Tides 



129 



Figure 11 5. Oscillations of 5°C 

 isothermal surface in Santa 

 Catalina Basin as inferred from 

 temperature variations meas- 

 ured during a 25-hour period 

 at Station 2988. From Emery 

 (1956ZJ, Fig. 5). 



JAlslUARY 28. 



22 OO 02 04 06 08 



I9S5 f|^ JANUARY 2 9. I95S 



diurnal tidal components, which in turn are 

 results of the shape of the ocean basin 

 (Marmer, 1932) or of other causes (Sverdrup, 

 Johnson, and Fleming, 1942, pp. 545-564). 

 The greatest diurnal asymmetry in spring 

 tides occurs near the summer and winter 

 solstices (June 22 and December 22) and the 

 least near the equinoxes (March 21 and 

 September 23). 



For the region the mean range is 3.7 feet, 

 but the extreme is 8.7 feet, reached during 

 the spring tides of the solstices (Fig. 116). 

 The annual mean of the lower of the two 

 low tides which occur each day is the datum 

 plane for hydrographic charts of the area — 

 mean lower low water. Some topographic 



maps of the land, however, are based on 

 mean sea level, 2.8 feet above mean lower 

 low water. The lowest of the low low tides 

 occur at the same time of the year as the 

 maximum daily range and are used to ad- 

 vantage by collectors of marine life along the 

 shore. These very low tides (called minus 

 tides when below the level of mean lower 

 low water) occur during winter in the after- 

 noon and during summer in the early morn- 

 ing hours before sunrise. The timing dur- 

 ing the summer is fortunate because, if the 

 lower shore were laid bare in the afternoon, 

 it would become heated and dried out so 

 much that many small animals restricted to 

 that zone would be killed. 



M 



M 



MOON 



Figure 116. Range between highest and lowest tide of each day during 1956 at San Pedro. Note close relationship of 

 dates of spring tides to times of new moon (black) and full moon (white). At the bottom is shown the time of day at 

 which minus (below mean lower low water) tides occur; note that extreme low tides occur in the afternoon during 

 winter and before sunrise during summer. 



