Variations In the Tides
The basic lunar tides with spring and neap tide modification are subject to further, more complex variations. Some of these variations are based on long-term cycles in the relative positions of the sun and the moon. One such cycle is about 18 years long and produces greater than usual spring tides. Other variations in the tides cause different tidal patterns to exist in different places on the earth at one time. In the Mediterranean Sea the range from high tide to low tide is usually less than three feet (0.9 m). The highest tides in the world occur in part of the Bay of Fundy, between New Brunswick and Nova Scotia, Canada. There, during spring tides, the water rises more than 50 feet (15 m) from low tide to high tide.
The earth's orbit around the sun is elliptical, as is the orbit of the moon around the earth. There are regular changes in the relative abilities of the sun and the moon to raise tides. The position of the moon above and below the earth's equator varies during the month, and the sun's position shows similar variation during the year. Because of these variations in position, the tidal bulges occur at different locations at different times.
On earth, factors such as the depth and breadth of the bodies in which tides occur and the configuration of shorelines affect the tides. Tides are also modified by the friction of the water against sea bottoms. The various combinations of all the factors affecting tides produce three distinct tidal patterns—semi-diurnal, diurnal, and mixed.
have two nearly equal high tides and two nearly equal low tides during a period of 24 hours and 50 minutes. High tides tend to follow one another at intervals of 12 hours and 25 minutes. The Atlantic Ocean has primarily semidiurnal tides.
occur where there is only one high tide and one low tide a day, with high tide occurring 24 hours and 50 minutes after the preceding high tide. The Gulf of Mexico has primarily diurnal tides.
occur where there are two high tides and two low tides of very unequal size in each period of 24 hours and 50 minutes. San Francisco Bay, for example, has a mixed tide pattern of very high tide, very low tide, moderately high tide, moderately low tide. The Pacific Ocean has primarily mixed tides.
