connected to a given source Is the rms voltage which 

 would be measured at its load terminals if they were 

 open-circuited. (4) 



OVERFALLS . Breaking waves caused by a conflict of 

 currents or by the wind moving against the current. 

 (14) 



OVERLOAD LEVEL . The overload level of a component 

 or system is that level at which operation ceases 

 to be satisfactory as a result of signal distortion, 

 overheating, etc. In an acoustical system, sound 

 pressure level is to be understood, unless other- 

 wise specified. (2) 



OVERT IDE . A harmonic tidal constituent with a 

 speed that is an exact multiple of the speed of one 

 of the fundamental constituents derived from the 

 development of the tide-producing force. The pres- 

 ence of overtides is usually attributed to shallow- 

 water conditions. The overtides usually considered 

 in tidal work are the harmonics of the principal 

 lunar and solar semidiurnal constituents M2 and $2 

 and are designated by the symbols l\, Mg, Mg, S4, 

 Sg, etc. The magnitudes of these harmonics rela- 

 tive to those of the fundamental constituents are 

 usually greater in the tidal current than in the 

 tide. (14) 



OVERTURN (CONVECTIVE OVERTURN) . The renewal of 

 bottom water that occurs annually in lakes and 

 ponds in regions wherever winter temperatures are 

 cold enough. As the surface waters are cooled in 

 the autumn and early winter, they become denser and 

 therefore sink, until the whole body of water is 

 at 4°C, the temperature of maximum density. Fur- 

 ther cooling is restricted to the surface layers, 

 since both ice and water colder than 4 C are less 

 dense than the underlying waters at 4°C. 



This phenomenon does not take place over most 

 areas in the ocean because of the salinity strati- 

 fication, and usually the resulting brine is dense 

 enough to sink to the bottom only in the extreme 

 polar regions, where extensive freezing occurs. 

 (12) 



OVERWASH . That portion of the uprush that carries 

 over the crest of a BERM or of a structure. (11) 



OVIPAROUS . In reproduction, animals are either 

 oviparous or viviparous . The oviparous forms de- 

 posit eggs that develop outside the mother's body, 

 while in the VIVIPAROUS forms the young are nour- 

 ished by the mother and are born alive in a post- 

 embryonic state. An intermediate condition exists 

 in the OVOVIVIPAROUS forms, where the eggs are in- 

 cubated and hatched within the body, as in certain 

 sharks, perch, and blennies . The term larviparous 

 is sometimes used to indicate that larval stages 

 are born. An embryo derives its nourishment from 



the yolk of the egg or directly from the mother, 

 whereas typically a larva is morphologically adapted 

 with mouth and digestive tract for the purpose of 

 seeking its own nourishment. (13) 



OVOVIVIPAROUS . See OVIPAROUS. 



OWN-SHIP'S NOISE . In many cases the limiting noise 

 registered by a sonar receiver is that set up by 

 the vessel itself or as a result of its motion. 

 This has come to be known as own-ship's noise. (4) 

 (Also known as self noise.) 



OXYGEN CONCENTRATION CELL . A cell established on 

 a metal surface caused by a difference in oxygen 

 concentration in the solution at one point as com- 

 pared to another. (35) 



OXYGEN DEFICIENCY . See ANOXIA. 



OXYGEN TOXICITY . Pure oxygen cannot be breathed 

 indefinitely at pressures greater than atmospheric. 

 Following a safe period which becomes shorter as 

 diving depth increases, symptoms of oxygen toxicity 

 occur. These include involuntary fine twitches 

 around the eyes and mouth that later extend to in- 

 clude larger muscle groups including the diaphragm, 

 causing abruptness of inspiration. Anxiety, and 

 apprehension may occur and some times loss of 

 lateral visual fields and ringing in ears. These 

 preliminary symptoms are followed by general con- 

 vulsions and unconsciousness. These can be avoided 

 by not exceeding the safe depth-time relationships 

 for pure oxygen breathing. The onset of oxygen 

 poisoning can usually be arrested by inhaling a few 

 breaths of a gas mixture with a high proportion of 

 nitrogen or other inert gas . Permanent after effects 

 from oxygen poisoning do not appear to occur, even 

 after repeated exposures. This of course does not 

 alter the risk for the underwater swimmer at depths 

 great enough to produce toxicity. 



The basic physiological mechanism of oxygen 

 poisoning and the effects of oxygen at depths from 

 15 to 60 feet, and greater, are subjects that re- 

 quire much more study, since oxygen poisoning is 

 the major factor limiting diving depth and duration 

 with mixed gas as well as pure oxygen apparatus . 

 (37) 



OYASHIO CURRENT . A North Pacific Ocean current 

 flowing southwestward from the Bering Sea, along 

 the southeast coast of Siberia and the Kuril 

 Islands, and then curving southward and eastward, 

 the greater part then joining the NORTH PACIFIC 

 CURRENT, and the remaining northern part continu- 

 ing eastward to feed the ALEUTIAN CURRENT. The 

 Oyashio is formed by the merging of those parts 

 of the Aleutian and ALASKA CURRENTS that enter 

 the Bering Sea from the south and flow northward 

 and northwestward to join with water flowing 

 southward through Bering Strait. (17) 



P 



PACK ICE . Any large area of floating ice which has 

 been driven closely together. (15) 



PAIGH. Pan American Institute of Geography and 

 History. 



Headquarters: Mexico City. 



Established in 1928 in Havana by a resolution 

 adopted at the 5th Conference of American States. 

 In 1903 recommendations were made for the creation 

 of a Pan-American organization dealing with Geo- 



graphy and Cartography and the establishment of an 

 Inter-American Institute of Oceanography. The 

 present organization is an outgrowth of this recom- 

 mendation. 



Activities: Principal scientific programs of 

 the Institute are conducted by three commissions 

 on cartography, geography and history. PAIGH has 

 established special committees on Oceanography, the 

 International Geophysical Year, Volcanology, etc. 



PALEOCRYSTIC ICE . PRESSURE ICE, usually more than 

 ten years old, well weathered, and irregularly 

 heaped and tumbled. The type locality is the 

 Lincoln Sea. (25) 



PAN . See PANCAKE ICE. 



PANCAKE ICE (PLATE ICE) . Circular pieces of newly- 



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