ASCIDIAN 



ascidian — (or sea squirt). One of a class (As- 

 cidiacea) of attached or buried tunicates having 

 a saclike body covered by a tough flexible tunic, 

 called a test. They are widespread and notable 

 fouling organisms. 



asea — On the sea ; at sea ; seaward. 



aseismic — Not subject to earthquakes, as an 

 aseismic region. ( 35 ) 



ash breeze — Absence of wind ; calm. ( 68) 



ash fall— (also called ash shoioer). 1. A rain of 

 airborne volcanic ash falling from an eruption 

 cloud, characteristic of volcanic eruptions. 



2. A deposit of volcanic ash resulting from 

 such a fall. (2) 



ash shower — See ash fall. 



aspect — The angle made by a target with the line 

 joining it to the observation point is known as 

 the aspect of the target. (28) 



astacin carotenoid — An animal pigment found 

 especially in the covering of adult planktonic 

 Crustacea. Measurement of this carotenoid 

 in sea water samples gives some measure of 

 microscopic animal matter. 



asthenosphere — See mantle. 



astronomical meteor — See meteor. 



astronomical position — A point on the earth 

 whose position has been determined by celestial 

 observations. (66) 



astronomical tidal constituent — See constitu- 

 ent. 



astronomical tide — (or astronomic tide). Tide 

 due to the attractions of the sun and moon, in 

 contrast to a meteorological tide, which is 

 caused mainly by wind and atmospheric pres- 

 sure. See equilibrium tide. 



astronomic tide — See astronomical tide. 



ASWEPS aircraft — A specially instrumented 

 flying laboratory which conducts thermal struc- 

 ture research in support of ASWEPS. 



asymptotic radiance distribution — The radiance 

 distribution which is the limit of the distribution 

 in the hydrosphere as the depth increases in- 

 finitely. It is symmetrical around the vertical 

 and independent of sun zenith distance. (8) 



Atlantic Ocean — The Atlantic Ocean extends 

 from Antarctica northward to the southern 

 limits of the Greenland and Norwegian Seas. 

 It is separated from, the Pacific Ocean by the 

 meridian of Cape Horn to the Antarctic Con- 

 tinent, and the boundary between the Atlantic 

 and Indian Oceans is placed at the meridian of 

 Cape Agulhas to the Antarctic Continent. 



The Equator separates the Atlantic Ocean 

 into the North and South Atlantic Oceans. 



The limits of the Atlantic Ocean exclude the 

 seas lying within it. 



ATLANTIS — A long-range study to determine 

 the feasibility of large ocean surveillance sys- 

 tems. 



atmosphere — This term when used as a pressure 

 term is defined as the pressure exerted per square 

 centimeter by a column of mercury 760 milli- 

 meters high at a temperature of 0°C where the 

 acceleration of gravity is 980.665 centimeters 

 per second per second. One atmosphere of pres- 

 sure equals 1.0133X10^ dynes per square centi- 

 meter. The actual effect of atmospheric pres- 

 sure is ignored when considering pressures with- 

 in the ocean. Surface pressure is considered to 

 be zero. 



atmospheric pressure — (also called harmnetric 

 pressure). The pressure exerted by the atmos- 

 phere as a consequence of gravitational force 

 exerted upon the "column" of air lying directly 

 above the point in question. ( 5 ) 



atmospheric tide — See tide. 



atoll — A ring-shaped organic reef that encloses a 

 lagoon in which there is no preexisting land, 

 and which is surrounded by the open sea. (56) 



atoll lagoon — See barrier lagoon. 



atollon — A large reef ring in the Maldive Islands 

 consisting of many smaller reef rings. The word 

 atoll was derived from this name. (2) 



atoll reef — A ring-shaped coral and limestone 

 reef often carrying low sand islands, enclosing 

 a body of water. (2) 



atom — The smallest particle of an element which 

 can enter into a chemical combination. All 



CONSPICUOUS FEATURES OF AN ATOLL AND ITS PERIPHERAL REEF 



1. Outer slope; 2. Reef front; 3. Seaward reef margin; 4. Reef flat; 5. Lagoon reef margin; 6. Lagoon slope; 

 7. Reef patch; 8. Lagoon floor; 9. Coral head; 10. Lagoon reef flat; 11. Lagoon beach; 12. Seaward beach. 



12 



