Tides are the rise and fall of sea levels, driven primarily by the gravitational influence of the Moon and, to a lesser extent, the Sun. The tide cycle that occurs twice a day, featuring two high-water marks and two low-water marks within a single lunar day, is called the Semidiurnal tide.
Semidiurnal Tides: The Twice-Daily Cycle
The semidiurnal pattern is the most widespread tidal cycle observed globally. This cycle is characterized by two high tides and two low tides occurring over a lunar day, which is approximately 24 hours and 50 minutes long. Consequently, the time between successive high tides is about 12 hours and 25 minutes.
In this pattern, the two high tides reach a nearly equal maximum height, and the two low tides fall to a nearly equal minimum height. This consistency in height gives the semidiurnal tide its name, which translates to “half-daily.”
The Mechanics of Tidal Bulges
The twice-daily cycle results from two tidal bulges forming on opposite sides of the Earth. The Moon’s gravitational pull is not uniform across the planet, and this difference in force, called the tidal force, stretches the Earth and its oceans into an elongated shape.
On the side of Earth facing the Moon, the gravitational attraction is strongest, pulling the ocean water toward the Moon to create the first high-tide bulge. On the opposite side, the gravitational pull is weakest due to the greater distance. The water bulges outward here because the solid Earth is pulled more strongly toward the Moon than the ocean water on the far side.
As the Earth rotates on its axis, any location passes through both bulges every lunar day. This rotation causes the water level to rise as the location enters a bulge (high tide) and fall as it moves away (low tide). The simultaneous presence of two bulges explains why a location experiences two high tides each day.
Diurnal and Mixed Tides
Not all locations experience the semidiurnal pattern; two other major classifications exist. A Diurnal tide features only one high tide and one low tide within a lunar day. This pattern is less common and often results from the shape and depth of the ocean basin amplifying the Moon’s angle relative to the equator.
The Mixed tide also has two high tides and two low tides each lunar day. However, the successive high tides and low tides in a mixed pattern are of significantly unequal heights, a difference known as diurnal inequality.
The Moon’s declination, its angle north or south of the Earth’s equator, heavily influences these variations. When the Moon is at its maximum declination, its asymmetrical position can cause one of the two tidal bulges to be more pronounced than the other. Local factors, such as continental landmasses and the resonant properties of local ocean basins, further modify global tidal forces to produce these three distinct patterns.