Science / Tides and Currents / Overtide: A harmonic tidal (or tidal current) constituent with a speed that is an exact multiple of the speed of one of the fundamental constituents derived from the development of the tide-producing force. The presence of over tides is usually attributed to shallow water conditions. The over tides usually considered in tidal work are the harmonics of the principal lunar and solar semidiurnal constituents M2 and S2, and are designated by the symbols M4, M6, M8, S4, S6, etc. The magnitudes of these harmonics relative to those of the fundamental constituents are usually greater in the tidal current than in the tide.