Tide are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of Earth.
Tide changes proceed via the following stages:
- Sea level rises over several hours, covering the intertidal zone; flood tide.
- The water rises to its highest level, reaching high tide.
- Sea level falls over several hours, revealing the intertidal zone; ebb tide.
- The water stops falling, reaching low tide.
Oscillating currents produced by tides are known as tidal streams. 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.
From the highest level to the lowest:
- Highest astronomical tide (HAT) – The highest tide which 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) and Chart Datum (CD) – The lowest tide which can be predicted to occur. Modern charts use this as the chart datum. Note that under certain meteorological conditions the water may fall lower than this meaning that there is less water than shown on charts.
- High Water – The Highest level reached by the sea during that tidal oscillations.
- Low water – The lowest level reached by the sea during that tidal oscillations.
- Range of tide – is the difference between the levels of successive high and lower waters.
- Tidal Stream – is the periodical horizontal movement of the sea waters due to the tide raising forces of the Moon and Sun
- Bore – is a rapid build up in the level of water due to a tidal wave of unusual height, in narrowing estuaries and rivers. The tidal wave is generated by the flood being held back by the sea ward outflow of the river. Bores occur within a few minutes of the predicted low water.
- Flood Tide – is the flow of water due to rising tide
- Height of Tide – is the vertical distance between the chart datum and the sea level at that time
- Ebb Tide – is the outflow of water due to falling tide