Tides are the daily rising and lowering of sea levels. They are caused by the gravitational pull of the moon, the Earth’s rotation, the gravitational pull of the Sun, and bathymetry – the shape and contours of the ocean floor and shoreline. Depending on the time of month, or even the time of day, tides can have different characteristics. Some places on Earth experience semidiurnal tides (twice daily), some experience diurnal tides (once daily), and some experience mixed tides. Nearly all places on Earth are subjected to Spring and neap tides twice a month.
The principal characteristics of all tides are basically similar. Flood tide occurs over several hours, usually flooding intertidal zones. When sea levels have risen to their highest level, high tide happens. At this point slack tide occurs, a period of time when the tide is not coming in or going out. After slack tide, as the tide begins to go out, it is said to be turning. Ebb tide occurs, again over several hours and exposing the intertidal zones. Water levels stop lowering at their lowest point indicating low tide. Slack tide happens again.
A semidiurnal tide occurs twice a day. It is the most common kind of tide found in the world, having two high tides and two low tides. A semidiurnal tide last about 12 hours and 25 minutes or half of a tidal day. Most of the time the high tides are relatively equal though not entirely of the same height, just as the low tides are relatively equal though also not of the same height.
A diurnal tide occurs once a day and takes much longer than a semidiurnal tide, an entire tidal day of roughly 25 hours. Diurnal tides can be found along the Gulf of Mexico, Alaska, and parts of the Asian Pacific coast. Diurnal tides have just one high tide and one low tide. Bathymetry is one of the most effective factors in forming a diurnal tide as land masses (think Gulf of Mexico) act as barriers against free moving waters and affect tidal frequency.
A mixed tide, or mixed semidiurnal tide, is found around the Caribbean, West Coast of the United States, and the west coast of India, among other places. A mixed tide has two high waters and two low waters of unequal size. In fact, one low tide is sometimes so high that a mixed tide may appear to be a single low tide following two high tides. On tide tables these inequalities are noted as the higher high water, lower high water, higher low water, and lower low water.
Twice a month, with full moon and new moon, the tides rise and lower more than normal. These are called Spring tides and are caused by the moon, Earth and Sun falling in a straight line creating a greater gravitational pull on the tides. This is called syzygy. If the moon is at its closest point to Earth – perigee – the tides are even more extreme.
Alternately, twice a month tides are also less extreme than normal. This occurs during a half moon when the Sun and Moon are at right angles to each other and is called a neap tide. The gravitational pull of the Sun and Moon combat each other and have less of an effect on the tides, creating a smaller difference between high and low tides.
Tides can also have great ranges from location to location. The greatest tidal range is found in Canada where one bay has 50 foot tides. Places in Georgia can have 13 foot tides but just a little further south in Florida some places have a tidal range of only a couple of feet.
The characteristics of tides can vary greatly depending on a number of factors such as the moon, the Sun, and location. Knowing the local tide characteristics helps mariners navigate waterways and the different characteristics of tides creates unique environments around the world.