By Kat Black Updated Mar 24, 2022
The Earth's oceans rise and fall in a predictable rhythm driven by the gravitational forces of the moon and the sun. When the water level at a specific point reaches its maximum, it's called high tide; when it reaches its minimum, it's low tide.
The moon’s gravity exerts the strongest influence on tides. When a location is aligned directly beneath the moon or on the opposite side of Earth, the ocean bulges, creating high tide. Perpendicular to this alignment, the water recedes, producing low tide. The entire cycle repeats every lunar day of 24 hours and 50 minutes, giving most places two high tides and two low tides each day.
The sun also pulls on Earth's waters. During new and full moons—when Earth, moon, and sun line up—the combined gravitational pull amplifies the tidal range, producing spring tides. These tides have the highest high tides and the lowest low tides, significantly more extreme than during ordinary phases.
Most coastal regions, including the U.S. East Coast, experience semidiurnal tides: two high and two low tides of roughly equal height each day. Areas such as the U.S. West Coast display mixed semidiurnal tides, where the two high or two low tides differ in magnitude. The Gulf of Mexico, on the other hand, exhibits diurnal tides—one high and one low tide per day.
The NOAA identifies locations with the greatest tidal ranges. Eight sites in the Bay of Fundy, spanning Nova Scotia and New Brunswick, dominate the top‑10 list, boasting differences exceeding 30 feet between low and high water. In contrast, places like Mt. Pleasant Plantation in South Carolina register ranges of less than 2 feet.
Understanding these patterns helps mariners, fishermen, and coastal communities anticipate water conditions and plan accordingly.
