EKMAN SPIRAL 



echinoderm — One of a phylum (Echinodermata) 

 of principally benthic marine animals having 

 calcareous plates with projecting spines form- 

 ing a rigid or articulated skeleton or plates and 

 spicules embedded in the skin ; the animals have 

 radial symmetry, usvuilly is a Hve-rayed body. 

 Some echiiioderms are the sea stars, sea 

 urchins, crinoids, and sea cucumbers. 



echinopluteus — The planktonic larva of a sea 

 urchin. 



echo — An acoustic signal which has been reflected 

 or otherwise returned with suiRcient magnitude 

 and time delay to be detected as a signal distinct 

 from that directly transmitted. ( 3 ) 



echogram — 1. The graphic presentation of echo 

 soundings recorded as a continuous profile of 

 the bottom. 



2. Often erroneously called a fathogram 

 when not recorded by a Fathometer*. 



echo ranging — The determination of distance by 

 measuring the time interval between transmis- 

 sion of a radiant energy signal (sound) and the 

 return of its echo. ( 66 ) 



echo sounder — See echo sounding. 



echo sounding — {or acoustic sounding) . Deter- 

 mination of the depth of water by measuring the 

 time interval between emission of a sonic or 

 ultrasonic signal and the return of its echo from 

 the bottom. The instrument used for this pur- 

 pose is called an echo sounder. ( 68 ) 



ecology — See marine ecology. 



eddy — A circular movement of water usually 

 formed, where currents pass obstructions, be- 

 tween two adjacent currents flowing counter to 

 each other, or along the edge of a permanent 

 current. 



eddy-built bar — Sediment deposit believed to be 

 formed by the rotating action of eddies in a tidal 

 lagoon. Ridges surrounding some emerged 

 Carolina bays may have developed in this wav. 

 (2) 



eddy conduction — See eddy heat conduction. 



eddy conduction coefficient — See eddy conduc- 

 tivity. 



eddy conductivity — (also called coefficient of 

 eddy diffusion, eddy condu<^tion coefficient). 

 The exchange coefficient for eddy heat con- 

 duction. (5) 



eddy current — See eddy. 



eddy diffusion — (or turbulent diffusion). The 

 transfer of matter by the turbulent eddies in a 

 fluid. 



eddy flux — The rate of transport (or flux) of 

 fluid properties such as momentum, mass, heat, 

 or suspended matter by means of eddies in a 

 turbulent motion; the rate of turbulent ex- 

 change. (5) 



eddy heat conduction — (or eddy heat fua\ also 

 called eddy conduction) . The transfer of heat 

 by means of eddies in turbulent flow, treated 

 analogously lio molecular conduction. (5) 



eddy heat flux— /See eddy heat conduction. 



eddy viscosity— The turbulent transfer of 

 momentum by eddies giving rise to an internal 

 fluid friction, in a manner analogous to the 

 action of molecular viscosity in laminar flow, but 

 taking place on a much larger scale. 



The value of the coefficient of eddy viscosity 

 is of the order 10^ square centimeters per second. 

 (5) 



edge wave — An ocean wave traveling parallel to 

 a coast, with crests normal to the coastline. 

 Such a wave has a height that diminishes rapidly 

 seaward and is negligible at a distance of one 

 wavelength offshore. (5) 



eel grass — See seagrass. 



effective acoustic center (apparent source) — 

 The effective acoustic center of an acoustic gen- 

 erator is the point from which the spherically 

 divergent sound waves, observable at remote 

 points, appear to diverge. ( 67 ) 



effective back radiation — See effective terres- 

 trial radiation. 



effective half-life — The time required for a radio- 

 active element fixed in the tissue of an animal 

 body to be diminished 50 percent as a result of 

 the combined action of radioactive decay and 

 biological elimination. (70) 



effective radiation — See effective terrestrial 

 radiation. 



effective sound pressure — The sound pressure 

 at a point is the root-mean-square value of the 

 instantaneous sound pressures over a time inter- 

 val at the point under consideration. 



effective terrestrial radiation — (or effective 

 radiation, effective hack radiation). The dif- 

 ference between the outgoing infrared terres- 

 trial radiation of the earth's surface and the 

 downward infrared counterradiation from the 

 atmosphere. (5) 



ejecta — (or ejectamenta) . Material thrown out 

 by a volcano, such as ash, lapilli, bombs. (2) 

 See tephra. 



ejectamenta — See ejecta. 



Ekman current meter — A mechanical device for 

 measuring ocean current velocity. A sensitive 

 impeller is turned hy current action and the 

 number of turns recorded on an attached dial. 

 Speed is measured indirectly from the number of 

 impeller revolutions by means of conversion 

 tables. Lead shot are dropped into a compass 

 box below the meter for a set number of impeller 

 revolutions. Current direction is determined 

 from the location of shot dropped in the com- 

 pass box. 



Ekman Spiral — A theoretical representation of 

 the effect that a wind blowing steadily over an 

 ocean of unlimited depth and extent and of 

 uniform viscosity would cause the surface layer 

 to drift at an angle of 45 degrees to the right of 

 the wind direction in the Northern Hempishere. 



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