Higher High Water (HHW)

Higher High Water (HHW) is the higher of the two high waters that occur during a tidal day where diurnal inequality is present. The two highs differ in height because the diurnal constituents K1 and O1 add to one high and subtract from the other; the larger is the HHW. It is the daily value averaged over the epoch to form Mean Higher High Water, the US upper charting datum. In a pure semidiurnal regime the two highs are nearly equal, so the HHW and lower high water labels carry little practical difference.