tide datum— See datum. 



tide gage— j\n instrument for measuring the rise and fall of tlie tide. 



tide-producing force— That part of the gravitational attraction of the Moon and Sun which is 

 effective in producing the tides on the Earth. The force varies approximately as the mass 

 of the attracting body and inversely as the cube of its distance. The tide-producing force 

 exerted by the Sun is a little less than one-half as great as that of the Moon. A 

 mathematical development of the vertical and horizontal components of the 

 tide-producing forces of the Moon and Sun is given in Schureman (1941). 



tide staff— A tide gage consisting of a vertical graduated staff from which the height of the 

 tide can be read directly. It is called a fixed staff when secured in place so that it cannot 

 be easily removed. A portable staff is one that is designed for removal from the water 

 when not in use. For such a staff a fixed support is provided, and the staff itself has a 

 metal stop secured to the back to maintain the same elevation when installed for use. 



tide station— The geographic location at wliich tide observations are conducted. Also, the 

 facilities used to make tide observations. These may include a tide house, tide gage, tide 

 staff, and tidal bench marks. See primary control tide station, subordinate tide station, 

 secondary control tide station, and tertiary tide station. 



tide tables— Tables which give daily predictions of tlie times and heights of high and low 

 waters. These predictions are usually supplemented by tide differences and constants 

 through which additional predictions can be obtained for numerous other places. 



tropic tides— Tides occurring semimontlily when tlie effect of the Moon's maximum 

 declination is greatest. At these times there is a tendency for an increase in the diurnal 

 range. The tidal datums pertaining to the tropic tides are designated as tropic higher high 

 water (TcHHW), tropic lower high water (TcLHW), tropic higher low water (TcHLW), 

 and tropic lower low water (TcLLW). 



tropical month— The average period of the revolution of the Moon around the Earth with 

 respect to the vernal equinox. Its length is approximately 27.321582 days. 



tropical year— The average period of the revolution of the Earth around the Sun with respect 

 to the vernal equinox. Its length is approximately 365.2422 days. The tropical year 

 determines the cycle of changes in the seasons, and is the unit to which the calendar year 

 is adjusted tlu-ough the occasional introduction of the extra day of leap years. 



tsunami— A shallow-water progressive wave, potentially catastrophic, caused by an under- 

 water earthquake or volcano. Also called seismic sea wave. 



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