Ocean tides represent the rhythmic, predictable rise and fall of sea levels across the globe, driven by celestial mechanics. These changes in water height follow different patterns depending on a coastal location’s geography. The semidiurnal pattern is the most widespread and commonly experienced, responsible for the twice-daily changes in water level familiar to many coastlines.
Defining the Semidiurnal Cycle
The term “semidiurnal” literally means “half-daily,” describing a tidal pattern where the sea level completes a full cycle twice over the course of one lunar day. A lunar day lasts approximately 24 hours and 50 minutes, the time it takes for a point on Earth to return to the same position relative to the Moon. Consequently, a pure semidiurnal cycle features two high tides and two low tides within this period.
A defining characteristic of this cycle is that the two high tides reach nearly the same maximum height, and the two low tides drop to nearly the same minimum height. The time elapsed between a high tide and the subsequent high tide is about 12 hours and 25 minutes. This consistent, balanced rhythm is typical of the Atlantic coasts of North America and Europe, representing the theoretical ideal for a planet covered entirely by water.
The Astronomical Drivers
The twice-daily rhythm of the semidiurnal tide originates from the gravitational forces, primarily from the Moon, and the motion of the Earth-Moon system. The Moon’s gravity pulls ocean water toward it, creating a bulge on the side of the Earth facing the Moon, which is the first high tide.
A second, equally important bulge forms simultaneously on the side of the Earth opposite the Moon. This indirect bulge occurs because the Earth and Moon orbit a common center of mass, generating an outward-pulling centrifugal force. On the far side of Earth, this force exceeds the Moon’s gravitational pull, effectively pulling the ocean water away from the planet’s surface to form the second high-tide bulge.
As the Earth rotates on its axis through these two bulges of water, any point on the planet passes through two high-water marks and two low-water marks. The high tides occur when the location is aligned with a bulge, and the low tides occur in the areas between the bulges.
Distinguishing Tidal Patterns
While the semidiurnal tide is the most common, not all coastal areas exhibit this balanced, twice-daily pattern. Tidal cycles are broadly classified into three types, determined by the local ocean basin shape, continental landmasses, and the Moon’s angle relative to the equator.
A Diurnal Tide is a pattern with only one high tide and one low tide occurring each lunar day. This cycle can be found in locations like the Gulf of Mexico.
In contrast, the Mixed Semidiurnal Tide maintains two high tides and two low tides per lunar day, but with a noticeable difference in height. This is known as diurnal inequality, where the heights of the two high tides and the two low tides are unequal. The Pacific coast of North America frequently experiences this mixed pattern, which is a blend of the semidiurnal and diurnal influences.