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. 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". 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
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