Semi-Diurnal Tide

A semi-diurnal tide is a tidal regime with two nearly equal high waters and two low waters each lunar day, a period of about 12 hours 25 minutes set by the principal lunar constituent M2 at 12.4206 h. It dominates the Atlantic and is the most common tide worldwide, distinguished from diurnal and mixed tides by the form number F, the ratio (K1 plus O1) over (M2 plus S2); F below 0.25 marks a semi-diurnal regime. The two highs stay close in height where the diurnal constituents are weak, so the diurnal inequality remains small and the rule of twelfths approximates the curve between high and low water.