83 



A characteristic fluctuation of tides of the mixed type is exempHfied 

 by the tide curve at San Francisco, CaHf., at the time of a tropic tide, 

 June 29, 1935, shown in figure 33. 



Hour-s. 



Figure 33.— Tropic tide, San Francisco, June 29, 1935. 



A is the lower low water (LLW), B the lower high water (LHW), Cthe 

 higher low water (HLW) , and D the higher high water (HH W) . 



151. As shown in paragraph 68, the mean speed of the lunar diurnal 

 part of the tide is mi, the speed of the lunar day. It is therefore 

 exactly one-half of the mean speed, m2, of the lunar semidurnal part. 

 Consequently, the resultant of the lunar diurnal components keeps in. 

 general step, from month to month, with the resultant of the lunar 

 semidiurnal components. In most regions, the lunar components 

 are so much larger than the solar that they determine the general shape 

 of the daily tide curves. Usually, therefore, the higher and lower high 

 and low waters at a tidal station always follow one another in the same 

 sequence. If the higher high follows the lower low water, the lower 

 high must follow the higher low, and vice versa, so that the sequence 

 is established either as "HHW to LLW" or as "LLW to HHW." 

 At San Francisco, for example, the sequence is HHW to LLW. At 

 some stations, but exceptionally, the sequence changes during the 

 5^ear. Such a condition is to be anticipated when the principal solar 

 diurnal component, Pi, is relatively large. 



152. Tropic and diurnal ranges, high- and low- water inequalities. — 

 The average difference, from month to month, in elevation between 

 the higher high and the lower low waters of tropic tides is called the 

 great tropic range, and the corresponding difference between the lower 

 high and the higher low waters of tropic tides is called the stnall tropic 

 range. The difference in the average heights of all higher high waters 

 and the average heights of all of the lower low waters from day to day 



