In major part of the Atlantic Ocean there are two high tides and two low tides during each solar day, occurring approximately every 12 hours and 25 minutes, each high tide being of the same level. Such tides are known as semi-diurnal tides.
Here, it may be pointed out that over much of the Pacific and Indian Oceans there are also two high and two low tides in a day. However, their amplitudes are different.
The height of the high tides in these oceans is not equal, while that of the low tide remains constant. Sometimes the opposite happens; the height of low tides differs and that of the high tides remains almost equal. Such a type of tide is known as mixed tides.
Besides the afore said types of tides, there are diurnal tides which are generated in certain specific bays and seas, for example, the Gulf of Mexico, the adjacent seas around the Philippines Islands, the coastal seas of Alaska, and the seas near the coast of China. In all these areas there is only one high tide and one low tide during each 24 hours.
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It is interesting to note that besides the above-mentioned categories of tides, there are certain types of tides characterized by anomalies. For example, at Southampton there is a double high tide, the second high tide occurs only after two hours of the first with the result that high water persists there for more than three hours.
The prolonged high tide occurs during both the spring and neap tides which is only about 2 meters. The double tide is perhaps due to the effect of shallow water and the peculiar configuration of the coast-line. This causes an unusual oscillation within the particular ‘tidal unit’.
On the contrary, a double low tide is experienced at Portland, on the coast of Dorset. Thus, from the above examples there is little doubt that the tidal range and the nature of tides are not the same everywhere. Therefore it is necessary to have a clear understanding of the tide-producing forces and their interaction with the oceans.