The moon's gravitational pull on the Earth causes water levels in the oceans to rise and fall in a consistent, predictable fashion. The point at which the water level in a specific location reaches its highest point is the high tide. Conversely, the lowest water level in a specific location is the low tide.
The Moon's Effect
The moon has the greatest effect on the oceans' tides due to its strong gravitational pull. High tides occur in the locations closest to and farthest from the moon. Low tides occur in locations at 90 degree angles to the moon. Tidal cycles have the same length as the lunar day, 24 hours and 50 minutes. In this time span, most locations will experience two high tides, when the location is close to and far from the moon, and two low tides.
The sun also has a gravitational pull on the Earth. This pull most strongly effects tides with new and full moons, when the Earth, moon and sun are aligned. When these three bodies are aligned, they cause spring tides. During spring tides, the low tides are much lower than normal, and the high tides are much higher than normal.
Types of Tidal Cycles
Most oceanic locations, including the east coast of the United States, have two low tides and two high tides of approximately equal heights per day. Scientists refer to this pattern as semidiurnal tides. Locations with two low tides and two high tides of different heights follow a pattern called mixed semidiurnal tides. The west coast of the U.S. experiences mixed semidiurnal tides. Other locations, such as the Gulf of Mexico, have only one high and low tide daily, a pattern called diurnal tides.
Ranges of Tides
The National Oceanic and Atmospheric Administration has compiled a list of the locations that experience the greatest variation between low and high tides. Eight locations in the Bay of Fundy, in Nova Scotia and New Brunswick, appear in the top 10 spots on this list. In these locations, there is a difference of more than 30 feet between water levels at low and high tide. As a comparison, the average range at Mt. Pleasant Plantation in South Carolina is less than 2 feet.