Highest Astronomical Tide (HAT)

The highest level of sea water that can be predicted to occur under average meteorological conditions and under any combination of astronomical influences.

The Highest Astronomical Tide (HAT) is a critical reference point in marine navigation and coastal planning. It represents the maximum tidal height that can be anticipated based solely on predictable astronomical factors, such as the moon’s gravitational pull and the sun’s relative positions to the Earth. Importantly, the HAT does not account for meteorological influences like storm surges, heavy rainfall, or strong winds, which can cause water levels to exceed this predicted maximum.

HAT is a static datum calculated over an extended period, often 18.6 years, corresponding to the lunar nodal cycle. This long-term approach ensures that the HAT reflects the most extreme tidal conditions likely to occur under normal astronomical circumstances. In some regions, it is used as a reference point for chart data, although many countries prefer to use the Lowest Astronomical Tide (LAT) for this purpose. 

In practical terms, HAT is significant for mariners, as it helps them understand the upper limits of tidal ranges when navigating shallow waters or planning port operations. Coastal engineers and urban planners also rely on HAT to design infrastructure such as seawalls, jetties, and flood defences, ensuring these structures can withstand the highest predictable water levels. While HAT is not frequent, its implications for safety and infrastructure resilience make it an essential concept in marine and coastal disciplines.