On the east coast of the United States, we experience semidiurnal tides. These tides produce a cycle of one high and one low tide every 12.42 hours; that is, two tide cycles per day. There are one-tide cycle areas (diurnal) in the Gulf of Mexico, the Philippines, and parts of Alaska. There are also mixed tides.
Every 14.3 days, approximately twice a month, the earth, sun, and moon are aligned so as to form a straight line. As you may readily imagine, the gravitational forces of each of these bodies reinforce each other to form higher than average tides. These tides are called spring tides, and occur during the new and full moon. Similarly, when the sun and moon are at right angles to the earth, every 14.3 days, the gravitational forces imposed on the earth are reduced, resulting in lower than normal tides called neap tides.
In general, at one of the two tidal cycles, there is a higher high tide. This higher high tide is called the spring tide. Therefore, the spring tide is not the result of snow or ice melt, or excess rain fall. If we can imagine that the moon is closer to the earth during the first tidal cycle of the day, then during the second semidiurnal cycle, the moon will not be as close to the earth and, therefore, the gravitational influence will be reduced resulting in a lower high tide.
In our sun, moon, earth system, the sun is the largest body. However, since the sun's distance is so great from the earth, its gravitational influence on the earth's tides is only forty six percent of that of the earth's moon. Consider the northwest winter winds blowing the ebb tide to the sea, our lowest low tides occur during the winter, and our highest high tides are spring tides that occur during the summer southeast wind influence.
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