Highest Astronomical Tide (HAT) (HAT)

The highest tidal level predicted to occur under average meteorological conditions from any combination of astronomical influences.

HAT is a reference level derived from long-term tidal prediction, not the highest water level that can ever occur. Wind, atmospheric pressure, waves, river flow and other non-astronomical effects can raise the observed sea above it. It is used as a vertical datum or reference in some charts, publications and coastal applications. The datum actually used for a charted height or clearance must always be checked, because HAT, Mean High Water Springs and other high-water levels are not interchangeable.