1. What causes the tides? Tides are caused primarily by the gravitational pull (sorry Newton, I meant gravitational attraction) on the earth by the sun and moon. Two high and two low tides occur each day. High tides occur approximately every 12 hours and 25 minutes, (12 lunar hours or 12.42 solar hours). 2. What's the Lunar Tide? Both the moon and sun control the tides, but as the moon is closer to the earth it has more than double the gravitational pull of the sun. As the moon orbits it pulls the earth in a 28 day orbit. Most people mistakenly believe the moon rotates around the earth, where in fact both the moon and earth rotate around a common axis or barycentre (centre of a mass system), this point is inside the earth about 4,700Km from it's centre), it is this point which follows an elliptical orbit around the sun and not the center of the earth. The gravitational forces between the earth and the moon are in perfect balance, however, the gravitational pull of the moon is stronger on the side of the earth closest to the moon and weakest on the side of the earth furthest from the moon, this is because one side of the earth is 12,756Km closer to the Moon. The result of this is anything free to move is attracted towards the moon where the gravitational pull is the strongest. The effect on earths oceans is to create a bulge in the mass of water closest to the moon and a similar bulge in the mass of water furthest from the moon. The second bulge is due to the lesser gravitational effect and the centripetal force (a center-seeking force analogous to swinging a ball on a string) of the earth's rotation. This is sometimes mistakenly referred to as a centrifugal force. An object traveling in a circular motion behaves as if it is experiencing an outward force, when in reality it is constantly accelerating and is therefore never in an inertial frame of reference. Since the force appears so real, it is often used as if it were. From Newton's first law, a body continues in a state of rest or uniform (straight line) motion unless acted on by another force. When a body travels in a circle a force must be applied to stop it from travelling in a straight line. This force is the centripetal force and is the only force necessary for circular motion. What is interpreted sometimes as a centrifugal force is the tendency of the object to follow in a straight line, which would move it outside of its circular trajectory. These two bulges correspond to the two high tides. Between these (roughly at right angles to the moon) are two depressions corresponding to the low tides. As the Earth rotates the two bulges move around the globe at approx. 1,600Km/Hr (at the equator) trying to keep up with moon. Each of these four bulges and depressions rotate around their own central or amphidromic point, these rotate anti-clockwise in the northern hemisphere and clockwise in the in the southern hemisphere. The exceptions to this rule are those amphidromic systems unconstrained by land masses, such as the South Atlantic, Mid Pacific and North Pacific, or where the system rotates around an island, such as Madagascar, Ceylon and New Zealand. The diurnal component of the lunar tide repeats every 24.82 Hours (1 lunar day). Some bodies of water have more than one amphidromic system, the North Sea has three. 3. What's the Solar Tide? Solar Tides are caused by the gravitational pull of the sun and operate in the same way as Lunar Tides. The gravitational pull of the sun is considerably smaller than (46 percent to be precise) that of the moon, as such it's effect is only seen on the size of the lunar tides, instead of the sun causing separate visible solar tidal bulges of its own. Generally, the solar tide either increases or reduces the size of the lunar tides. During new moon or full moon, when the sun is on the same or opposite side of the earth as the moon (alignment or syzygy) the tidal range is increased. With a first or third quarter moon, where the sun is at right angles to the moon (quadrature) the tidal range is reduced. (see Neap and Spring tides below) 4. So... It's just the Sun and moon that control the tides? No not exactly, in addition to the lunar and solar tides weather conditions serve to increase or reduce the tidal effects. Storm surges caused by wind and atmospheric pressure can increase high tide levels several metres. 10 millibars can result in a difference of 0.1 metres on the tide level - The change takes time to develop and applies to the average pressure over a large area of the sea. In addition, wave movement which is purely wind generated adds to the tide levels. Statistical models use the significant wave height (average of the highest 1/3 of waves) in an attempt to predict the wave movement effects. 5. Two Tides or one? The lunar tide always causes two bulges and two depressions in the worlds oceans, so why do we sometimes see only one tide or none at all at some places around the world? The answer to this is due to the localised effect of land mass and of ocean depths (or bathymetry) effecting the ebb and flood times. The bulging masses of water have to move around the globe to generate the tides but because of the coastline and bathymetry, the flow of water is restricted, sometimes disrupted completely, giving each location a unique tidal pattern. For example, here in the UK, Southampton experiences double-high water (a high water peak followed by a small drop, then a second high water peak followed by low water). Also in the UK Portland experiences a similar double-low water effect. These effects (known as harmonics) are small in most places and have little or no effect on the tides, however, in the shallow waters of the Hampshire and Dorset coast the harmonics are significant in size and in phase with each other. 6. What are Spring and Neap tides? The difference between the height of high and low water is known as the tidal range. The tidal range builds up to a maximum and falls to a low twice a month. The tides with the maximum range are called spring tides and those with the smallest tidal difference are called neap tides (meaning low). They are caused by the combined effects of the sun (solar tide) and moon (lunar tide) canceling out or reinforcing each other. Spring tides occur within 36 to 48 hours of a new or full moon (syzygy - when the sun and moon are in alignment) with neap tides occurring around seven days later (quadrature - when the sun and moon are at right angles to each other). The gravitational effect is strongest during New Moon (Sun and Moon on the same side of Earth) and weakest during Full Moon (Sun and Moon on opposite sides of Earth). Spring tides have the highest high-tides and lowest low-tides, whereas Neap tides have the lowest high-tides and highest low-tides. 7. Are spring tides higher around the Equinoxes? Yes, the Spring tides nearest to the equinoxes (21st March and 23rd September - when day and night are both 12 hours long) are slightly bigger than usual. 8. Can you get tides in open bays? Yes, a miniature amphidromic point develops forming a small tidal cell. If the mass of water is too small this is difficult if not impossible to measure. If the mass of water is large enough, this can develop into a rotary current. In reality for significant tides to be generated, there must be a large body of water through which the tide raising forces are exerted. The major oceans of the world are large enough for this. Others such as the Mediterranean have only small tides and the Baltic has no tides. 9. What's a Surge? A surge is caused by meteorological conditions such as wind and atmospheric pressure. For example, a strong onshore wind effectively piles water up around the coast. If this coincides with the normal high water, a higher high tide develops, likewise, an offshore wind at low water results in a lower low-water level than would normally be expected. Wind has an increased effect on shallow water such as the North Sea. High atmospheric pressure regions also have the effect of pushing water toward lower pressure regions. 10 millibars can cause a difference of 0.1 metres on the tide level. A Storm surge tends to travel at the same speed as the tidal bulge. Large storm surges develop as a result of distant weather and can effect the high/low tide times. A south westerly gale in the Hebrides may result in a local sea level change of only 0.2 metres, this will travel clockwise around the UK arriving at the Thames Estuary around 18 hours later (due to the water depth restricting the wave speed) - it can easily build up to a large storm surge of over 2 metres. 10. What is Slack water? This a period of little or no movement in the water level. It occurs between the ebb of one high water and the flood of the next (called slack before flood), also before the ebb of high water (called slack before ebb). i.e. around high water and low water there is a variable period (depending on local geography) before the next high/low cycle starts. At this time tidal water currents are at their weakest. The duration of slack water can be in minutes or extend several hours, it is unique to each location. 11. Where are the highest tides in the World? The highest tidal range in the world is 12.9 metres and occur at Burncoat Head (Bay of Fundy) Nova Scotia, Canada. The second largest occur at Avonmouth (Bristol Channel) in the UK, here the average spring range is 12.3 metres, with high water over 14 metres. 12. Over time, are there any repeated tidal patterns? Yes and No, similar patterns occur due to astrological effects, but they don't result in an exactly repeated pattern. For example, maximum tidal force occurs every 1600 years and we experience larger high tides every 18.6 years caused by the zero declination angle of the moon (which oscillates between an 18.5 and 28.5 degree angle). 13. Do the tides have any major impact on the Earth? Yes, the Earth's rotation is decelerating at a rate of 1.5 to 2 milliseconds per day per century. This is due to the friction of the tides on the worlds surface. At present the Earth's rotation is about 2 milliseconds slower than UTC per day, so every 450 to 500 days the time difference reaches 0.9 seconds and a leap second is added to UTC to keep UTC aligned with the "real time". 14. Are there always 60 seconds in a minute? No there can be 61. If a leap second is needed to correct for the slowing of earth's rotation by the tides, this is usually inserted after the 60th second of the last minute of 31st December or 30th June. 15. How far ahead can tides be predicted? Because the tides are caused by the effect of the moon and the sun, they can be predicted very accurately years ahead. However, the further in time we go the less accurate these predictions become. This is because the predictions are also based on local effects, like the oceans bathymetry (depth and contours) which is constantly changing, and the global sea level (which is rising). Truly accurate tide predictions, taking into account local atmospheric conditions, can only really be made 24 to 36 hours ahead. 16. What's tidal lag? This is where the two tidal bulges traveling around the earth don't lie directly in line with the moon. With the earths rotation at a constant angular velocity, the actual speed of rotation will increase from virtually zero at the poles to a maximum at the equator of 460m/s. In any given depth of water there is a maximum speed a wave can travel, this is related directly to the depth of the water. The average depth of the oceans is around 4Km, this gives a maximum wave speed of 200m/s. At a latitude of 26 degrees, the speed at the tidal bulges should be 400m/s. The tidal bulge can only travel at half this speed (200m/s), and so the tide lags by 90 degrees (6 hours 12 minutes). At higher latitudes the lag will be less, reducing to zero at around 65 degrees. At lower latitudes the lag will be 90 degrees. The ocean's depth would need to be 22Km to allow the shallow water tidal bulge to travel at 460m/s at the equator. Tides at latitudes above 65 degrees have no lag in the tidal bulge, these tides are called direct tides. Tides at latitudes below 26 degrees have a 90 degree lag, these tides are called indirect tides. 17. What's the maximum wave speed? The maximum wave speed for any given wave depends on the depth of the water the wave is traveling in. The tidal bulges generated by the lunisolar attraction are shallow water waves. A wave is classified as a shallow water wave if the ratio of the depth (the vertical distance of the still water level from the bottom) to the wave length (the horizontal distance between crests) is less than 0.04. Surface waves are limited by the density of water and the pull of gravity. They can only travel along the surface and their wavelength can at most be twice the average depth of the ocean (2 x 4000m). The fastest surface waves observed, are those caused by tsunami. The tsunami caused by an submarine earthquake in Chile in May 1960, covered the 11,000km to New Zealand in about 12 hours, traveling at a speed of about 900 km/h. Shallow Water Waves The relationship between wave speed (phase velocity) and depth of long surface waves in shallow water is given by: c x c = g x d x (p2 - p1) / p2 or,
c x c = g x d for water/air Where:
Deep Water Waves For deep water waves the relationship between speed and wavelength is given by:
l = g x t x t / (2 x pi) l = t x c (or t x c = g x t x t / (2 x pi)) c = g x t / (2 x pi) t = c x 2 x pi / g or t = c x 0.641 (s) Where:
To calculate c and l from wave period t (in sec): c = t x 1.56 m/s= t x 5.62 km/hr = t x 3.0 knot l = 1.56 x t x t (metres) Waves with a period of 10 seconds travel at 56 km/hr with a wavelength of 156m. A 60 knot (110 km/hr) gale can produce waves with periods of 17 seconds and wavelengths of 450m. These waves travel close to the wind speed (97 km/hr). A tsunami traveling at 200 m/s has a wave period of 128s, and a wavelength of 25,600m. These formula state wave speed increases with wave depth and the relative difference in density. For an ocean depth of 4000m, a wave's speed (celerity) would be about SQR(10 x 4000) = 200 m/s = 720 km/hr. Surface waves could theoretically travel much faster on larger planets or in a more denser medium than water. 18. What are tidal harmonic constants? All of the repeating tidal events (solar and lunar effects) occur on a fixed cycle called a tidal harmonic constituent. "Time Series" Tidal observations (manually recording the tide levels) over time for a particular location, yield the two harmonic constants of phase and amplitude (or time and size) for the tide, this observation includes bathymetrical, meteorological and astronomical components and is unique for the particular location. By comparing the harmonic constituents of lunar and solar effects with the observed time series, it is possible to determine the localised tide times by weighting the times derived solely from the lunar/solar harmonic constituents. This allows a realistic local tide estimation to be made taking into account the bathymetrical and sometimes average meteorological effects. 19. What are the Solar/Lunar constants? The constants making up the solar/lunar components are shown below, some of these have a small effect of the tides and are not included by tide models. Over 390 coefficients have been identified, but the effect of the coefficients other than those for the sun and moon amounts to less than 4% of the total tidal effect.
20. What are the Fundamental Tidal Frequencies? The fundamental tidal frequencies are used in tide calculations, the values (in degrees per hour) are shown below:
21. What are the Principal Tidal Constituents? The Principal Tidal Constituents are used in tide calculations, these are shown in the table below:
22. When's the maximum possible tides? The maximum possible tidal force occurs once every 1600 years, this is when:
23. Why do tidal rivers lag behind the river estuary? In rivers the effects of tides are dampened by friction of the river bed/walls and the river water flow. Flood and ebb times are generally slowed down and gradual. Sometimes a tidal river bore can develop with a fast high water flood (incoming tide). 24. What's a tidal river bore? A tidal river bore is where a large tidal (high water) flood current surges up the river getting stronger as the river gets narrower and shallower (width and depth). The incoming tide forces the wave front to travel faster than it can freely travel, given the depth of water, the crest then overtakes the trough causing the peak to sharpen and ultimately the wave to break which results in a tidal bore. The incoming high-tide is classified as a tidal bore only if it produces a discernible head wave within the river. There are over 100 rivers around the world known to produce bores. The largest bore is the Qiantang Tidal River Bore (Hangzhou Bay) in China this has reached a maximum height of 8.9m (30ft) and speed of 40Km/h. The River Severn (Avon/Gloucester) in the UK is the home of bore surfing, the bore has reached a height of 2.8m and speeds of 16Km/h, the best ones occur around the equinoxes. Bores can be small and barely noticeable to large waves moving fast and spectacularly upriver, the River Severn has over 250 bores a year. To read more about tidal river bores, go to the Tidal Bore Research Society or the Proudman Oceanographic Laboratory 25. What's bore surfing? Bore surfing is surfing a tidal river bore - being carried along a river by the bore as it crashes its way upstream. Not for the weak hearted, but extreme fun none the less. The home of bore surfing is here in the UK on the River Severn Tidal Bore. Not all bores are suitable for safe surfing, the Qiantang Tidal River Bore (known as the Qiantang Dragon) is a little on the wild side, only attempt this beast if you know what you're doing (and have safety boat support), up times for this bore are in the seconds (the longest I've heard of is 11 seconds). Safe bore surfing is concentrated on the smaller bores like the River Severn in the UK, Dordogne Mascaret (France), Pororroca (Amazon Basin) and Turnagain Arm in Alaska. To read more on Severn bore surfing check out Boreriders.com 26. How do I catch a bore? Surfing a bore is not really that difficult. Just remember people don't do it for bravado, to impress or such like, like surfing it's a connection between you and the wave - you'll either get it or you won't, if you don't you'll probably never will (so no worries). You only have a few chances a year to catch a good bore. This means you have to be prepared, you're best with a long or foamy board. Get there early, suit up and wait in the river (Remember the Severn bore can be up to 20 mins early and up to 30 mins late of its predicted time - so you could be sitting in the river for an hour!!!). If you've never surfed, It's imperative you get some tuition first. Or you run the risk of injuring yourself, other riders or at best missing it completely. The bore moves relatively slow, but with the sight and sound of a giant wave heading toward you, it seems like a train doing 100mph - don't panic and try not to be clucked (intimidated) by it. As soon as you see it coming, start paddling (up river), as soon as the leading edge of the bore catches up, you should feel the board lift and you'll be up and planing - watch out for overhanging branches. The worse part is when you loose the bore, you'll need to swim through the (up to 15km) current and all the debris, to the river bank - where, hopefully, you'll get a van ride up river to catch it again. Always remember the basic Surfing Rules:
The next few don't really apply to bores, but I've included them anyway.
27. Do the tides matter to me? More or less yes, most people can find use for the tide times. If you're in to water-sports of any description then tide times are an invaluable tool, particularly if visiting tidal rivers or launch sites only accessible for part of the tidal range. You may fancy a walk on the beach, maybe visiting parts only accessible during very low water. Surprisingly enough, even when planning a beach holiday, using the tide tables is very useful. If you're considering visiting a popular beach and high tide is around noon, you know there's a whole lot less beach to go around, whereas seven days later you know high tide will be early in the morning and you'll have a lot bigger beach to play with at mid-day. Even when scuba diving you will know water currents will be at their weakest during the neap tides and can use the tables to find slack water (period of little movement in the water level). 28. Are there any other movements of the ocean's? Yes, there's the ocean thermohaline circulation system - sometimes called the Great Ocean Conveyor (thermo = temperature and haline = salinity). This is where water from hotter climates is circulated around the globe. One noticeable part of this in the UK is the Gulf stream. The thermohaline carries over a billion megawatts of heat to the northern hemisphere, it is this which gives rise to higher temperatures in Europe than would normally be expected. And one of the reasons why scuba diving of the North-West coast of Scotland is so good. The gulf stream is thought to contribute six degrees Celsius to the average UK winter temperatures. London is located at the similar latitudes to Calgary and Moscow yet has warmer winters than even New York, which is hundreds of miles further south. The ocean conveyor occasionally shuts-down causing a "little ice-age". Coincidentally and as recently as February 2002, scientists have been saying the salinity of the North Atlantic is approaching a density very close to critical point - This is the point where the conveyor is expected to shutdown! If you are interested in the moon why not check out www.inconstantmoon.com for some cool pics. Get the tide times from the Proudman Oceanographic Laboratory or North Shields To read more about the ocean circulation system check out WHOI and Role of the Ocean Conveyor Belt as a Cause of Global Multidecadal Climate |
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