Standard pressure

The predicted tide height can vary by a foot or more depending on atmospheric pressure. Wind and other local factors are important too, but less easy to second-guess unless you know your area. All tidal height predictions assume ‘standard pressure’ on the barometer. You can take this to be 1013 millibars. If the pressure rises 30mb above this the extra ‘weight’ of air will sit on the tide and depress it by about one foot, or 0.3 metres. At 30mb below 1013, the tide will rise 0.3m higher than predicted. That’s 0.6m between 983 and 1043. In between is pro rata, and remember, any variation affects the whole tide – high water, low water, and all levels in between.