Home' Afloat : September 2013 Contents Take monthly with water September 2013 53
*Malcolm Riley is the Public and Marine Officer for the Bureau of
Meteorology in Hobart. He has worked in all States with the exception
of Qld and is a Master V. He gives education courses on Marine
by Malcolm Riley*
The Bureau of Meteorology has updated the way it displays
tide information on its web pages. Mariners can now access
a map of Australia and zoom in on their area of interest to
view the stations available. The navigation of this map is similar
to google maps. See http://www.bom.gov.au/australia/tides/
The first thing mariners will notice is a lot more tide stations
that have tide times and heights; previously many only gave a
time correction from a standard port. Tide stream information
for Torres Strait and Port Phillip Heads can also be accessed.
There are cautionary notes that come with some of the new
tide heights and it is worthwhile to explain how the Bureau of
Meteorology calculates tide information.
First and foremost the heights and times you read from this
site and any tide book refer to that part of the sea level that is
caused by astronomical influences only.
This is a complicated calculation of the gravitational pull of
the moon and the sun. This gravitational pull varies as the moon
and the earth travel on their orbits. The earth also 'wobbles'
and rotates on its axis and has an irregular orbit around the
sun. This irregular orbit can vary the distance between the sun
and earth by five million kilometres. All of these movements
are predictable and occur in cycles. One cycle takes 21,000
years to complete, however, its effect is small and effectively
to note that it is not just the world's oceans that rise and fall.
The earths crust rises and falls as well, near the equator by as
much as 55 centimetres.
The Bureau of Meteorology works closely with the Australian
Hydrographer. Most of the navigation charts in circulation have
their datum as the LAT (Lowest Astronomical Tide).
If your chart shows a depth of 2 metres at spot X in a tide
port, this is the depth that occurs at the LAT. The tide prediction
for that port/area for a given day may be a high tide of 2.2 metres
and a low of 1.1 metres. When on your vessel over the spot X, at
high tide the water depth would be 4.2 metres and at low tide
3.2 metres. You may have some old charts that have different
datum; check in your chart title.
There are other factors other than the tide that can increase
or decrease the sea level in a given area. The air pressure
changes the depth
of the water. High
pressures 'push' the
water level down and
low pressure allows
it to rise, in the
latitudes the water
level changes about
one centimetre per
can either blow water into an area causing the water level to rise
or blow it away causing it to fall. An extreme example of wind
and air pressure working together to change sea level heights
is a storm surge from a tropical cyclone which can add many
metres to the sea level.
Large eddies in the ocean cause the sea level to rise or fall if
these eddies move near the coast it can effect the littoral zone
sea level. In the southern hemisphere anticlockwise eddies cause
the sea level to rise and clockwise eddies reduce the sea level.
In some large bays seiching can occur. This is the natural wave
harmonic of a particular bay. It is bit like try to carry a large tray
of water; the water slops back and forth. This seiching causes
the water level to rise and fall.
The graph at left is from the tide gauge at Spring Bay in
Tasmania. In the top portion the red line is the predicted
astronomical tide and the black line is the actual sea level
recorded. The reason for the difference in this case was a
large high pressure system sitting over Tasmania (around 1030
hectopascals). This high 'pushed' the water level down around
The bottom part of the graph is the sea level with the
astronomical influences removed. The fairly regular rises and
falls in water level on this graph are caused by seiching in the
bay and occur about every 30 minutes and vary the water level
by about 20 centimetres.
Pittwater Wooden Boats will commence classes
in October in Fundamentals of Wooden Boat
Building, both Traditional and Modern. Places are
still available for evening and day classes.
For more information, please contact
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