There are three main tidal patterns: semidiurnal, diurnal, and mixed. Most shorelines on the planet experience semidiurnal tides (two high tides and two low tides per lunar day), making it so that each transition between low to high lasts approximately 12 hours and 25 minutes.
Sir Isaac Newton (1687) explained that tides are caused [mainly] by the effects of gravity – that is, by the attraction of a given mass for another. According to the law of universal gravitation, tides are protuberances of water induced by the pull that the sun and the moon exert over the Earth. When the Earth moves around the sun, it maintains its orbit due to the equilibrium of two forces: gravitational and centrifugal. These very same forces are present in the moon-Earth system, making it so that the combination of all forces, from both systems, together with an array of environmental factors, are responsible for creating the phenomenon of tides on our planet.
Tides are events that happen in cycles. A tidal cycle consists of four main stages: the sea level rises over the course of a few hours (flood tide), reaches its highest point (high tide), falls over several hours (ebb tide), and hits its lowest level (low tide). These cycles all follow the same order but the duration of each stage may vary within a lunar day (25 hours and 50 minutes, the time it takes for the moon to complete a full rotation around the Earth), which enables different tidal patterns.
The most common tidal patterns are: (a) semi-diurnal: two high tides and two low tides each lunar day; (b) diurnal: a single high tide and low tidal per lunar day; (c) mixed: usually two high tides and two low tides each lunar day, featuring significantly different heights. Most shorelines on the planet experience two high tides and two low tides per lunar day (semidiurnal), making it so that each transition between low to high lasts approximately 12 hours and 25 minutes.
If the Earth was a perfect sphere covered by a single ocean with uniform depth, there would be two high tides of different amplitudes each lunar day, thus making the semi-diurnal tide the closest thing to an “ideal” pattern. Alternatively, the existence of diurnal and mixed tides can be explained by the fact that the Earth has a very irregular surface, with land masses that separate the oceans in irregular ways, often blocking the passage of tidal bulges as the planet rotates, which creates complex tidal patterns within and across ocean basins.
Ryan is the founder of Tideschart.com. Originally from New Zealand, Ryan has traveled to more than 20 countries and has combined more than 10 years of development experience with his passion for the ocean.