Tides are the rise and fall of sea levels caused by the combined effects of gravitational forces exerted by theMoon, Sun, and rotation of the Earth.
The times and amplitude of tides at a locale are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean, by the amphidromic systems of the oceans, and the shape of the coastline and near-shore bathymetry . Some shorelines experience a semi-diurnal tide - two nearly equal high and low tides each day. Other locations experience a diurnal tide - only one high and low tide each day. A "mixed tide"; two uneven tides a day, or one high and one low, is also possible
Characteristics
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.
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 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.
The tidal forces affect the entire earth, but the movement of the solid Earth is only centimeters.
The atmosphere is much more fluid and compressible so its surface moves kilometers, in the sense of the contour level of a particular low pressure in the outer atmosphere
Timing
The tidal forces due to the Moon and Sun generate very long waves which travel all around the ocean following the paths shown in co-tidal charts. The time when the crest of the wave reaches a port then gives the time of high water at the port. The time taken for the wave to travel around the ocean also means that there is a delay between the phases of the moon and their effect on the tide. Springs and neaps in the North Sea, for example, are two days behind the new/full moon and first/third quarter
moon. This is called the tide's age
Aisha Alsalhi
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