102 



U. S. COAST AND GEODETIC SURVEY 



TYPES OF TIDE 



From place to place the characteristics of the rise and fall of the tide generally 

 differ in one or more respects; but according to the predominating features the 

 various kinds of tide may be grouped under three types, namely, semidiurnal, 

 diurnal, and mixed. Instead of semidiurnal and diurnal the terms "semidaily" 

 and "daily" are frequently used. 



The semidiurnal type of tide is one in which two high and two low waters occur 

 each tidal day with but little diurnal inequality; that is, morning and afternoon 

 tides resemble each other closely. Figure A may be taken as representing this 

 type of tide, and this is the type found on the Atlantic coast of the United States. 



In the diurnal type of tide but one high and one low water occur in a tidal day. 

 Do-Son, French Indo-China, may be cited as a place where the tide is always of 



Fig. B.— Tide curves, San Francisco, Calif., October 18 and 24, 1922 



the daily type, but it is to be noted that there are not many such places. When 

 the moon's declination is zero the diurnal tidal forces tend to vanish, and there 

 are generally two high and two low waters during the day at such times. Gal- 

 veston, Tex., and Manila, P. I., may be mentioned as ports at which the tide is 

 frequently diurnal, while St. Michael, Alaska, may be cited as a port at which 

 the tide is largely diurnal. 



The mixed type of tide is one in which two high and two low waters occur 

 during the tidal day but which exhibits marked diurnal inequahty. Several 

 forms may occur under this type. In one form the diurnal inequality is exhib- 

 ited principally by the high waters; in another form it is the low waters which 

 exhibit the greater inequality; or the diurnal inequality may be features of both 

 high waters and low waters. 



It is to be noted that when the tide' at any given place is assigned to any 

 particular type, it refers to the characteristics of the predominating tide at that 

 place. At the time of the moon's inaximum semimonthly declination the semi- 

 diurnal type exhibits more or less diurnal inequality and thus approaches the 

 mixed type; and when the moon is on or near the Equator the diurnal inequality 



