Diurnal Inequality

Diurnal inequality is the difference in height between the two high waters, or the two low waters, of a single tidal day, the signature of the diurnal constituents K1 and O1 beating against the semidiurnal M2 and S2. It is largest when lunar declination is greatest, because the two daily tidal bulges then fall at different latitudes, and it nearly disappears at zero declination. Where the inequality is strong the four water levels become the higher high water, lower high water, higher low water, and lower low water that define the MHHW and MLLW chart datums. Mariners must use the lower of two daily lows for under-keel clearance.