POLYNYA 



flowmeters and closing devices, or are enclosed 

 within devices that may permit one or more of 

 the following: high-speed towing, quantitative 

 sampling, multiple sampling, and strip 

 sampling. 



plankton pump — A device to raise water from de- 

 sired depths in order to collect plankton exist- 

 ing at those depths ; the water is filtered at the 

 surface. 



plankton recorder — 8ee plankton sampler. 



plankton sampler — (or plankton recorder, flank- 

 ton indicator) . A device for collecting plank- 

 ton, usually designed to take quantitative sam- 

 ples. See plankton net. 



plankton snow — See sea snow. 



plankton tow — See plankton haul. 



planula — A young ovoid planktonic larva of a 

 coelenterate. 



plastic flow — A phenomenon in which bottom 

 sediments under pressure of a weight may flow 

 out from under the weight allowing partial or 

 complete burial. 



plastic limit — See Atterberg limits. 



plateau — A comparatively flat-topped elevation 

 of the sea floor of considerable extent across the 

 summit and usually rising more than 100 fath- 

 oms (200 meters) on all sides. (62) 



plate ice — See pancake ice. 



platform — Any manmade structure (aircraft, 

 ship, buoy, or tower) from or on which oceano- 

 graphic instruments are suspended or installed. 



platform reef — ^An organic reef with a flat upper 

 surface. (2) See table reef. 



pleuston — A community of macroorganisms float- 

 ing on the surface of the sea, for example 

 siphonophores, barnacles, isopods, gastro- 

 pods, etc. See neuston. (44) 



plunge point — 1. For a plunging wave, the point at 

 which the wave curls over and falls. {See fig- 

 ure for surf zone.) 



2. The final breaking point of the waves just 

 before they rush up on the beach. 

 (61) 



plunging breaker — See breaker. 



pluteus — A free-swimming larva of the sea ur- 

 chins and brittle stars, from supposed resem- 

 blance to an upturned easel. 



Plutonic rock — Igneous rock which has cooled 

 some distance below the surface and usually is 

 characterized by a coarse grained structure. 

 (35) 



pneumatocyst — (or vesicle, air Hadder, -float). 

 An air or gas bladder or float; structures so 

 called in siphonophores and in several species 

 of brown algae. 



pneumatophore — In certain organisms, a gas- 

 filled, saclike structure, such as the float of the 

 Portuguese man-of-war, which generally serves 

 as a buoyant mechanism. 



pocket — See blind lead. 



pod — (or school). A number of animals (as seals 

 or whales) closely clustered together. ("We 



lowered for a pod of four or five whales." — 

 Herman Melville) . School (pod is usually used 

 for groups smaller than a school, as, "pods of 

 15 to 25 seals detached from the main group." — 

 M. E. Stansby). 



point — The extreme end of a cape; or the other 

 end of any area protruding into the water, us- 

 ually less prominent than a cape. (61) 



polar air — A type of air whose characteristics are 

 developed over high latitudes, especially within 

 the subpolar highs. Continental polar air {cP) 

 has low surface temperature, low moisture con- 

 tent, and, especially in its source regions, has 

 great stability in the lower layers. It is shallow 

 in comparison with arctic air. Maritime polar 

 air {niP) initally possesses similar properties to 

 those of continental polar air, but in passing 

 over warmer water it becomes unstable with a 

 higher moisture content. (5) 



polar anticyclone — See arctic high. 



polar cap ice — See polar ice. 



polar convergence — A line along which cold polar 

 intermediate water sinks under the warmer 

 subpolar water in its movement toward lower 

 latitudes. It is marked by a sharp change in 

 surface temperatures, particularly in the South- 

 em Hemisphere. See Arctic Convergence, 

 Antarctic Convergence. 



polar drift ice — Drift ice composed exclusively 

 of polar ice. 



polar fast ice — 1. Fast ice formed by the ground- 

 ing and cementing together of polar ice. (74) 

 2. Fast ice of more than one winter's growth 



polar high — See arctic high. 



polar ice — [or polar-cap ice) . 1. Sea ice that is 

 more than one year old (in contrast to winter 

 ice). It is usually the thickest form of sea ice, 

 occasionally exceeding a thickness of ten feet. 

 The WMO Code defines it as any sea ice more 

 than one year old and more than 3 meters (9.8 

 feet) thick. 



2. See arctic pack (sense 3). 



3. A Russian term for any sea ice more than 

 two years old. 



pole or inaccessibility — See ice pole. 



pollutants — Waste products from industry or in 

 sewage. (35) 



polychaete — One of an order (Polychaeta) of 

 annelids which includes most of the marine seg- 

 mented worms, some of which are the tube- 

 worms of fouling. Some of these worms are 

 luminescent during spawning. See tubeworm. 



polyconic projection — A projection where the 

 latitude curves are developed as a series of tan- 

 gent cones. The scale is chosen to be true along 

 a selected central meridian. This projection is 

 neither conformal nor equal area. 



polynya — {or clearing, ice clearing). 1. A water 

 area enclosed in ice, generally fast; this water 

 area remains constant and usually has an oblong 

 shape; sometimes limited to one side by the 

 coast. (74) 



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