Semidiurn

Semidiurn, or semidiurnal, is a term used in tidal science to describe phenomena that occur twice in each day. In the context of tides, a semidiurnal tide produces two high tides and two low tides within roughly a 24-hour period. The time between successive high tides is about 12 hours 25 minutes, corresponding to half of the average lunar day of about 24 hours 50 minutes. This pattern results from the combined gravitational influence of the Moon and the Sun acting on Earth’s oceans, together with the rotation of the planet and the shaping effects of coastlines and seabed topography.

In many locations the two high tides and two low tides are of similar magnitude, yielding a nearly symmetric cycle of sea-surface height. The primary tidal constituents responsible for semidiurnal tides are the lunar semidiurnal M2 (period about 12.42 hours) and the solar semidiurnal S2 (period 12.00 hours). Local variations in latitude, bathymetry, and coastline geometry can affect the relative heights of the two highs and two lows, sometimes producing diurnal tides (one high and one low per day) or mixed tides (two highs of differing heights).

Semidiurn thereby contrasts with diurnal tides, where only a single high and a single low occur per day, and with mixed tides, where two highs and two lows differ in amplitude. The term is used primarily in oceanography and coastal geography, though the concept of two cycles per day appears in other geophysical and atmospheric contexts as well.