APPENDIX 



GENERAL CHARACTERISTICS OF TIDES AND CURRENTS 



[Reprinted from United States Coast and Geodetic Survey Special Publication No. Ill] 



I. TIDES, GENERAL CHARACTERISTICS 



DEFINITIONS 



The tide is the name given to the alternate rising and falling of the level of 

 the sea which at most places occurs twice daily. The striking feature of the 

 tide is its intimate relation to the movement of the moon. High water and low 

 water at any given place foUow the moon's meridian passage by a very nearly 

 constant interval, and since the moon in its apparent movement around the 

 earth crosses a given meridian, on the average, 50 minutes later each day, the 

 tide at most places likewise comes later each day by 50 minutes, on the average. 

 The tidal day, like the lunar day, therefore, has an average length of 24 hours 

 and 50 minutes. 



With respect to the tide, the "moon's meridian passage" has a special signifi- 

 cance. It refers not only to the instant when the moon is directly above the 

 meridian, but also to the instant when the moon is directly below the meridian, 

 or 180° distant in longitude. In this sense there are two meridian passages in a 

 tidal day, and they are distinguished by being referred to as the upper and lower 

 meridian passages or upper and lower transits. 



The interval between the moon's meridian passage (upper or lower) and the 

 following high water is known as the "high-water lunitidal interval," Likewise 

 the interval between the moon's meridian passage and the following low water 

 is known as the "low-water lunitidal interval." For short they are called, 

 respectively, high-water interval and low-water interval and abbreviated as 

 foUows: HWI and LWI. 



In its rising and falling the tide is accompanied by a horizontal forward and 

 backward movement of the water, called the tidal current. The two move- 

 ments — the vertical rise and fall of the tide and the horizontal forward and 

 backward movement of the tidal current — are intimately related, forming parts 

 of the same phenomenon brought about by the tidal forces of sun and moon. 



It is necessary, however, to distinguish clearly between tide and tidal current, 

 for the relation between them is not a simple one nor is it everywhere the same. 

 At one place a strong current may accompany a tide having a very moderate 

 rise and faU while at another place a like rise and fall may be accompanied by 

 a very weak current. Furthermore, the time relations between current and tide 

 vary widely from place to place. For the sake of clearness, therefore, tide 

 should be used to designate the vertical movement of the water and tidal current 

 the horizontal movement. 



It is convenient to have a single term to designate the whole phenomenon 

 which includes tides and tidal currents. Unfortunately no such distinct term 

 exists. For years, however, "the tide" or "the tides," or even "flood and ebb," 

 have been used in this general sense, and usually no confusion arises from this 

 usage, since the context indicates the sense intended; but the use of the term 

 "tide" to denote the horizontal movement of the water is confusing and is to be 

 discouraged. 



With respect to the rise and faU of the water due to the tide, high water and 

 low water have precise meanings. They refer not so much to the height of the 

 water as to the phase of the tide. High water is the maximum height reached 

 by each rising tide and low water the minimum height reached by each falling 

 tide. 



It is important to note that it is not the absolute height of the water which 

 is in question, for it is not at all infrequent at many places to have the low water 

 of one day higher than the high water of another day. Whatever the height 

 of the water, when the rise of the tide ceases and the fall is to begin, the tide is 

 at high water; and when the fall of the tide ceases and the rise is to begin, the 

 tide is at low water. The abbreviations HW and LW are frequently used to 

 designate high and low water, respectively. 



