AEROGRAPHER'S MATE 3 & 2 



60' 



50 



LATITUDE 

 30° 



20* 



£ 3,000 



209.320 

 Figure 16-3. — North-south distribution of a simple three-layered ocean (North Atlantic) in winter. 



Areas in which precipitation or evaporation 

 dominate vary with latitude and with seasonal 

 changes. 



Latitudinally a salinity minimum exists 

 near the equator where precipitation is abundant, 

 increasing to a salinity maximum near 20° 

 latitude N and S where evaporation far exceeds 

 precipitation. Salinity decreases again toward 

 the polar seas where heavy precipitation and 

 the fresh water inflow from melting ice and 

 river runoff cause a minimum. 



Greatest evaporation occurs when the air 

 mass is colder than the underlying water; 

 therefore on a seasonal basis, surface salinity 

 values are higher in early spring and lower 

 in late fall. 



PRESSURE 



The pressure of sea water is measured in 

 terms of decibars. A decibar is one-tenth of 

 a bar. As mentioned in chapter 12, a bar is 

 1 million dynes per square centimeter. 



Pressure in the oceans is essentially a 

 function of depth, with a 1-meter increase in 

 depth equal to a 1-decibar increase in pressure. 

 The numerical value of pressure in decibars 

 is equal to the water depth in meters. There- 

 fore, the range in pressure is from zero at 



the surface to over 10,000 

 deepest part of the ocean. 



DENSITY 



decibars in the 



As has been mentioned previously in this 

 manual, density refers to mass per unit volume. 

 The density of sea water is usually expressed 

 in grams per cubic centimeter. Density is 

 dependent upon temperature, salinity, and 

 pressure. 



Density increases when the salinity or 

 pressure increases, but it decreases when the 

 water expands with increasing temperature. When 

 these properties are known, the density can 

 be determined readily from standard density 

 tables. 



Under given conditions of temperature and 

 pressure, ocean waters that have the greatest 

 salinity also have the greatest density. Conse- 

 quently, these waters have a tendency to sink. 

 Since sea water is slightly compressible, as a 

 mass of water sinks, the temperature of the 

 mass increases adiabatically without an actual 

 gain of heat. This occurs in the same manner 

 as is observed in a parcel of air that is brought 

 from a higher elevation to a lower elevation 

 with a consequent adiabatic increase in tem- 

 perature due to greater pressure. Pressure 



376 



