Causes, Predictions and Effects.
The tides are
the regular, predictable
changes in sea level which every boater must know
is vitally important, for
coastal Navigation, to understanding and be able to predict tidal
Why does the
sea rise and fall?
The rise and
fall of sea levels are caused
by the gravitational pull of the sun and moon and are also affected by
centrifugal forces generated by the earth’s rotation.
The effect is
subject to the relationship
between the earth, the moon and the sun at any given time.
The moon being
closer to the Earth has a
greater influence on the oceans than the sun.
As the moon
rotates around Earth a bulge in the seas occurs beneath the moon where its gravitational pull is
the opposite side of the
globe, where the moons pull is negative the centrifugal force of the
earth’s rotation creates a second bulge away from the
These are the
high tides, between which
are the low waters.
It is also why
there are normally two
tides every 24 hours and 50 minutes, which is the time it takes for the
rotate around Earth.
And it is that
extra 50 minutes which mean
the times alter from day to day.
are modified by the ‘solar-tides’ caused by the gravitational pull
of the sun.
It is this
reinforcing or partially
canceling out of the ‘lunar-tide’ which causes springs and
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experience a high water every 12 hours 25
minutes with a corresponding low water in between, these are known as
are also influenced by the
shape of the coastline and sea bed.
geography can, in some places,
cause double high waters, or double low waters.
There are even
some coasts which have only
one, ‘diurnal’ phase per day and places where it can vary
between ‘diurnal’ and ‘semi-diurnal’.
As the sea level rises (floods)
and falls (ebbs) it produces what are known as tidal
water’ is the interval at high and low water where the
ceases momentarily before changing direction.
the highest high waters and
corresponding lowest low waters occur a day or so after the full moon.
least, occur shortly after the Moon is in its First and Third
The time between high and
low water is
approximately the same whether it’s a neap or spring tide.
So, this means that at
springs the water has to
rise further so the ‘stream’ must be faster, than at
The largest tidal ranges of
the year, the
occur shortly after the new and full
moon closest to the equinoxes.
The spring or ‘vernal’
usually around the
21st of March, and the autumnal
around the 23rd of September.
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the rising and falling of the ocean is caused by the
gravitational influences of the Earth-Moon-Sun and as the behavior of
orbits are known extremely accurately, so their influence on the oceans
However, as the
topography also has an
influence, each location has to have its own record of sea level behavior
using a tide-gauge.
By combining all this
accurate predictions for any date in the future, at a given location,
However, there are
factors such as changes in
seabed, rises in sea level, as well as the weather that will affect
behavior at any given time.
Tables generally give a
list of the maximum
and minimum sea levels heights and times for each day.
However, the heights
given are related to a
calculate the actual
depth of water at a
given point, add the charted depth to the height in the
Areas which dry will be
shown on the chart as
having negative values these should be subtracted from the tabulated
You must always check
that your charts and
tables are based on the same datum.
The most commonly used
‘lowest astronomical tide’ and ‘mean lower low
N. O. A. A. In the
United States uses the
mean-lower-low-water, MLLW, as its datum.
This means that some
tidal states may show as
having a negative height.
Most other national
agencies use the lowest-astronomical-tide (LAT) as their
The advantage of using
LAT is that all
tabulated heights are shown as positive numbers.
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It should be remembered
that the tables for a
given location are predictions based on average historical data.
There are other
factors which contribute
to the rise and fall of the sea level.
effects such as wind and
atmospheric pressure can also contribute.
A storm surge
for instance can add several
meters to the predicted water level.
Wind can speed
up or retard the time of
high or low water depending on whether it is with or against the tidal
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The most famous
example of an extreme
tidal range is that of the Bay of Fundy, where the difference between
low water, the range, can exceed 16 meters.
This is closely
followed by the Bristol
Channel where the extreme range at Avonmouth is just over 15
While on the
other hand areas like the
Mediterranean Sea and the Baltic Sea have no significant rise and fall
close to their narrow entrances.
fast streams can
occur anywhere where
tide flowing from one large area of water to another has to pass
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