Tides
Tides:
- causes of the tides:
- although tides were known to the ancients and the Roman scholar Seneca
linked them with the moon, an understanding of them did
not come until Sir Isaac Newton's Principia in 1687.
- the rising tide is called a flood (from 13thC English sailors term flod)
& the falling tide is called an ebb
- tides are governed by the relative positions of the moon, and secondarily
of the sun, both of which exert a gravitational pull and attract the waters
of the earth towards them as well as many other factors including ocean
depth, coastal topography, tilt of the earth, all of which create some 120
tides of which a few are dominant. Wind and storm
surge also adds to tide heights;
- the sun exerts a gravitational pull 180x as strong as the moon's
gravitational pull on earth, but the variation in the moon's force is
2.2x larger than the variation in the sun's force, resulting in the moon
creating the major variance in tides.
- although it is the gravitation of the moon & the sun that raises
the tides, the energy of the churning waters is extracted from the
rotational energy of the earth spinning on its axis.
- see also:
- tidal friction:
- tidal friction is the friction of the earth's waters moving over the
earth's solid surface as a result of the tides.
- tidal friction both lengthens the day by 1 second every 50,000yrs (or
2msec per century and contributes to the need to add leap
seconds) and
increases the size of the orbit of the moon (and how long it takes to
revolve around the earth). In a few billion years, the length of the day
will have increased to the same length as the moon's revolution around
the earth of approx. 40 days, and the moon will appear stationary in our
skies. The solar tides will then cause the moon to approach the earth
once more.
- in fact, the earth exerts a much greater gravitional pull on the moon
resulting in tides on the moon which slowed down the rotation of the
moon to be in synch with its revolution around earth so that both take
27 days and thus always has the same side facing earth, thus we always
see the same part of the moon!
- in addition, the daily tides create much smaller daily variations to
earth's rate of rotation of +/- 40microseconds.
- height of tides:
- the water level with time is approximately a sinusoidal curve with a
wavelength of 12-13hrs, ie. there are two high tides and two low tides every
25hrs
- tidal ranges are dependent on:
- distance of moon-earth (greatest at perigee):
- as the moon's orbit around the earth is an ellipse & not a
circle, when it is at its closest to earth (perigee) once a
lunar month, it's tidal influence is 30-48% greater than when it
is at its far point (apogee) as although it only comes 9-14%
closer at perigee than apogee, gravitational forces vary
inversely with the square of the distance and the variation in
gravitational forces vary inversely with the cube of the
distance. Indeed, at Minas Basin, it is this factor that has a
more important influence on the tidal range than the spring or
neap tides , as the tidal range increases by 3-6m at perigee.
- part of the lunar cycle (greatest near New Moon - the spring
tide):
- highest high tides and lowest low tides each month occur around the time
of the new moon and secondarily around the time (usually after) the full
moon as the gravitational pull of moon and sun are in the same direction,
these are called "spring tides"
although they have no relation to the season spring.
- a proxigean spring tide is an unusually high tide at New
Moon when moon is at perigee and occurs at most once every 1.5yrs.
- lowest high tides and highest low tides occur around the time of the first
& last quarter moons as the pull of the moon is at right angles to the
pull of the sun, these tides are called "neap tides".
- sizes, boundaries & depths of oceanic basins
- resonance of the tidal system
- latitude + season + part of lunar cycle (greatest when sun and
moon are at zenith in the sky)
- highest tidal ranges occur at:
- Minas basin,
the eastern extremity of the Bay of Fundy between New
Brunswick and Nova Scotia:
- avg. range is 12m but can reach 16m, at low tide, Wolfville's
harbour is empty!
- near the mid-point of an incoming tide, a tidal bore may be
seen tumbling upstream in some of the rivers flowing into the
basin. A tidal bore forms when the incoming tide pushes
its way upstream against the outgoing freshwater flow of the
river.
- at Cape Split, the awesome display of the roaring incoming
tides and its currents exceed 8 knots (4m/s) and the flow in the
deep 5km wide channel on the north side of Cape Split equals the
combined flow of all streams & rivers of earth (4 cubic km
per hour). 3 hours later the spectacle pauses & then begins
flowing in the opposite direction. The 14 billion tonnes of
water entering actually causes the Nova Scotia countryside to
tilt slightly under the immense load.
- highest tides occur a day or two after the astronomical peak.
- this is mainly due to resonance of the tidal system which is
effectively bounded at this outer end by the edge of the
continental shelf with its ~40:1 increase in depth. The system
has a natural period of approx. 13hrs which is close to the 12h
25m period of the dominant but relatively gentle lunar tide of
the Atlantic Ocean.
- north-west coast
of WA - avg. range is ~10m
- Sea of Okhotsk
- the Bristol Channel on the west coast of England
- Ungava Bay in northern Quebec
- timing of tides:
- each subsequent day, the high tide of the same part of the day is 34 to
71min later than that on the previous day depending on the phase of the moon
and whether it is low tide or high tide as they are not fully symmetrical.
- the reason for this is the revolution of the moon around the earth
which takes ~27 days and as there are 24hours in a day, high tide,
ignoring other effects such as the sun, local topography, etc, would
occur 24/27 of an hour later each day, at the time the moon is highest
in the sky or some 12h 25m later when it is directly underneath us.
- thus if you want to get low tide at sunrise or sunset for photography,
this timing will approx. recur in Victoria coast every new moon or full
moon,
although the actual height will depend on the position of moon & sun
and the exact timing depending on the season.
- In actual fact on the Victorian surf coast, the low tide will be
approx. at sunrise at new moon & full moon for Oct-March with the
subsequent low tide approx. 12.5hrs later (ie. before sunset). In March
& Sept, during the equinoxes when daylight duration = night
duration, low tide will occur at both sunrise and sunset during a full
moon or new moon. From April to September, low tide tends to occur at
sunset at new moon & full moon with the preceding low tide ~12.5hrs
earlier (ie. before sunrise).
- local lag times:
- topographic features such as bays, estuaries have "lag
time" from the regional oceanic high tide depending on the
distance from the opening to the ocean, the narrowness of the
opening and the volume of the bay.
- for example Port Phillip Bay in Victoria:
- some places such as Gulf of Carpentaria have only one high tide a
day as the lag time is about 12hrs as it is a long way from the
Indian Ocean "entrance" and the Pacific Ocean
"entrance" is too small to be effective at allowing rapid
tidal equilibrium. This 12hr lag time coincides with the 12hr cycle
of oceanic tides and one tide each day gets nullified. (see as the
Karumba tides)
- tide prediction charts:
- the 1st official tide charts were issued by the British Admiralty in
1833 & the US began to compile data in 1853.
- for Australia, see National Tidal Centre tide
charts
- tidal currents:
- high velocity tidal currents can be produced at inlets to seas, bays,
which can make many of these impossible to navigate at times when the
current is not slack (eg. the Saltens Maelstrom, Norway). Ships caught
in a swirl of water can be hurled against rocks at high speed &
crushed to pieces.
- in some places, tidal currents are being used to generate electricity
Storm
surges and storm tides:
- in addition to the above "astronomical tides", in storm
conditions such as tropical cyclones, a "storm surge" long
wavelength tide may be created and when added to the astronomical tide will
result in a "storm tide"
Earth tides:
- in addition to water tides, the crust of the earth also has a tidal range
due to the sun and moon's gravitational pull which results in a tidal range
of some 8" to 12" twice a day which we cannot detect without
highly sensitive instruments.
- the water tides in themselves distort the earth's crust as the movement of
water results in transient depression of the crust at high tide due to the
weight of water.
Air tides:
- Laplace discovered tides in the air, but these were not related to
gravitational effects of the sun and moon, but more to the heating of the
air by the sun creating a "high tide" at 10am & 10pm, and low
tides at 4am and 4pm.
- there are minimal gravitational air tides as well.
Tidal chart
lag times
- Victorian coastal:
- lag times as measured at Port Phillip Heads:
- Apollo Bay -0h 25m
- Lorne -0h 20m
- Barwon Heads Bridge +0h 15m
- Rip Bank -0h 15m
- Seal Rocks - Woolamai 0h 00m
- Venus Bay 0h 00m
- Queenscliff Pier +0h 30m
- Portsea Pier +1h 20m
- Sorrento Pier +2h 10m
- Portarlington Pier +3h 10m
- Rosebud, Dromana, Mornington +3h 15m
- Frankston, Sandringham, St Kilda, Altona +3h 15m
- Melbourne (Williamstown) +3h 20m
- Geelong +3h 30m
- NSW coastal lag times as measured from Middle Head, Sydney Harbour Lat.
33º 50' Long 151º 15' E:
- Tweed Heads, river entrance 0h 04m
- Ballina, river entrance 0h 07m
- Maclean 2h 00m
- Coffs Harbour jetty -0h 07m
- Macksville 2h 00m
- Port Macquarie river entrance 0h 19m
- Harrington 0h 11m
- Taree 2h 25m
- Forster, harbour entrance 0h 01m
- Nelson Bay, Port Stephens 0h 30m
- Stockton Bridge 0h 20m
- Swansea lake mouth 0h 00m
- Woy Woy Bridge 2h 15m
- Little Patonga (Juno Point) 0h 09m
- Homebush Bay 0h 23m
- Canterbury Bridge, Cooks River 0h 40m
- Port Kembla -0h 01m
- South Shoalhaven Heads 0h 55m
- Nowra Bridge 2h 00m
- Sussex Inlet, Coastal Patrol Jetty 0h 35m
- Batemans Bay, Princes Hway Bridge 1h 15m
- West Tuross Head 1h 40m
- Narooma Old Municipal Wharf 0h 40m
- Wallaga Lake, Regatta Point 2h 50m
- Eden, Fishing Co-op Jetty 0h 08m
- Merimbula Lake, Bonnie Doon 2h 40m
- Merimbula Wharf 0h 40m
- Pambula Lake Tea Tree Point 1h 10m
- Qld coastal lag times:
- KARUMBA TIDES in
the Gulf of Carpentaria - why is there only one high tide a day here?