HAT (Highest Astronomical Tide)

Highest Astronomical Tide (HAT) is the highest water level predicted to occur under any combination of astronomical conditions and average meteorological conditions. It is computed from the full harmonic constituent set over a 19-year cycle, not observed once, so it bounds the upper tidal envelope the way Lowest Astronomical Tide (LAT) bounds the lower. HAT is the standard reference for vertical clearance under bridges and overhead cables on Admiralty charts. Real water can exceed HAT when low pressure, onshore wind, or surge adds to the astronomical tide, so HAT is not an absolute maximum.