The tidal force causes Earth—and its water—to bulge out on the side closest to the moon and the side farthest from the moon. These bulges of water are high tides. High tide left and low tide right in the Bay of Fundy in Canada. Image credit: Wikimedia Commons, Tttrung. Photo by Samuel Wantman. As the Earth rotates, your region of Earth passes through both of these bulges each day. When you're in one of the bulges, you experience a high tide.
When you're not in one of the bulges, you experience a low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world. This animation shows the tidal force in a view of Earth from the North Pole.
As regions of Earth pass through the bulges, they can experiences a high tide. Tides are really all about gravity, and when we're talking about the daily tides, it's the moon's gravity that's causing them.
As Earth rotates, the moon's gravity pulls on different parts of our planet. The moon's gravity even pulls on the land, but not enough for anyone to tell unless they use special, really precise instruments. When the moon's gravity pulls on the water in the oceans, however, someone's bound to notice. Water has a much easier time moving around, and the water wants to bulge in the direction of the moon. This is called the tidal force.
Because of the tidal force, the water on the side of the moon always wants to bulge out toward the moon. This bulge is what we call a high tide. As your part of the Earth rotates into this bulge of water, you might experience a high tide. An illustration of the tidal force, viewed from Earth's North Pole. The lunar day is 50 minutes longer than a solar day because the moon revolves around the Earth in the same direction that the Earth rotates around its axis. High tides occur 12 hours and 25 minutes apart.
It takes six hours and Unlike a hour solar day, a lunar day lasts 24 hours and 50 minutes. 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. The second graph is a tidal curve from Lowestoft showing the mixed nature of the tidal range on a daily basis.
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