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RADIO ACOUSTIC RANGING 



632 



the temperature approaches a minimum and becomes relatively constant, usually at 

 a depth not greater than 400 fathoms. The temperatures in this layer are relatively 

 stable and are generally only slightly affected by the seasonal variation in atmospheric 

 temperature. This layer might be called the stationary thermocline. (See fig. 130.) 



Above the stationary thermocline the temperature is unstable throughout a layer 

 whose thickness varies with latitude and other characteristics of the area. Its tem- 

 perature varies rapidly with respect to depth, and in summer the seasonal thermal 

 progression raises its temperature far above that of the stationary thermocline. It 

 might be called a summer thermocline. The summer thermocline may extend from the 

 surface downward to join the stationary thermocline, but it is unstable, because wind, 

 surface waves, and changes in density cause mixing in the surface layers. This mixing 

 has been observed to extend to a depth of 35 fathoms during a storm. Consequently, 

 above the summer thermocline there is generally a layer of variable thickness in which 

 the physical characteristics, excepting hydrostatic pressure, are nearly uniform. 



c. Coastal waters.— The temperature conditions in waters on the Continental Shelf, 

 or near land, are generally different from those found in the open ocean, for the tem- 

 peratures throughout the entire depth of water generally experience a seasonal cycle 

 and the stationary thermocline is seldom evident. The summer thermocline is generally 

 more pronounced in shoal water than in the open ocean. Occasionally there is a layer 

 of colder surface water that has been moved inshore from the open ocean by winds 

 causing what is known as a temperature gradient reversal. Wliere such temperature 

 reversals are experienced, R.A.R. is usually difficult in operation. 



In some localities and over offshore shoal areas, where there are strong currents 

 and winds, the temperature curve is vertical (if plotted as shown in fig. 129) during 



TEMPERATURE DEGREES CENTIGRADE 



10 11 12 



14 



. SKOGSBERG "Hydrography of Monterey Bay.' 



Figure 129.— Annual cycle of temperature distribution in sea water near the surface. 



