Before discussing the tide-generating forces, it would be better to understand the meaning of different technical terms that shall be used while discussing the various aspects of tides.
First, the tides represent the vertical movement of sea-water. When the sea-level, under the influence of the solar and lunar attraction, is raised, it is called high tide. Conversely, when the sea-level falls, the low tide occurs. In the open ocean the difference in height between the high and low tides is known as the tidal range.
The time interval between two high tides is called the tidal interval. When due to tides, horizontal currents are generated in a bay or inlet, they are known as tidal currents.
The tidal currents heading towards the coast are called flood tide, and those returning from the coast towards the sea are known as ebb tide. The tidal currents have been discussed in detail in the latter part of this chapter.
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In the open ocean the tidal range is between 30 and 60 cm, but in shallow marginal seas it can increase to 10 meters. In certain constricted tidal estuaries the tidal range may be more than 13 meters.
The highest tidal range is recorded in the Bay of Fundy in north-eastern Canada; at the mouth of the bay the range hardly exceeds 3 meters, but near its head the range is of the order of more than 20 meters.
However, 15 to 19 meter range is common. On the contrary, in some partially enclosed seas like the Mediterranean and the Baltic Seas, the tidal range is very small.