IRRADIANCE 



a gradual density change, somewhere in the in- 

 terior of the fluid and not at the free upper 

 surface wliere the surface waves have their maxi- 

 mum amplitude. ( 61 ) 



Because of the small density gradient in- 

 volved, compared to that at the surface, a rel- 

 atively small amount of energy is required to set 

 up and maintain an internal wave. Wave 

 heiglits, periods, and lengths are usually large as 

 compared to surface waves. 



International Gravity Formula — A development 

 of the formula for theoretical gravity, based 

 on the assumptions that the spheroid of reference 

 is an exact ellipsoid of revolution having the 

 dimensions of the International Ellipsoid of 

 Reference, rotating about its minor axis once 

 in a sidereal day ; that the surface of the ellipsoid 

 is a level surface; and that gravity at the Equa- 

 tor equals 978.049 gals. 



international low water — (abbreviated ILAV). 

 A plane of reference below mean sea level by 

 the following amount; half the range between 

 mean lower low water and mean higher high 

 water multiplied by 1.5. 



International Synoptic Code — A synoptic code 

 approved by the World Meteorological Orga- 

 nization in which the observable meteorological 

 elements are encoded and transmitted in "words" 

 of five numerical digits length. ( 5 ) 



intersecting waves — {ov sugarloaf sea, pyramidal 

 sea) . One of the component waves which, when 

 superimposed on others, produces cross swells. 



interstitial water — Water contained in the pore 

 spaces between the grains in rock and sediments. 



interstratified — iS'ee interbedded. 



intertidal — See littoral. 



intertidal zone — ( also called littoral zone) . Gen- 

 erally considered to be the zone between mean 

 high water and mean low water levels. {See 

 figure for classification of marine environ- 

 ments.) 



intracellular bioluminescence — A widespread 

 form of biological light production usually as- 

 sociated with special luminous organs (photo- 

 phores) or luminous cells which contain photo- 

 genic granules. In some organisms intracel- 

 lular light may be produced by luminous bacteria 

 living within special sacs or organs. In all orga- 

 nisms, the light emitted is internal, below the 

 body surface. 



intrainment force — (or fricfional drag) . Motion 

 induced in water in physical contact witli a re- 

 gion of relatively high velocity. ( 20) 



intrusive rocks — (or irrupt ive rocks). Igneous 

 rocks which while molten, have penetrated into 

 and between other rocks but which have solidi- 

 fied before reaching the surface of the earth or 

 the sea floor/water interface. They may either 

 have penetrated fissures in the original rocks or 

 absorbed and replaced the original rocks. 



inverse cylindrical orthomorphic projection — 

 See transverse Mercator projection. 



inverse estuary — An estuary in which evapora- 

 tion exceeds land drainage plus precipitation, 

 with resulting mixture of high salinity estuarine 

 water and sea water. 



inverse law — See spreading of sound. 



inverse Mercator projection — See transverse 

 Mercator projection. 



inversion layer — A layer of water in which tem- 

 perature increases with depth. ( 25 ) 



invertebrate — Any animal without a backbone or 

 spinal column. 



inverted tide — See reversed tide. 



ion — An electrically charged group of atoms either 

 negative or positive. The dissolved salts in sea 

 water dissociate into ions. 



ion exchange — A chemical process involving the 

 reversible interchange of ions between a solution 

 and a particular solid material such as an ion ex- 

 change resin consisting of a matrix of insoluble 

 material interspersed with fixed ions of opposite 

 charge. (70) 



ionization — Any process by which a neutral atom 

 or molecule loses or gains electrons, thereby 

 acquiring a net charge ; the process of producing 

 ions. (41) 



ionizing particle — A particle that directly pro- 

 duces ion pairs in its passage through a sub- 

 stance. In practice it is a charged particle 

 having considerably greater kinetic energ-y than 

 the ionizing energy appropriate to the medium. 

 (41) 



ionizing radiation — Any electromagnetic or par- 

 ticulate radiation capable of producing ions, 

 directly or indirectly, in its passage through 

 matter. (70) 



ion pair — A positive ion (cation) and a nesrative 

 ion (anion) (usually, an electron) that have 

 charges of the same magnitude and are formed 

 from a neutral atom or molecule by the action of 

 radiation. A primary ion pair is an ion pair 

 produced by a primary particle or photon. 



Irish moss — (or canvgeen, carragheen) . One of 

 several species of red algae, but particularly 

 CKondrus crisfus, having a short bushy form 

 and often forming a carpet on rocks in the 

 lower intertidal zone. Carrageenin, or carra- 

 gheenin is prepared from this alga. 



Irminger Current — An ocean current that is one 

 of the terminal branches of the Gulf Stream 

 system (part of the northern branch of the 

 North Atlantic Current) ; it flows west oft" the 

 south coast of Iceland. 



A small part of the Irminger Current turns 

 clockwise around the west coast of Iceland but 

 the greater amount turns southwest, and joins 

 the East Greenland Current. 



irradiance — (or irradiation). The total radiant 

 flux received on a unit area of a given real or 

 imaginary surface. This is a general term for 



87 



