Predictions are referred to being computed for ‘average barometric pressure’. What is ‘average’, and how do tides react to differences from this average?
Tidal predictions are computed for average barometric pressure at the particular place concerned. The average barometric pressure for certain places is given in Admiralty Sailing Directions and information is also given in some instances concerning the changes in level which can be expected under different conditions. A difference from the average of 34 millibars can cause a difference in height of about 0.3m. A low barometer will tend to raise sea level and a high barometer will tend to depress it. The water level does not, however, adjust itself immediately to a change of pressure and it responds, moreover, to the average change in pressure over a considerable area. Changes in level due to barometric pressure seldom exceed 0.3m but, when mean sea level is raised or lowered by strong winds or by Storm Surges (wind-induced long period waves causing higher and lower-than-predicted levels to occur), this effect can be important.
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