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.
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
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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