POLYNYA OFF EDGE OF SHORE ICE 



2. Any enclosed sea water area in pack ice 

 other than a lead, not large enough to be called 

 open water. If a polynya is found in the same 

 region every year, for example, off the mouths 

 of big rivers, it is called a recurring polynya. A 

 temporary small clearing in pack ice which con- 

 sists of small ice floes and brash ice in con- 

 tinuous local movement is called an unstable 

 polynya; an opening which is flanked by large 

 floes and therefore appears to be relatively 

 stable is called a stable polynya. When frozen 

 over, a polynj^a becomes an ice skylight from 

 the point of view of the submariner. (7) 



polynya oflf edge of shore ice — A polynya be- 

 tween shore ice and drift ice formed by squeez- 

 ing winds and currents. (74) 



polyp — An individual sessile coelenterate. 



polytrocular larva — An advanced stage of the 

 planktonic young of certain annelids, in which 

 several segments, each bearing a ring of cilia, 

 are present. 



Polyvinyl (PVC) cor er— See Hydroplastic 

 corer. 



polyzoa — See bryozoan. 



polyzoan — See bryozoan. 



P, constituent — The principal solar diurnal con- 

 stituent of the theoretical tide-producing 

 forces. (See figure for partial tide.) 



pool — Any enclosed relatively small sea area in 

 pack ice, drift ice other than a lead or lane. 

 (74) See polynya, puddle. 



population — The total collection of units being 

 considered statistically. (22) 



porosity — The ratio of the aggregate volume of 

 pore space in a rock or sediment to its total vol- 

 umej usually expressed as a percentage. (2) 



porpoise — A small to moderate sized member of 

 the cetacean suborder Odontoceti. The name is 

 used interchangeably with dolphin by some. 

 More properly it is given to the small and beak- 

 less members of the family Delphinidae, which 

 have a triangular dorsal fin and spade-shaped 

 teeth. 



port plan — A geographical outline of a port area 

 showing piers, railroad extensions, repair facili- 

 ties, pilot office, customhouse, and other applica- 

 ble non-navigational features. 



positive estuary — An estuary in which there is 

 a measurable dilution of sea water by land 

 drainage. 



positive gradient — A positive rate of change with 

 depth. 



potential density — The density that a parcel oi 

 water would have if raised adiabatically to the 

 surface, that is, if determined from the parcel's 

 in situ salinity and potential temperature. 



potential energy— The energy resulting from the 

 elevation or depression of the water surface 

 from the undisturbed level. This energy ad- 

 vances with the wave form. ( 61 ) 



potential temperature — In oceanography, the 

 temperature that a water sample would attain 



if raised adiabatically to the sea surface. For 

 the deepest points of the ocean, which are just 

 over 10,000 meters, the adiabatic cooling would 

 be less than 1.5°C. (5) 



potrero — An accretionary ridge separated from 

 the coast by a lagoon and barrier island, as 

 along the Texas coast. ( 2 ) 



potted — Instruments, connections, or fittings en- 

 capsulated in a waterproof plastic material. 



power gain — The amount (in decibels) by which 

 the output power level exceeds the input power 

 level. Thus, if the output power of a device 

 is 10 times that of the input, the power gain 

 is 10 decibels. On the other hand, if the out- 

 put is one hundred times that of the input, the 

 gain is 20 decibels. (3) 



power level — Power level, in decibels, is 10 times 

 the logarithm to the base 10 of the ratio of a 

 given power to a reference power. The refer- 

 ence power must be indicated. ( 6 ) 



power spectrum — The decomposition of the vari- 

 ance of a random process over frequency inter- 

 vals. 



precision — See accuracy. 



pressure fluctuation — See bottom pressure 

 fluctuation. 



pressure force — (or pressure gradient force). 

 The force due to differences of pressure within a 

 fluid mass. The force per unit volume is equal 

 to the pressure gradient, and the force per unit 

 mass is equal to the product of the volume force 

 and the specific volume. (5) 



pressure gage — A tide gage that is operated at 

 the bottom of a body of water and which records 

 tide heights by the change in pressure due to the 

 rise and fall of the tide. (50) 



pressure gradient — The rate of decrease (gradi- 

 ent) of pressure in space at a fixed time. The 

 term is sometimes loosely used to denote simply 

 the magnitude of the gradient of the pressure 

 field. (5) 



pressure gradient force — See pressure force. 



pressure gradient hydrophone — A hydrophone 

 in which the electric output substantially corre- 

 sponds to a component of the gradient (space 

 derivative) of the sound pressure. (69) 



pressure hydrophone — A hydrophone in which 

 the electric output substantially corresponds to 

 the instantaneous sound pressure of the im- 

 pressed sound wave. (69) 



pressure ice — ( or screw ice ) . Sea ice ( or river or 

 lake ice) whicli has been deformed or altered by 

 the lateral stresses of any combination of wind, 

 water currents, tides, waves, and surf. Tliis 

 may include ice pressed against the shore or over 

 another piece of ice. Rafted ice, tented ice, 

 pressure ridges, and hummocks are forms of 

 pressure ice. 



pressure ice foot — An ice foot formed along a 

 shore by the freezing together of stranded pres- 

 sure ice. (68) 



126 



