Tides are the periodic rise and fall of sea level resulting from the differential gravitational forces exerted primarily by the Moon and the Sun, combined with inertial effects associated with the Earth–Moon system’s orbital motion and the Earth's rotation.
While these astronomical forcings generate the fundamental tidal potential, actual observed tides are strongly modified by terrestrial factors, including the geometry of ocean basins, continental boundaries, bathymetry, the coriolis effect, frictional dissipation within shallow seas and the tidal resonance of coastlines.
Tides vary on timescales ranging from hours to years due to a number of factors, which determine the lunitidal interval. To make accurate records, tide gauges at fixed stations measure water level over time. Gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference (or datum) level usually called mean sea level. Do not confuse with the astronomical lunar day on the Moon. A lunar zenith is the Moon's highest point in the sky.
While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to change from thermal expansion, wind, and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts.
Tidal phenomena are not limited to the oceans, but can occur in other systems whenever a gravitational field that varies in time and space is present. For example, the shape of the solid part of the Earth is affected slightly by Earth tide, though this is not as easily seen as the water tidal movements.
Characteristics Ocean tides are cyclic, rising and falling approximately twice a day. Four stages in the tidal cycle are named: The water stops falling, reaching a local minimum called low tide. Sea level rises over several hours, covering the intertidal zone; flooding. The water stops rising, reaching a local maximum called high tide. Sea level falls over several hours, revealing the intertidal zone; ebbing.
Oscillating currents produced by tides are known as tidal streams or tidal currents. The moment that the tidal current ceases is called slack water or slack tide. The tide then reverses direction and is said to be turning. Slack water usually occurs near high water and low water, but there are locations where the moments of slack tide differ significantly from those of high and low water.
Tides are commonly semi-diurnal (two high waters and two low waters each day), or diurnal (one tidal cycle per day). The two high waters on a given day are typically not the same height (the daily inequality); these are the higher high water and the lower high water in tide tables. Similarly, the two low waters each day are the higher low water and the lower low water. The daily inequality is not consistent and is generally small when the Moon is over the Equator.
Reference levels
The following reference tide levels can be defined, from the highest level to the lowest: Highest astronomical tide (HAT) – The highest tide that can be predicted to occur. Note that meteorological conditions may add extra height to the HAT. Mean high water springs (MHWS) – The average of the two high tides on the days of spring tides. Mean high water neaps (MHWN) – The average of the two high tides on the days of neap tides. Mean sea level (MSL) – This is the average sea level. The MSL is constant for any location over a long period. Mean low water neaps (MLWN) – The average of the two low tides on the days of neap tides. Mean low water springs (MLWS) – The average of the two low tides on the days of spring tides. Lowest astronomical tide (LAT) – The lowest tide which can be predicted to occur.
Range variation: springs and neaps
The semi-diurnal range (the difference in height between high and low waters over about half a day) varies in a two-week cycle. Approximately twice a month, around new moon and full moon when the Sun, Moon, and Earth form a line (a configuration known as a syzygy), the tidal force due to the Sun reinforces that due to the Moon. The tide's range is then at its maximum; this is called the spring tide. It is not named after the season, but, like that word, derives from the meaning "jump, burst forth, rise", as in a natural spring. Spring tides are sometimes referred to as syzygy tides.
When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the Earth (in quadrature), and the solar tidal force partially cancels the Moon's tidal force. At these points in the lunar cycle, the tide's range is at its minimum; this is called the neap tide, or neaps. "Neap" is an Anglo-Saxon word meaning "without the power". Old English (example given from AD 469: forðganges nip – without the power of advancing). The Danish niptid is probably from the English. The English term neap-flood (from which neap tide comes) seems to have been in common use by AD 725. Neap tides are sometimes referred to as quadrature tides.
Spring tides result in high waters that are higher than average, low waters that are lower than average, "slack water" time that is shorter than average, and stronger tidal currents than average. Neaps result in milder tidal conditions. There is about a seven-day interval between springs and neaps.
Tidal constituents
Tidal constituents are the net result of multiple influences impacting tidal changes over certain periods of time. Primary constituents include the Earth's rotation, the position of the Moon and Sun relative to the Earth, the Moon's altitude (elevation) above the Earth's Equator, and bathymetry. Variations with periods of less than half a day are called harmonic constituents. Conversely, cycles of days, months, or years are referred to as long period constituents.
Tidal forces act on the entire Earth system. In Earth's crust, these forces produce periodic vertical displacements of centimeters, a phenomenon known as Earth tide. In the atmosphere, gravitational forcing by the Moon and Sun, together with solar heating, generates global-scale oscillations in pressure, density, and wind known as Atmospheric tide. Whereas Earth tides involve elastic deformation, atmospheric tides are observed primarily as oscillations in pressure gradients and wind patterns.
Principal lunar semi-diurnal constituent
In most locations, the largest constituent is the principal lunar semi-diurnal, also known as the M2 tidal constituent or M2 tidal constituent. Its period is about 12 hours and 25.2 minutes, exactly half a tidal lunar day, which is the average time separating one lunar zenith from the next, and thus is the time required for the Earth to rotate once relative to the Moon. Simple tide clocks track this constituent. The lunar day is longer than the solar day because the Moon orbits in the same direction as the Earth spins.
The Moon orbits the Earth in the same direction as the Earth rotates on its axis, so it takes slightly more than a day—about 24 hours and 50 minutes—for the Moon to return to the same location in the sky. During this time, it has passed overhead (culmination) once and underfoot once (at an hour angle of 00:00 and 12:00 respectively), so, in many places, the period of strongest tidal forcing is the above-mentioned, about 12 hours and 25 minutes. The moment of highest tide is not necessarily when the Moon is nearest to the zenith or nadir, but the period of the forcing still determines the time between high tides.
Because the gravitational field created by the Moon weakens with distance from the Moon, it exerts a slightly stronger than average force on the side of the Earth facing the Moon, and a slightly weaker force on the opposite side. The Moon thus tends to "stretch" the Earth slightly along the line connecting the two bodies. The solid Earth deforms a bit, but ocean water, being fluid, is free to move much more in response to the tidal force, particularly horizontally (see equilibrium tide).
As the Earth rotates, the magnitude and direction of the tidal force at any particular point on the Earth's surface change constantly; although the ocean never reaches equilibrium—there is never time for the fluid to "catch up" to the state it would eventually reach if the tidal force were constant—the changing tidal force nonetheless causes rhythmic changes in sea surface height.
When there are two high tides each day with different heights (and two low tides also of different heights), the pattern is called a mixed semi-diurnal tide.
Lunar distance
The changing distance separating the Moon and Earth also affects tide heights. When the Moon is closest, at perigee, the range increases, and when it is at apogee, the range shrinks. Six or eight times a year, perigee coincides with either a new or full moon causing perigean spring tides with the largest tidal range. The difference between the height of a tide at perigean spring tide and the spring tide when the moon is at apogee depends on location but can be as large as a foot higher.
Other constituents About 62 constituents are of sufficient size to be considered useful in marine tide prediction, these include solar gravitational effects, the obliquity (tilt) of the Earth's Equator and rotational axis, the inclination of the plane of the lunar orbit and the elliptical shape of the Earth's orbit of the Sun.
Shallow-water constituents include overtides, which are higher harmonics of a single dominant tidal frequency, and compound tides, which result from interactions between the primary lunar and solar constituents. Although typically smaller in amplitude than the fundamental tidal potential, these constituents modify asymmetries between rising and falling tides in estuaries and continental shelf regions.
Phase and amplitude
Because the M2 tidal constituent dominates in most locations, the stage or phase of a tide, denoted by the time in hours after high water, is a useful concept. Tidal stage is also measured in degrees, with 360° per tidal cycle. Lines of constant tidal phase are called cotidal lines, which are analogous to contour lines of constant altitude on topographical maps, and when plotted form a cotidal map or cotidal chart. High water is reached simultaneously along the cotidal lines extending from the coast out into the ocean, and cotidal lines (and hence tidal phases) advance along the coast. Semi-diurnal and long phase constituents are measured from high water, diurnal from maximum flood tide. This and the discussion that follows is precisely true only for a single tidal constituent.