Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by moon and the sun and the rotation of the Earth. Other natural cycles such as the lunar nodal cycle also affect tides.
Tides changes in stages. The sea level rises over several hours, covering the intertidal zone. This is the flood tide. The water rises to its highest level called high tide. Then sea level falls over several hours, exposing the intertidal zone. This is the ebb tide. When the water stops falling low tide is reached.
Tides produce oscillating currents known as tidal streams. Slack water or slack tide is the moment when the tidal current ceases. The tide then reverses direction and starts to turn. Slack water usually occurs near high water and low water.
Tides commonly have two high waters and two low waters each day. This is called semi-diurnal, One tidal cycle per day is referred to as diurnal. The two high waters on any given day are not the same height. This is difference in heights is called the daily inequality and these tides are described as the higher high water and the lower high water in tide tables. The two low waters each day are referred to as 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.
Spring tides
The combined tide raising forces of the Moon and the Sun are at their greatest effect when the Sun and the Moon are in line with the Earth. This happens at new Moon and full Moon, the first and last quarters of the Moon’s cycle. These variations affect the high tidal wave and hence the range of the tide. The range of tide is the difference in level between successive high and low waters. Shortly after the full or the new Moon a locality will experience its highest high waters and lowest low waters of the lunar month. Tides in this period are called Spring Tides not after the season but from the word spring which has the meaning "jump, burst forth, rise", as in a natural spring.
Neap tides
The lowest high waters and the highest low waters of the lunar month occur twice a month in the first and third quarters of the moon. These tides are called Neap Tides when the difference between the high and low tide is the least. At this time the Sun and Moon are at right angles to the Earth and their total gravitational pull on the Earth’s water is weakened because it comes from two different directions. The origination of Neap is uncertain.
Tides tables are available on the United Kingdom Hydrographic Office Admiralty EasyTide website.
