 ner by measuring a physical property of the sub- 

 stance that varies with the amount of water vapor 

 absorbed. The lithium chloride humidity strip and 



carbon-film hygrometer element are examples of the 

 latter. (24) 



ABSORPTION LOSS . Absorption loss is that part of 

 the transmission loss which Is due to dissipation 

 or the conversion of sound energy into some other 

 form of energy, usually heat. This conversion may 

 take place within the medium itself or upon a re- 

 flection at one of its boundaries. (9) 



ABYSS . A very deep, unfathomable place. The term 

 is used to refer to a particularly deep part of the 

 ocean, or to any part below 3000 fathoms. (17) 



ABYSSAL-BENTHIC ZONE . See BENTHIC DIVISION. 



ACANTHIN . The strontium sulphate material which 

 makes up the skeletons of some plankton organisms 

 (especially the Acantharia) . (13) 



ACCELEROMETER . A device which measures the forces 

 of acceleration acting upon a body. (35) 



ACCPO. Administrative Committee on Coordination 

 Pertaining to Oceanography. 



ACCRETION (AGGRADATION) . May be e 

 artificial. Natural accretion is 

 up of land over a long period of t 

 action of the forces of nature, on 

 sltion of water- or air-borne mate 

 accretion Is a similar build-up of 

 of an act of man, such as the accr 

 a GROIN, BREAKWATER, or beach fill 

 mechanical means. (11) 



ither natural or 

 the gradual build- 

 ime solely by the 



a BEACH by depo- 

 rial. Artificial 



land by reason 

 etion formed by 



deposited by 



ACCUMULATION . In glaciology, the quantity of snow 

 or other solid form of water added to a glacier or 

 snowfield by ALIMENTATION; the opposite of 

 ABLATION. (24) 



ACLINIC LINE (DIP EQUATOR, MAGNETIC EQUATOR) . The 

 line through those points on the earth's surface 

 at which the magnetic INCLINATION Is zero. The 

 aclinic line is a particular case of an ISOCLINIC 

 LINE. 



In South America the aclinic line lies about 

 15° South latitude; while from central Africa to 

 about Indochina it coincides approximately with 

 the parallel of 10° North latitude. (24) 



ACM. A designation for a U. S. Navy Minelayer, 

 Auxiliary Ship. 



ACMRR . Advisory Committee on Marine Resources 

 Research of the Food & Agriculture Organization of 

 the United Nations. 



ACOUSTIC. ACOUSTICAL . These two qualifying adjec- 

 tives can be confused and in fact are often mis- 

 used. The qualifying adjective acoustic is used 

 when the term which it modifies designates some- 

 thing which has the properties, dimensions, or 

 physical characteristics associated with sound 

 waves. The adjective acoustical, on the other 

 hand, is used when the term being qualified does 

 not innately contain some property, dimension, or 

 physical characteristic which Is intimately asso- 

 ciated with sound. Thus, we speak of an acoustic 

 impedance, but we speak of the Acoustical Society 

 of America. (9) 



ACOUSTIC ATTENUATION CONSTANT . The acoustic atten- 

 uation constant Is the real part of the acoustic 

 propagation constant. The commonly used unit is 

 the neper per section or per unit distance. (2) 



ACOUSTIC AXIS ■ See Axis of Acoustic Symmetry. 



