Chapter III 

 CHARACTERISTICS OF THE TIDES 



Paragraphs 



Types of tides 139-140 



M2 component, semidiurnal tides 141-142 



S2 component, spring tides 143 



Phase age 144 



N2 component, perigean tides 145 



Parallax age 146 



Combined effect of S2 and N2 components 147-148 



Ki and Oi components, tropic tides, and diurnal inequalities 149-152 



Diurnal age '. 1 153 



Overtides 155 



139. Types of tides. — The tides throughout the world are of three 

 general types, which are determined by the relative magnitude of the 

 semidiurnal part of the tide (as indicated by the amplitudes of the 

 principal semidiurnal components, M2, S2, and N2) and of the diurnal 

 part of the tide (as indicated by the principal diurnal components, 

 Ki and Oj). These types are: 



(a) The semidiurnal or semidaily tides. — Tides of this type have two 

 nearly equal high waters and two nearly equal low waters each lunar 

 day of 24 hours 50 minutes. They occur when the amplitudes of the 

 diurnal components are small in comparison with those of the semi- 

 diurnal components. This type is found along both coasts of North 

 and South Atlantic Oceans, and at other places as well. 



(6) Mixed tides. — This type is characterized by two markedly un- 

 equal high waters, or two markedly unequal low waters, or both, on 

 each lunar day, during most of the month. Tides are of this type 

 when the amplitudes of the diurnal components are considerable in 

 comparison with those of the semidiurnal components, but do not 

 greatly exceed the latter. Such tides are common, but not universal, 

 along the coasts of the Pacific Ocean. 



(c) Diurnal tides. — Tides of this type have but one high water and 

 but one low water each day during a substantial part of or all of the 

 month. Such tides are common along the coasts of large enclosed 

 seas with restricted entrances, such as the Gulf of Mexico, the Carib- 

 bean, the waters of the East Indies, and the Mediterranean; and some- 

 times at oceanic islands. Diurnal tides are usually quite small and 

 irregular. 



140. Tides of the semidiurnal type usually have some diurnal in- 

 equality during the two periods in each month when the moon is 



192750—40 6 (75) 



