AEROGRAPHER'S MATE 3 & 2 



sets up a vertical thermohaline circulation in 

 the water mass resulting in an increase in the 

 depth of the mixed layer, 



EVAPORATION. — Evaporation causes the 

 surface water to lose water vapor and heat. As 

 the water vapor escapes into the atmosphere, 

 the solids are left behind in the uppermost 

 layer of water. This situation causes the upper- 

 most layer of water to become more dense 

 than the underlying water. This sets up vertical 

 density currents resulting in an increase in 

 the mixed layer depth. Evaporation is the most 

 effective cooling process and the single most 

 important factor in "instability mixing" of sea 

 water. Instability mixing of sea water results 

 from an increase in density and salinity which 

 is produced by evaporation. 



TEMPERATURE GRADIENT 



Temperature gradient refers to the rate 

 of temperature change with distance. Vertical 

 gradients refer to changes with depth, and 

 horizontal gradient refers to changes along a 

 horizontal plane. Within a given water mass 

 vertical gradients are most important, while 

 at the boundaries between water masses or 

 currents, horizontal gradients are significant. 



The gradient is called positive if the tem- 

 perature increases with depth; negative if it 

 decreases. The temperature gradient is of 

 considerable interest to the forecaster since 

 it is the primary factor considered when 

 determining the quality of sound transmission. 



With a zero gradient, the sound rays are 

 very nearly straight, having only a slight 

 upward curvature due to the effect of pressure. 



With a strong negative temperature gradient 

 from the surface downward, the sound beam 

 will curve down in an arc, creating a shadow 

 zone into which very little sound penetrates 

 except by scattering. In this case, an echo 

 ranging vessel may not be able to detect a 

 target located in the shadow zone. However, 

 as soon as the target comes within the direct 

 beam, the echoes will come in loud and clear. 

 This is the situation illustrated in figures 16-4 

 and 16-5 in which the submarine in figure 16-4 

 is in the direct beam and the one in figure 

 16-5 is in the shadow zone. 



In the ocean it is relatively common to find 

 a mixed layer overlying a negative temperature 



gradient. In such cases the echo range on a 

 target in the mixed layer will be long, but 

 the part of the sound beam that enters the 

 negative gradient will be refracted downward, 

 resulting in reduction of range. With slighter 

 negative temperature gradients and generally with 

 any gradient underlying a mixed layer, the 

 shadow zone will not be clearly defined. 



SOUND RAY TRANSMISSION 

 PATHS UNDER WATER 



Before the Aerographer's Mate can correctly 

 analyze the conditions under which a sound 

 beam (packet of acoustic energy) will be received 

 as an intelligible signal after it has been 

 transmitted through the various mixtures of 

 sea water, he must be able to determine the 

 paths that the sound rays are likely to take. 

 This includes determining energy loss due to 

 reflection, absorption, and spreading along the 

 way. 



Snell's Law 



In order to determine underwater ray paths, 

 you should realize that sound rays will always 

 bend in the direction of minimum sound velocity. 

 This is the fundamental principle of sonar 

 range prediction and is derived from SNELL'S 

 LAW. 



Based on Snell's law, the ray path of sound 

 in water will refract upward if the velocity 

 of sound increases with depth. The ray path 

 will refract downward if the velocity of sound 

 decreases, 



Basic Sound Ray Patterns 



Figure 16-6 illustrates some of the basic 

 patterns which might be encountered in the 

 transmission of sound rays, 



A brief discussion of the basic patterns 

 shown in figure 16-6 is as follows: 



1. Straight line rays. This example shows 

 a very slight negative temperature pattern as 

 indicated by the sample bathythermogram trace 

 at the left side of the figure (decrease of about 

 0.2°/100 feet of depth) which results in an 

 isovelocity (no change in velocity) structure. 

 The corresponding sound pattern shows rays 

 leaving the sound source in straight lines which 

 show very little change in the angle as the 



380 



