The difference between the heights of the two high tides (high water inequality) or the two low tides (low water inequality) during a tidal day, or the difference in speed between the two flood or the two ebb currents during a tidal day. The average diurnal inequality is called tropic high water inequality and tropic low water inequality when referred to the high waters and low waters, respectively, of tropic tides. Mean diurnal high water inequality is half the average difference between the heights of the two high waters of each tidal day over a 19-year period. Mean diurnal low water inequality is half the average difference between the heights of the two low waters for a similar period. Also called declinational inequality.