Tide

Tides are the periodic rise and fall of sea levels across the globe. This phenomenon results from differential gravitational forces exerted primarily by the Moon and the Sun. These forces combine with inertial effects from the Earth–Moon system's orbital motion and the Earth's rotation. While astronomical forces generate the fundamental tidal potential, local factors strongly modify what we actually observe. These terrestrial factors include the geometry of ocean basins, continental boundaries, and bathymetry, which is the study of underwater depth. Other influences include the Coriolis effect, frictional dissipation in shallow seas, and the tidal resonance of coastlines.

To understand the mechanism, we must look at how gravity acts on a sphere. The gravitational field created by the Moon weakens with distance. Therefore, the Moon exerts a slightly stronger force on the side of the Earth facing it. This pulls the water toward the Moon, creating a high tide. On the opposite side of the Earth, a second bulge occurs. This happens because the Moon pulls the solid Earth away from the water. The Moon essentially "stretches" the Earth along the line connecting the two bodies.

While the solid Earth deforms slightly, the fluid ocean moves much more easily. As the Earth rotates, the magnitude and direction of this tidal force change constantly. This causes the rhythmic changes in sea surface height we call tides.

Tidal cycles consist of four distinct stages. First, the water stops falling and reaches a local minimum called low tide. Next, the sea level rises over several hours, a process known as flooding. This covers the intertidal zone, which is the area between high and low water. The water then stops rising and reaches a local maximum called high tide. Finally, the sea level falls over several hours, a process called ebbing.

The oscillating currents produced by these movements are known as tidal streams or tidal currents. When the current ceases, it is called slack water or slack tide. This moment of slack water usually occurs near high or low water.

Tides are categorized by their frequency and patterns. Most locations experience semi-diurnal tides, which feature two high and two low waters each day. Some areas see mixed semi-diurnal tides, where the two daily high waters have different heights. Others experience diurnal tides, which consist of only one tidal cycle per day. The difference between the two high waters in a day is called daily inequality. This inequality is generally small when the Moon is positioned over the Equator.

The magnitude of tides changes in a two-week cycle based on the Moon's phase. Approximately twice a month, the Sun, Moon, and Earth form a straight line. This configuration is known as a syzygy. During a syzygy, the solar tidal force reinforces the lunar force. This results in a spring tide, which features the maximum tidal range. Spring tides produce higher high waters and lower low waters than average.

Conversely, when the Sun and Moon are at a 90-degree angle, known as quadrature, we see neap tides. The term "neap" is an Anglo-Saxon word meaning "without the power." During neap tides, the solar force partially cancels the Moon's force, resulting in milder conditions.

Scientists use specific reference levels to measure these changes accurately. These include the Highest Astronomical Tide (HAT), the highest tide predicted to occur. They also use Mean Sea Level (MSL), which is the constant average sea level for a location. Other levels include Mean High Water Springs (MHWS) and Lowest Astronomical Tide (LAT). To track these, tide gauges at fixed stations measure water levels over time. These gauges ignore short variations like waves that last less than a few minutes.

Tidal forces act on the entire Earth system, not just the oceans. In the Earth's crust, these forces produce vertical displacements of centimeters called Earth tides. In the atmosphere, gravitational forcing and solar heating create atmospheric tides. These are observed as oscillations in pressure, density, and wind patterns. Furthermore, the distance between the Moon and Earth affects height. When the Moon is at perigee, it is closest to Earth, increasing the tidal range. At apogee, the Moon is farthest away, and the range shrinks.