632 HYDROGRAPHIC MANUAL PaGE 576 



For use in R.A.R. surveys the apparent horizontal velocity of sound (see 6343) 

 should be known with sufficient accuracy so that no error in a plotted distance is intro- 

 duced from this source in excess of 1.0 mm, regardless of scale. The accuracy with 

 which the velocity must be known depends, therefore, on the scale of the survey and 

 the maximum length of the R.A.R. distances: for example, to plot a distance equivalent 

 to 20 seconds of travel time on a 1:80,000 scale survey sheet with this accuracy, the 

 velocity must be known within 4 meters per second. Obviously it is impossible to 

 specify accuracy requirements for all of the various cases for R.A.R., but generally 

 the hydrographer should endeavor to determine the velocity of sound within 3 to 5 

 meters per second. (See 6322 and 6352.) 



632. Temperature of Sea Water 



Of the physical characteristics of sea water, temperature affects the velocity of 

 sound to the greatest degree, particularly in shoal water. A hydrographer using echo 

 sounding or R.A.R. must know with considerable accuracy the vertical distribution 

 of temperature from the surface to the bottom, and the seasonal change in temperature 

 in a given area. Temperature distribution in sea water is best studied by means of 

 graphs. A graph of observed temperatures at a given station plotted with reference 

 to depth is known as a temperature curve (see 6321). The character of the temperature 

 curve varies with latitude, and depth of water and ocean currents have a pronounced 

 effect on it. 



Solar radiation is the principal agent affecting, either directly or indirectly, the 

 temperature of sea water. Water has a high thermal capacity, consequently the water 

 near the surface is at a maximum temperature following the hot season of the year and 

 at a minimum temperature following the cold season. In the waters usually surveyed 

 by the Coast and Geodetic Survey the summer maximum is generally reached in late 

 September or early October, the winter minimum occurring in late February or early 

 March. 



a. Thermocline. — The temperature of the waters near the terrestrial poles is 

 nearly uniform from surface to bottom, and there is very little seasonal variation. 

 In lower latitudes, during the warmer months, the surface water is heated and there is 

 generally a layer of warm water above the- colder water. A decrease of temperature 

 is, therefore, to be expected in sea water as the depth increases. The surface heat is 

 transferred downward by conduction, convection currents, and the stirring of the 

 surface water by wind and waves. The thickness of this upper layer of warm water 

 varies with locality, but it is generally less than 40 fathoms (see figs. 129 and 130). 

 It has been called the epithalassa, the colder water below being known as the hypo- 

 thalassa. Between the warm surface water and the colder w^ater below, there is a 

 layer in which the temperature changes rapidly with depth. Such a temperature 

 gradient is known as a thermocline. Thermoclines are very important in subaqueous 

 sound ranging, for when they are marked, as they generally are during the summer 

 months, a sound wave is refracted rapidly downward. During the colder months the 

 temperature gradient is much less marked. Hence, thermoclines have a great effect 

 on the range at which subaqueous sound can be received, and on the apparent hori- 

 zontal velocity. 



b. In open-ocean areas. — In the open ocean, to moderately high latitudes, there is 

 generally a layer of water of variable depth, below approximately 75 fathoms, in which 

 the temperature decreases rather rapidly with depth. At the bottom of this layer 



