TROPIC CURRENTS 



logaritlim of the reciprocal of the transmission 

 factor is written with a positive sign and desig- 

 nated as a transmission gain. (28) 



transmission level — Tlie energy at any point in 

 an energy transmission system is the rate of flow 

 of that energy as expressed in terms of (1) a 

 specified reference rate of flow and of (2) the 

 transmission loss by which tlie actual rate of 

 flow must be reduced to equal the reference rate. 

 (28) 



transmission loss — The energy lost in the trans- 

 mission of sound from one point to another; the 

 loss usually is expressed in decibels. In passive 

 sonar the loss is one way, whereas in active 

 sonar it is two way (travel from projector and 

 return of echo) . 



transmittance — The ratio of the transmitted ra- 

 diant flux to the incident radiant flux (in either 

 irradiance or radiance form ) . ( 8 ) 



transparency — That property of water to trans- 

 mit light of different wavelengths. Transpar- 

 ency sometimes is measured in percent of radia- 

 tion that penetrates a distance of one meter; 

 sometimes it is expressed as the average depth 

 at which a Secchi disc disappears and reappears. 



transponder — An automated receiver/transmitter 

 for transmitting signals when triggered by an 

 interrogating signal. 



transport — 1. The process by which a substance 

 or quantity is carried past a fixed point, or across 

 a fixed plane. In oceanography and meteorol- 

 ogy, such quantities are : heat, momentum, mass, 

 dissolved nnpurities, suspended particles, etc. 



(5) 

 2. See flux, 

 tranverse bars — Slightly submerged sand ridges 

 which extend at right angles to the shoreline. 



(2) 



transverse cylindrical orthomorphic projec- 

 tion — See transverse Mercator projection. 



transverse Mercator projection — (also called 

 trcm.'iverse cylindrical orthomorphic projection^ 

 inverse Mercator projection, inverse cylindrical 

 ortlioinorphic projection^ . A conformal cylin- 

 drical map projection in which points on the 

 surface of a sphere or splieroid, such as the eartli. 

 are conceived as developed by Mercator princi- 

 ples on a cylinder tangent along a meridian. 

 This projection is particularly useful for charts 

 of polar regions and for those extending a rela- 

 tively short distance from the tangent meridian. 

 It is frequently used for star charts. (68) 



transverse wave — (also called distort ional wave). 

 A wave in which the direction of propagation 

 of the wave is normal to the displacements of the 

 medium, for example, a vibrating string. The 

 gravity wave in which fluid parcels move in 

 circular orbits is an example of a mixed trans- 

 verse-longitudinal wave. (5) 



traumatic air embolism — See air embolism. 



travel time — The time necessary for waves to 

 travel a given distance from the generating area. 



trawl — 1. A bag- or funnel-shaped net to catch 

 bottom fish by dragging along the bottom. 



2. A large research net designed on bottom 

 trawl principles to catch large zooplankton and 

 fishes by towing in intermediate depths. 



treibeis — See pack ice. 



trench — A long, narrow and deep depression of 

 the sea floor, with relatively steep sides. (62) 

 See foredeep, hadal. 



triangulation — The measurement of a series of 

 angles between points on the surface of the earth, 

 for the purpose of establishing relative posi- 

 tions of the points in surveying. (68) 



Tridacna — See giant clam. 



trim — 1. The attitude of a submerged submarine 

 with respect to neutral buoyancy and fore-and- 

 aft balance. 



2. In a more restrictive sense, the adjustment 

 of submarine buoyancy. 



3. The relation of the draft of a ship at the 

 bow and stern. (68) 



tripton — (or abioseston) . Collectively, all of the 

 dead suspended particulate matter in aquatic 

 habitats. (44) 



Tritium — (abbreviated ^H^ or T) . The hydrogen 

 isotope having one proton and two neutrons in 

 the nucleus (mass number 3) . (70) 



trochoidal wave — A progressive oscillatory wave 

 whose form is that of a prolate cycloid or tro- 

 choid. It is approximated by waves of small 

 amplitudes. (61) 



trochophore — (or trochosphere) . The free-swim- 

 ming pelagic larval stage of some annelids and 

 molTusks. 



trochosphere — See trochophore. 



trophic level — A successive stage of nourishment 

 as represented by links of the food chain. Pri- 

 mary producers (phytoplankton) constitute the 

 first trophic level, herbivorous zooplankton the 

 second trophic level, and carnivorous organisms 

 the third trophic level. 



tropical air — A type of air whose characteristics 

 are developed over low latitudes. Maritime 

 tropical air {inT), the principal type, is pro- 

 duced over the tropical and subtropical seas. It 

 is very warm and humid, and is frequently car- 

 ried polewai'd on the western flanks of the sub- 

 tropical highs. Continental tropical air {cT) 

 is produced over subtropical arid regions, and is 

 liot and very dry. (5) 



tropical cyclone — The general term for a cyclone 

 that originates over the tropical oceans. At ma- 

 turity, tlie tropical cyclone is one of the most 

 intense and feared storms of the world; winds 

 exceeding 175 knots (200 mph) have been meas- 

 ured, ancl its rains are torrential. See hurri- 

 cane. (5) 



tropic currents — Tidal currents occurring twice 

 monthly when the effect of the moon's maximum 

 declination is greatest. Greatest diurnal inequal- 

 ities between speeds and durations of successive 



169 



