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Full moon rising over water
 

Where are the highest tides in the world?

In Canada there has been some rivalry between Arctic Quebec and the Canadian Maritimes over who has the world's highest ocean tides. The Canadian Hydrographic Service has declared a tie between the famous tides of the Bay of Fundy and those of Ungava Bay on the northern coast of Quebec.

It has long been recognised that the tides at Burntcoat Head on the shore of Minas Basin, Bay of Fundy can in extreme reach a range of 17 metres. After 200 days of measurements at Leaf Basin in the southwest corner of Ungava Bay it is estimated that in the extreme the tides there could have a range of 16.8 metres. It is of course possible that points near Burntcoat Head or Leaf Basin, as yet unmeasured by tide gauges, could have slightly higher tides so lacking further data a dead heat has been declared.


The next highest tides are in the Bristol Channel where the extreme range at Avonmouth is just over 15 metres. 

Facts About Lunar Tides

  • The gravitational force of the moon is one ten-millionth that of earth, but when you combine other forces such as the earth's centrifugal force created by its spin, you get tides.
  • The sun's gravitational force on the earth is only 46 percent that of the moon. Making the moon the single most important factor for the creation of tides.
  • The sun's gravity also produces tides. But since the forces are smaller, as compared to the moon, the effects are greatly decreased.
  • Tides are not caused by the direct pull of the moon's gravity. The moon is pulling upwards on the water while the earth is pulling downward. Slight advantage to the moon and thus we have tides.
  • Whenever the Moon, Earth and Sun are aligned, the gravitational pull of the sun adds to that of the moon causing maximum tides.
  • Spring tides happen when the sun and moon are on the same side of the earth (New Moon) or when the sun and moon are on opposite sides of the earth (Full Moon).
  • When the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker; these are called neap tides.
  • Spring tides and neap tide levels are about 20% higher or lower than average.
  • Offshore, in the deep ocean, the difference in tides is usually less than 1.6 feet
  • The surf grows when it approaches a beach, and the tide increases. In bays and estuaries, this effect is amplified. (In the Bay of Fundy, tides have a range of 44.6 ft.)
  • The highest tides in the world are at the Bay of Fundy in Nova Scotia, Canada.
  • Because the earth rotates on its axis the moon completes one orbit in our sky every 25 hours (Not to be confused with moon's 27 day orbit around the earth), we get two tidal peaks as well as two tidal troughs. These events are separated by about 12 hours.
  • Since the moon moves around the Earth, it is not always in the same place at the same time each day. So, each day, the times for high and low tides change by 50 minutes.
  • The type of gravitational force that causes tides is know as "Tractive" force.

Watch a time lapse video showing a tide cycle in the Bay of Fundy, Nova Scotia, Canada.

Are the Tides moving the Moon away from the Earth?

As the earth spins, the gravity of the moon pulls water, air and even land up towards it in a bulge called the 'tide.' On the oceans, this tide often rises several feet. But the spinning earth pulls the tide 'ahead' of the moon. So the tide is not truly directly under the moon, but just east of it. Then the increased gravity effect from the tide, racing ahead of the moon, gradually, and ever so imperceptibly pulls the moon forward in its own orbit even faster.

The end result is a kind of tidal 'sling shot' effect that pulls the moon forward in its orbit, faster and faster over time, causing it to speed up. As it speeds up, it moves farther away from the earth. This is the mechanism that has caused the moon to 'recede' from the earth by many hundreds of thousands of kilometers, in the billions of years since it was formed, after the earth underwent a collision with another, Mars sized planet, in the early days of the solar system. 


 


 

Tides


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A tide is the regular and predictable movement of water caused by astronomical phenomena - the way the earth, moon and sun move in relation to each other and the force of gravity. These are the values that you can see in tide tables.
Movement of water caused by meteorological effects (for example winds and atmospheric pressure changes) are called surges.  These are not easily predictable and require powerful computers and sophisticated software to predict just 36 hours in advance. These are the reasons why tide table predictions do not always agree with observations. POL develops storm surge models for flood forecasting that have been run at the Met Office since 1978. (A large positive storm surge can add a few metres to the predicted water level.)
There is also wave movement which is purely wind generated and impossible to predict accurately. Therefore statistical values are used such as significant wave height which is the average of the highest 1/3 of waves.

The tides which we see in the oceans are due to the pull of the Moon and the Sun. The simplest explanation is that the water on the side of the Earth closest to the Moon is pulled, by the Moon's gravitational force, more strongly than is the bulk of the Earth; whereas the water on the side furthest from the Moon is pulled less strongly than the Earth. The effect is to make bulges in the water on opposite sides of the Earth. The effect of the Sun's pull is similar and the tides that we see are the net effect of both pulls.

Most people think the moon rotates round the earth. In reality, the earth and the moon rotate about a common centre just inside the Earth's surface (indicated by the light red dot on the diagram). At the centre of the earth the two forces acting: gravity towards the moon and a rotational force away from the moon are perfectly in balance. They have to be otherwise the earth and moon would not stay in this orbit.

The 'tide-generating' force is the difference between these two forces. On the surface of the earth nearest the moon, gravity is greater than the rotational force, and so there is a net force towards the moon causing a bulge towards the moon. On the opposite side of the earth, gravity is less as it is further from the moon, so the rotational force is dominant. Hence there is a net force away from the moon. It is this that creates the second bulge away from the moon. On the surface of the earth, the horizontal tide generating forces are more important than the vertical forces in generating the tidal bulges.

When the pull from the Sun adds to that of the Moon the tides are large and we call them Spring tides whereas when the pulls are at 90 degrees the tides are small and we call them Neap tides. The heights of spring tides are governed by the distance of the Moon from the Earth, being largest at Perigee (when the Moon is closest to the Earth) and smallest at Apogee (when the Moon is at its furthest).

Because the Sun's pull is aligned with that of the Moon at New Moon and Full Moon these are the times when Spring Tides occur. The pull of the Sun is less than half that of the Moon and so the frequency of the tides is determined by the apparent passage of the Moon around the Earth which takes just over a day. We, therefore, in most places on the Earth have two tides a day with the time of each becoming later from one day to the next by just under an hour a day. (The actual period is, of course, determined by the rotation of the Earth and the orbit of the Moon.)

You might expect that as Britain passes under the bulge of water, time of high water would be roughly the same for all points on the coast, but it isn't. The problem is caused by the land that 'gets in the way' of the moving water. As the earth rotates, the water has to move to generate the high tides but because of the shape of coastlines and the variation in sea depth (bathymetry), there is a lag. Every location has a unique coastline and bathymetry - which gives each location its unique tidal pattern.

In UK waters, approximately every 12 hours 25 minutes. You may wonder why it is not exactly 12 hours, but you must remember that the moon is also orbiting around the earth. By the time a point on the earth's surface has completed 1 rotation (12 hours) the moon has also moved a small amount, so the earth has to rotate for an extra 25 minutes  to be under the high water bulge once again.

A phenomenon which is generally not realised is that the air and solid landmasses also move up and down due to the tidal forces. Although the movement is much less in the land than that in the sea it can amount to a metre of vertical shift. It might be expected that the time of high tide would be when the Moon is on the meridian. This is not so. The reason is that, because of the Earth's rotation and friction, the tidal bulge gets left behind a little.

Predicting the Tides around the UK

Since the tide is caused by the astronomy of the earth-moon-sun system which is known very accurately and can be predicted well into the future, the tides can also be predicted well into the future. So if you want to plan your sailing club events for the next year, get in touch and (for a small fee) Applications Team will provide you with the tide table.

When trying to predict well into the future, we have to take into account the rise in global sea level. The further into the future we try and predict, the more significant this effect can become.

UK tides information for all standard and secondary ports is provided to the BBC Weather Centre by the UK Hydrographic Office (UKHO).



UK tides on BBC website

This allows them to provide an on-line six day tidal prediction service aimed at leisure mariners, holidaymakers and enthusiasts of coastal pursuits. Find out more about tides in their tidal features sectio. The locations are loosely divided up into 'regional' sections around the coastline, but these may not correspond exactly with geographic, political or BBC TV regional boundaries.

Tidal Power generation

Tidal energy can be extracted by two means: inserting a water turbine into a tidal current, or building ponds that release/admit water through a turbine. In the first case, the energy amount is entirely determined by the timing and tidal current magnitude. However, the best currents may be unavailable because the turbines would obstruct ships. In the second, the impoundment dams are expensive to construct, natural water cycles are completely disrupted, ship navigation is disrupted. However, with multiple ponds, power can be generated at chosen times. So far, there are few installed systems for tidal power generation (most famously, La Rance by Saint Malo, France) which faces many difficulties. Aside from environmental issues, simply withstanding corrosion and biological fouling pose engineering challenges.

Tidal power proponents point out that, unlike wind power systems, generation levels can be reliably predicted, save for weather effects. While some generation is possible for most of the tidal cycle, in practice turbines lose efficiency at lower operating rates. Since the power available from a flow is proportional to the cube of the flow speed, the times during which high power generation is possible are brief.




Underwater tidal turbine diagram

Intertidal ecology

Intertidal ecology is the study of intertidal ecosystems, where organisms live between the low and high water lines. At low water, the intertidal is exposed (or ‘emersed’) whereas at high water, the intertidal is underwater (or ‘immersed’). Intertidal ecologists therefore study the interactions between intertidal organisms and their environment, as well as among the different species. The most important interactions may vary according to the type of intertidal community. The broadest classifications are based on substrates — rocky shore or soft bottom.

Intertidal organisms experience a highly variable and often hostile environment, and have adapted to cope with and even exploit these conditions. One easily visible feature is vertical zonation, in which the community divides into distinct horizontal bands of specific species at each elevation above low water. A species' ability to cope with desiccation determines its upper limit, while competition with other species sets its lower limit.

Humans use intertidal regions for food and recreation. Overexploitation can damage intertidals directly. Other anthropogenic actions such as introducing invasive species and climate change have large negative effects. Marine Protected Areasare one option communities can apply to protect these areas and aid scientific research.




Rocky shore at low tide

Biological rhythms

The approximately fortnightly tidal cycle has large effects on intertidal organisms. Hence their biological rhythms tend to occur in rough multiples of this period. Many other animals such as the vertebrates, display similar rhythms.

Examples include gestation and egg hatching. In humans, the menstrual cycle lasts roughly a month, an even multiple of the tidal period. Such parallels at least hint at the common descent of all animals from a marine ancestor.


 
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