Tide predictions are calculated based on a standard barometric pressure of 1013 hectopascals or millibars. When the pressure rises above that level, tides will be lower than their predicted heights.
If the barometric pressure goes below 1013 hectopascals, then tidal levels will rise accordingly and exceed predicted levels.
According to Land Information New Zealand (LINZ), every one hectopascal difference from the average of 1013 hectopascals will result in a sea level height difference of one centimetre, either higher or lower, depending upon whether the pressure is greater or less than the average. This is because one cubic meter of air at sea level weighs about one kilogram. LINZ reports that sea level changes due solely to barometric pressure rarely exceed 30 centimetres, or about one foot, either way.
