The regularity of astronomical forcing, combined with the geometry and friction of the real oceans result in spring tides occurring between one to two days after new or full moon. For any specific location, high water at spring tides occurs at approximately the same time of day: for example, at Liverpool spring high tides are always around midday and midnight.
Neap means low. Tides can be predicted far in advance and with a high degree of accuracy. Tides are forced by the orbital relationships between the Earth, the moon and the Sun. These relationships are very well understood and the position of the celestial bodies can be forecast very accurately into the future. However, as sea levels rise , the periodicity and range of the tide will be altered due to different bathymetry underwater depth and topography the physical features of an area.
Therefore predicting tides a long way into the future could be less accurate. Storm surges are short term sea level changes caused by the weather winds and atmospheric pressure that also affect tidal predictability.
Storm surges can only be forecast with the same time horizon as weather forecasting about two to five days. The predictability of planetary motion means that we can also reconstruct tides in the past. For instance, we know that the disastrous flooding of the Bristol Channel on 30 January New Style occurred at 9am — exactly the time of high water. This, combined with records of high winds, allows us to rule out a tsunami as the cause of the disaster.
Tidal knowledge also explains the phases of fighting in the famous Battle of Maldon 10 August New Style : the ebbing tide allowed Vikings to cross a causeway in the River Blackwater in Essex where they then slaughtered the Anglo-Saxon Brythnoth and his men.
The tidal force generated by other planets is negligible. The nearest approach of Venus to Earth is more than a hundred times further than the moon. The tidal force is approximately 0. The next most significant effect is from Jupiter, with a tidal force of 0. Even if all the planets aligned such that their effects combined the additional force would be insignificant. In UK waters, high tides occur approximately every 12 hours 25 minutes. It takes 24 hours and 50 minutes a lunar day for the same location on Earth to re-align with the moon.
This is because the moon orbits the Earth in the same direction that the Earth rotates on its axis. This extra 50 minutes means that the same location will experience high tides every 12 hours 25 minutes. This varies between different locations as the local geography has an effect on tidal dynamics. Although most coastal locations in Britain experience two tides a day there are some places which experience what is known as a double-high water for example, Southampton or double-low water for example, Weymouth.
Dynamical effects the mathematics governing water motion combine with the bathymetry water depth to create higher frequency tidal harmonics that interact with the primary tidal forces to create these more complex tides. The tide gauge at Lowestoft displays a mixed semidiurnal tide where the diurnal daily tidal constituent is large enough to cause significant changes to the high and low tides every lunar day.
The diagrams below are tidal curves for Liverpool on the west coast, Lowestoft on the east coast and Weymouth on the south coast. The first tidal curve is from Liverpool where there is a semi-diurnal tidal regime. These bulges represent high tides while the flat sides indicate low tides. A lunar day is how long it takes for one point on the Earth to make one complete rotation and end up at the same point in relation to the moon.
The reason that a lunar day is longer than a normal hour day is because the moon rotates around the Earth in the same direction that the Earth is spinning. Tides are very long waves that move across the oceans. They are caused by the gravitational forces exerted on the earth by the moon, and to a lesser extent, the sun. When the highest point in the wave, or the crest, reaches a coast, the coast experiences a high tide. When the lowest point, or the trough, reaches a coast, the coast experiences a low tide.
On the side of Earth farthest from the moon, the moon's gravitational pull is at its weakest. At the center of Earth is approximately the average of the moon's gravitational pull on the whole planet.
Arrows represent the force of the moon's gravitational pull on Earth. To get the tidal force—the force that causes the tides—we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth.
The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges. Arrows represent the tidal force. It's what's left over after removing the moon's average gravitational pull on the whole planet from the moon's specific gravitational pull at each location on Earth.
These two bulges explain why in one day there are two high tides and two low tides, as the Earth's surface rotates through each of the bulges once a day. The Sun causes tides just like the moon does, although they are somewhat smaller. When the earth, moon, and Sun line up—which happens at times of full moon or new moon—the lunar and solar tides reinforce each other, leading to more extreme tides, called spring tides. When lunar and solar tides act against each other, the result is unusually small tides, called neap tides.
There is a new moon or a full moon about every two weeks, so that's how often we see large spring tides. When the gravitational pull of the Sun and moon are combined, you get more extreme high and low tides. This explains high and low tides that happen about every two weeks. Note: this figure is not to scale. The Sun is much bigger and farther away.
Wind and weather patterns also can affect water level. Strong offshore winds can move water away from coastlines, exaggerating low tides.
Onshore winds can push water onto the shore, making low tides much less noticeable. High-pressure weather systems can push down sea levels, leading to lower tides.
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