The tides on a regular open coast change rapidly as a stonn approaches, and the time history of this change is important in computing dangerously rugh tides in bays; in addition, this change influences the maximum wave height offshore. Simple physfcal assumptions are used (and justified) to define a "bathystrophic" flow, and it is shown mathematically that this flow is of great importance in the computation of storm tides. Using only the traditionally accepted formulae and constants for wind stress and bottom friction, examples of the time change of the tide have been calculated. These results agree with observations as well as can be expected. Deviations from observations may be due to an edge wave.