AEROGRAPHER'S MATE 3 & 2 



ISOVELOCITY GRADIENT. — Values of sound 

 velocity are the same in all parts of a given 

 water column; no change in sound velocity with 

 depth. 



MIXED LAYER. — The layer of water which 

 is mixed through wave action or therm ohaline 

 convection. Frequently, this is referred to as 

 the layer. 



MDCED LAY<iR DEPTH. — The depth of the 

 bottom of the mixed layer. Frequently, this is 

 referred to as the layer depth. 



NEGATIVE TEMPERATURE GRADIENT.— 

 A decrease in temperature with depth. 



POSITIVE TEMPERATURE GRADIENT.- 

 increase in temperature with depth. 



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REFLECTION. — Sound rays transmitted in 

 the sea eventually reach either the surface or 

 the bottom. As these boundaries are abrupt 

 and very different in sound transmitting 

 properties from the water, sound energy along 

 a ray path striking these boundaries will be 

 returned (reflected) to the water. 



REFRACTION. — The bending or curving of a 

 sound ray that results when the ray passes 

 obliquely from a medium of one velocity to a 

 medium of another velocity. The sound wave 

 will be bent toward the medium of lower 

 velocity. When sound rays enter the layer of 

 differing velocity perpendicular to the layer 

 (90° angle of incidence), no refraction occurs. 



REVERBERATION. — Sound from a source 

 that has been scattered back towards the 

 source, principally from the ocean surface 

 (surface reverberation) or bottom (bottom 

 reverberation), andfrom small scattering sources 

 in the medium such as bubbles of air and 

 suspended solid matter (volume reverberation). 



RISE. — A long broad elevatio.i rising gently 

 from the ocean bottom. 



SALINITY. — The salinity of sea water 

 represents the total amount of dissolved solid 

 material (in grams) contained in one kilogram 

 (1,000 grams) of water. It usually is expressed 

 in parts per thousand (%o). 



SCATTERING. — The random dispersal of 

 sound energy after it is reflected from the sea 

 surface or bottom and/or off the surface of 

 solid, liquid, or gaseous particles suspended 

 in the water. 



SEAMOUNT. — A submerged, isolated, 

 mountainlike structure rising from the ocean 

 bottom . 



SHADOW ZONE. — A region into which very 

 little sound energy penetrates. 



SILL. — A submerged elevation separating two 

 basins. 



SILL DEPTH. — The greatest depth at which 

 free horizontal exchange of matter between 

 basins can take place. 



SOUND VELOCITY. — In sea water, the rate 

 of propagation of sound energy as a function 

 of temperature, salinity, and pressure. 



SURFACE DUCT. — Where the sound velocity 

 at some depth near the surface is greater 

 than at, the surface, sound rays are refracted 

 toward the surface where they are reflected. 

 Tlie rays alternately are refracted and 

 reflected along the duct to considerable dis- 

 tances from the sound source. 



THERMOCLINE. — The layer in the sea where 

 the temperature decreases rapidly with depth. 



TRANSPARENCY. — The ability of water to 

 transmit light or be seen through. 



TRENCH. — A long narrow depression in the 

 ocean bottom having steep sides. 



T-S DIAGRAM. — (Temperature-Salinity dia- 

 gram.) The plot of temperature versus salinity 

 data of a water column. It identifies the water 

 masses in the column and indicates the stability 

 of the water column. 



VELOCITY GRADIENT, — The rate of change 

 of sound velocity with depth in the ocean. 



VISCOSITY. — The resistance of a substance 

 to flow or be moved; viscosity is higher in 

 cold water than in warm water. 



WATER MA.SS. — A body of water having a 

 distinct range of temperature and salinity. It 



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