INTRODUCTION 



The monthly mean and yearly mean and extreme densities of 

 sea water given on the following pages are based on hydrometer 

 readings which usually were made once each day from water sam- 

 ples drawn from near the surface. These observations were made 

 at tide stations maintained by the U. S. Coast and Geodetic 

 Sui*vey along the Pacific Coast of the United States, Alaska, 

 and the Hawaiian Islands, and at stations maintained coopera- 

 tively in Chile and Peru. A supplemental table is included 

 which shows average monthly surface salinities at places along 

 the California coast as furnished by the Scripps Institution 

 of Oceanography, 



The UNIT OF DENSITY is the density of fresh water at a tem- 

 perature of 4° C (39.2° F) . The actual density of the water 

 may vary from a little less than unity for fresh water at a 

 temperature other than 4° C to approximately 1.0310 for the 

 heaviest sea water. Since the density of sea water, as ob- 

 served, depends not only upon the amount of soluble matter 

 contained in a unit volume, but also upon the temperature of 

 the water at the time the reading was made, it is necessary to 

 reduce the obsei'ved densities to some standard temperature in 

 order that they may be comparable and indicate the amount of 

 matter held in solution. IN THIS PUBLICATION THE OBSERVED 

 DENSITIES HAVE BEEN REDUCED TO A STANDARD TEMPERATURE OF 15° C 

 (59° F) . The density of pure water at a temperature of 15° C 

 is taken as 0.9991. 



The SALINITl of sea water is defined as the number of grams 

 of salts contained in 1000 grams of sea water. While the total 

 amount of salts contained in a given volume of sea water varies 

 in different places, the relative portions of the different 

 kinds of salt are nearly constant in all parts of the ocean. 

 The salinity of sea water may be determined by several different 

 methods, one of the simplest being based upon the density of 

 the water as obtained from use of the hydrometer. The table 

 on the inside back cover gives the salinity corresponding to 

 different values of density at the standard temperature of 15° C 

 to which all densities in this publication are referred. 



In the following table the mean and extreme densities for 

 each series are given on the line labeled "Mean density" when- 

 ever the series covers two years or more. On the next line the 

 corresponding salinities are given. Other density values in 

 the table can be converted to salinity, if desired, by means 

 of the table in the back of the book. The seasonal variation 

 in salinity is shown graphically for Coast and Geodetic Survey 

 stations where the observations cover five years or more. 



Other publications in this series, which are available 

 from the U. S. Coast and Geodetic Survey, are as follows: 



TW-1. Surface Water Temperatures, Atlantic and Gulf Coasts. 

 TW-2. Surface Water Temperatures, Pacific Coast. 

 DW-1. Density of Sea Water, Atlantic and Gulf Coasts. 



