Tides are often classified according to the number of tidal cycles that happen in one lunar day: semidiurnal, diurnal, and mixed. Most coastal regions in the Atlantic and the Indian Ocean experience semidiurnal tides, with only a small part of South America, the Arabian Sea, and the west coast of Australia and Indonesia experiencing mixed tides.
Most of us who have spent a day at the beach have noticed, at some point, the influx and outflux of seawater on the shore.The gravitational pull exerted by the sun and the moon on the Earth “attract” the water and causes it to bulge – a movement that creates the phenomenon of tides. Tides are cyclical events defined by the transition from the highest level of water (high tide) to its lowest level (low tide). These cycles follow the same order but the duration of each stage may vary within a lunar day (24 hours and 50 minutes, the time it takes for the moon to complete a lap around the Earth).
There are three different classifications regarding the number of tidal cycles per lunar day: semi-diurnal (two high tides and two low tides); diurnal (one high and one low tide); mixed (two high and two low tides of different heights). Most coastline areas experience semi-diurnal tides, when the transition from low to high lasts approximately 12 hours and 25 minutes.These different types of tide are mainly due to the fact that the Earth has a very irregular surface, with continents separating the oceans in irregular ways, which in turn block the passage of tidal bulges as the planet rotates.
Most coastal regions that are bathed by the Atlantic and Indian Ocean experience semi-diurnal tides, with only a small part of South America, the Arabian Sea, and the west coast of Australia and Indonesia experiencing mixed tides. Mixed tides occur mostly on the Pacific Ocean, prevailing along the west coast of the U.S., Alaska, and the Pacific Islands. Meanwhile, diurnal tides are less common and typically occur in partially enclosed basins, such as the Gulf of Mexico. In some occasions, different places in the world have a variety of tides throughout the tidal month: the tide may be a semi-diurnal for a few days, gradually change to a mixed tide, and finally show signs of being a diurnal tide before gradually changing back to a semi-diurnal cycle.