Sea level

Bermuda experiences semi-diurnal astronomical tides, which means that sea level rises and falls twice daily in response to the gravitational pull of the sun and the moon. High and low tides are 6.21 hours apart and the average tidal range or height (between low and high tide) is 0.75 m (2.5 ft). During a typical one month lunar cycle, this range varies from approximately 0.4 m to 1.2 m (see chart below).

Sea level is also affected by barometric pressure. In order to maintain hydrostatic equilibrium (constant pressure on the sea floor), the ocean compensates for low atmospheric pressure by rising at a rate of 1.01 cm for each 1 millibar increase in atmospheric pressure. The result is significant variations (tens of cms) in sea level in response to meteorological conditions. A large contribution to storm surge, such as in a hurricane, is related to the steep decline in atmospheric pressure and related “bulge” in sea level.

“Ordnance Datum” is a surveyor’s baseline used for determining land elevations relative to sea level. Average sea level between 2000 and 2007 was 0.15 m (0.5 ft) above ordnance datum (AOD). The discrepancy can largely be explained by a rise in sea level (see page 3) since the original datum was established at the Royal Naval Dockyard in the 19th Century.