84 



for one or more tropical months is called the great diurnal range, or 

 the diurnal range. The corresponding difference between the average 

 heights of all of the lower high waters and the higher low waters is 

 called the small diurnal range. These are all called declinational 

 ranges, since they depend on the declination of the moon. 



The diurnal high water inequality (DHQ) is defined as the difference 

 between mean higher high water and mean high water. The diurnal 

 low water inequality (DLQ) is similarly the difference between mean 

 low water and mean lower low water. It is apparent that the great 

 diurnal range is equal to the mean range plus (DHQ+DLQ) and the 

 small diurnal range is equal to the mean range minus (DHQ+DLQ). 



153. Diurnal age. — The diurnal age is the interval between the in- 

 stant at which the moon is at its maximum monthly declination, either 

 north or south of the Equator, and the time of tropic tides. 



Since the Ki and Oi components of the equilibrium tides are in con- 

 junction when the moon is at its maximum declination, the phases of 

 these components of the actual tides then differ by the difference of 

 their epochs and these components of the actual tides are in con- 

 junction (Ki°— Oi°)/(ki — Oi) hours later. Therefore: 



Diurnal age (in hours) = (K°i-Oi°)/l. 098 = 0. 911 (Ki°-Oi°) (108) 



For example, at Fort Hamilton, New York Harbor, Ki°=:104° 

 and Oi°=98°. The diurnal age at this station is therefore 



0.911 (104-98) =5.5 hours. 



The diurnal age at a station may amount to several days, and not in- 

 frequently is negative. 



154. As was shown in paragraph 40, the amplitude of the semi- 

 diurnal part of the tide decreases as the declination of the moon 

 increases, while that of the diurnal part increases with the declination. 

 As a consequence the mean daily tidal range tends to decrease with 

 the declination, but this decrease is overshadowed by the increasing 

 range from lower low to higher high water. In tides of the semidiurnal 

 type, the diurnal components do not obscure, to any marked degree, 

 the spring and neap, perigean, and apogean variations due to the 

 S2 and N2 components. In tides of the mixed type the variations 

 in higher high and lower low waters, culminating twice a month in 

 the tropic tides, become the outstanding characteristic, and obscure, 

 more or less completely, the spring, neap, perigean, and apogean 

 tides. In tides of the diurnal type, the diurnal components completely 

 dominate the semidiurnal during a considerable part of the month. 

 The fluctuations of the diurnal tides are, however, frequently so small 

 that meteorological disturbances become their outstanding 

 characteristic. 



