u 



Udden grade scale — A grade scale for particle 

 size, with 1 millimeter as the reference point and 

 involving the fixed ratio 2 or %, depending on 

 whether the scale is increasing or decreasing, as 

 1/4,1/2,1,2,4. (5) 



ultraplankton — Plankton smaller than 5 mi- 

 crons; includes bacteria and smaller flagellate 

 forms. 



ultrasonics — The technology of sound at frequen- 

 cies above the audio range ; that is, above 20,000 

 cycles per second. (3) 



ultraviolet radiation — Electromagnetic radia- 

 tion of shorter wavelength than visible radiation 

 but longer than X-rays; roughly, radiation in 

 the wavelength interval from 10 to 4,000 ang- 

 stroms. (5) 



umbrella — (or hell). The gelatinous body of a 

 jellyfish, usually bearing prominent tentacles. 

 It may be bell-, dome-, bowl-, or saucer-shaped 

 according to species. 



unbroken ice — Sea ice which has not been dis- 

 turbed since its formation. It is usually fast 

 ice, although a single smooth ice floe could be 

 said to be unbroken ice. 



unconformity iceberg — An iceberg consisting of 

 more than one kind of ice, such as blue water- 

 formed ice and neve. Such an iceberg often 

 contains many crevasses and silt bands. (68) 



unconsolidated sediments — See sediment. 



uncovers — (or dries) . An area of a reef or other 

 projection from the bottom of a body of water 

 which periodically extends above and below the 

 surface. (30) 



unda — The part of the ocean floor which lies in 

 the zone of wave action, in which the bottom 

 sediments are repeatedly stirred and reworked ; 

 the topographic expression is termed undaf orm, 

 and the rock unit is termed undathem. 



undaform — See unda. 



undathem — See unda. 



undercurrent — A water current flowing beneath 

 a surface current at a different speed or in a 

 different direction. (5) 



underflow — See bottom flow. 



undermelting — The melting from below of any 

 floating ice. (5) 



undersea satellite — A basketball-size device con- 

 taining a gauge (to measure tides) , a seismom- 

 eter, and a transmitter. 



undertow — 1. A seaward flow near the bottom of 

 a sloping beach. 



2. The subsurface return by gravity flow of 

 the water carried up on shore by waves or break- 

 ers. See rip current. (50) 



underwater gradient — See slope, gradient. 



underwater ice — See anchor ice, frazil ice. 



underwater sound — See sound. 



underway — The condition in which a ship is mak- 

 ing headway against the seas; as opposed to 

 hove-to. 



undulation — A continuously propagated motion 

 to and fro, in any fluid or elastic medium, with 

 no permanent translation of the particles them- 

 selves. 



uniform flow — Any current in which neither con- 

 vergence nor divergence is present. 



unilateral transducer — A transducer than can- 

 not be aotuated at its outputs by waves in such 

 a manner as to supply related waves at its in- 

 puts. (69) 



unprotected thermometer — A reversing ther- 

 mometer (for sea water temperature) which is 

 not protected against hydrostatic pressure. The 

 mercury bulb is therefore squeezed, and the 

 amount of mercury broken ofl' on reversal is a 

 function both of temperature and hydrostatic 

 pressure. 



Wlien compared with the simultaneous read- 

 ing of a protected thermometer, which is af- 

 fected by temperature only, the unprotected 

 thermometer reading can be converted to pres- 

 sure, and then, by applying the mean density 

 of the water, to depth. (5 ) 



unstable polynya — See polynya. 



unstable-type gravimeter — A gravity meter 

 which utilizes a moving system which ap- 

 proaches a point of instability such that small 

 changes in gravity produce relatively large mo- 

 tions of the system. 



upcoast — In United States usage, the coastal di- 

 rection generally trending toward the north. 

 (2) 



updrift — The direction opposite that of the pre- 

 dominant movement of littoral materials. (61) 



uplifted reef — A coral reef exposed above water 

 level. (2) 



upper transit — See transit. 



uprush — The rush of water up onto the beach fol- 

 lowing the breaking of a wave. See swash, run- 

 up. (61) {See figure for surf zone.) 



upstream — Generally, in the direction from which 

 a fluid is flowing; the opposite of downstream. 

 (5) 



172 



