IRRADIAKCE 



the flux density of electromagnetic radiation, 

 and is to be distinguished from illuminance. 



(5) 



irradiance (at a point of the surface) — The radi- 

 ant flux incident on an infinitesimal element of 

 surface containing tlae point under considera- 

 tion, divided by the area of that element. Unit 

 of measurement is watt per square meter 

 (W/m=). (8) 



irradiance meter — A radiant flux meter with 

 plane (usually circular) cosine collecting sur- 

 face (usually an opal glass) of effective area A. 

 If F is the radiant flux recorded by the meter, 

 then the associated irradiance is by definition 

 E=F/A. (8) 



irradiance (on a vertical plane) — The radiant 

 flux on an infinitesimal element of a vertical 

 surface (90 degrees) containing the point under 

 consideration, divided by the area of that ele- 

 ment. Unit of measurement is watt per square 

 meter (W/m^). (8) 



irradiance ratio (reflectance) — The ratio of the 

 upward to the downward irradiance at a depth 

 in the sea. (8) 



irradiation (at a point of a surface) — ^The prod- 

 uct of an irradiance and its duration. Unit 

 of measure is Joule per square meter (J/m^). 



(8) 



irrotational wave — A wave in which the particles 

 have an irrotational movement, that is, the rota- 

 tion vector is nil and consequently the speeds 

 are derived from potential energy. 



In a rotational movement the rotation vector 

 is not nil, there is no velocity potential ; any in- 

 finitely small element of the liquid is moved 

 not only by translation along three axis (pro- 

 ducing a change in form) but also by a rotation. 



irruptive rocks — See intrusive rocks. 



Isaac-Kidd midwater trawl — A device designed 

 to collect actively swimming marine organisms 

 from subsurface ocean layers. The midwater 

 trawl consists essentially of a towing bridle, a 

 net, and an inclined-plane surface placed in 

 front of the net to act as a depressor. When 

 streamed, the trawl shape is that of an asym- 

 metrical cone with a large pentagonal mouth 

 opening on one end and a small perforated col- 

 lecting can on the other. 



isabnormal — See isanomal. 



isallotherm — Lines connecting points in which 

 an equal temperature variation is observed 

 within a definite interval. 



isanomal — A line connecting points of equal 

 variations from a normal value. 



isaurore^ — See isochasm. 



isentropic — Of equal or constant entropy with 

 respect to either space or time. (5) 



island — A body of land surrounded by water; 

 relatively smaller than a continent. (30) 



island arc — A term used for a group of islands 

 usually having a curving archlike pattern, gen- 

 erally convex toward the open ocean, with a 



deep trench or trough on the convex side and 

 usually enclosing a deep sea basin on the con- 

 cave side; not considered a recommended term 

 by the ACUF. 



island ice — An ice sheet covering an island. See 

 ice island. (68) 



island shelf — (or insular shelf). See conti- 

 nental shelf. 



island slope — (or insular slope). See conti- 

 nental slope. 



islet — A small island. (30) 



isoanomal — See isanomal. 



isoballast lines — A set of lines, on the submarine 

 bathythermograph chart, starting from a set of 

 selected points on the temperature scale and 

 passing through all points for which the net 

 change in buoyancy resulting from changes in 

 water temperature and depth, is zero for a 

 submarine of given compression. 



isobar — 1. A line on a chart connecting all points 

 of equal or constant pressure; an isopleth of 

 pressure. (5) 



2. One of several nuclides having the same 

 number of nucleons in their nuclei and hence 

 having approximately the same atomic mass. 

 For example, P^^ and S^^ are isobars. Com- 

 monly, one of several nuclides of the same mass 

 number but different atomic numbers. Origi- 

 nally called isobares but the name isobars is 

 now general employed. (41, 70) 



isobaric surface — A surface where the pressure 

 is everywhere the same. This is not necessarily 

 a horizontal surface. If several parallel equally 

 spaced plane surfaces are used to cut an isobaric 

 surface into horizontal sections a pattern of 

 isobaric lines is formed. If the lines are closely 

 spaced, the pressure gradient is strong, less close 

 spacing indicates a weaker gradient. 



isobath — (sometimes called fathom curve, depth 

 contour, and depth curve) . A contour line con- 

 necting points of equal water depths on a chart. 



isobathytherm — A line or surface showing the 

 depths in oceans or lakes at which points have 

 the same temperature. Isobathytherms are 

 usually drawn to show cross sections of the 

 water mass. (5) 



isochasm — ( also called isaurore ) . A line connect- 

 ing points having the same average frequency 

 of auroras. (68) 



isochrone^A line on a chart connecting all points 

 having the same time of occurrence of particular 

 phenomenon or of a particular value of a quan- 

 tity, for example, a line representing all points 

 having the same time difference in the reception 

 of signals from two radio stations such as the 

 master and slave stations of a Loran rate. 



isoclinic line — A line drawn through all points on 

 the earth's surface having the same magnetic 

 inclination. The particular isoclinic line drawn 

 through points of zero inclination is given the 

 special name of aclinic line. (5) 



88 



