sound speed in surface sea water at 0°C and with salinity of 35 parts 

 per thousand is thus 1449 meters per second as contrasted with 1403 

 meters per second for fresh water at the same temperature and pressure. 



Equation (1) shows that change in temperature has the greatest 

 effect on v. In the upper 10 meters of the ocean considerable tempera- 

 ture variations may exist. Below 100 meters in deep water the temperature 

 generally decreases more regularly. Often in the upper 100 meters a 

 surface layer of water that is almost isothermal (the surface duct) 

 overlies a thermocline of rapidly decreasing temperature. Areas where 

 variations in salinity become important are: near the mouths of larger 

 rivers or in estuaries and deltas, where appreciable amounts of fresh 

 water run into the sea; in the vicinity of large ocean currents; in 

 shallow water or water close to the surface (where rain and evaporation 

 have greatest effect) ; and near fields of melting ice. Other serious 

 possibilities about which little is known may occur on a smaller scale 

 due to thermohaline convection or "salt fingers". 



Variation of the velocity with depth caused by the increasing 

 pressure is quite regular, i.e., 0.017 meters per second increase per 

 meter increase in depth. This effect is masked by temperature effects in 

 the upper region of the ocean except in the case of an isothermal surface 

 layer. However, at depths greater than about 1000 meters, the temperature 

 is very nearly constant (nearly the temperature of maximum density) , and 

 the velocity then increases at the pressure effect rate. Thermal anomalies 

 may exist at abyssal depths and in any water column, from a few meters above 

 the bottom to the bottom. 



For sonar operations a knowledge of the sound speed profile 



3rtbur ZD.IUttkllnr. 



