In turn, the relative strength of spring, neap or any other part of the tide will vary. Under certain conditions, when the sun and the moon are both closest to Earth and aligned, extreme tides can occur. This causes many complications in the tides and related currents, especially in coastal areas. The change in the tidal waveform depends on many things, but especially the shape, depth and position of the continental shelf and the land. There are three basic types of tidal patterns.
Most areas have two highs and two lows a day.
If the two highs and the lows are each respectively about the same amplitude, the pattern is called a semi-daily or semidiurnal tide. When the highs and the lows each respectively differ in height, the pattern is called a mixed tide. The West Coast of the United States tends to have mixed tides, whereas a semidiurnal pattern is more typical of the east coast. The typical tidal sequence then is a spring tide followed by lesser tides leading about a week later to neap tide, which in turn is followed by rising tides leading about a week later to spring tide and so on. Thus about every two weeks the tidal pattern will be repeated.