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A
tide chart is a graphical representation of the
tidal cycle. Because the laws of planetary motion,
the forces of gravitational attraction and the other
tide-causing forces are so well known, we can
predict the height of sea level with pretty good
accuracy. These predictions are published by various
governmental and private agencies into little
booklets, called tide tables, or calendars, called
tide charts.
The
tide chart shown below represents the tidal pattern
for one day of tides in Moss Landing, California.
Time appears along the X-axis (the horizontal axis)
and the height of the tide appears along the Y-axis
(the vertical axis). The hills and valleys shown on
the chart represent the height of the tide at a
given time. Studying this chart carefully, we can
see the presence of two "hills" or high tides and
two "valleys" or low tides. As mentioned above, we
name the high tides and low tides according to their
relative height. Thus, we have the the low low, the
low high, the high low and the high high tides from
midnight until midnight.
 So
far, we have taken for granted this concept of a
tide height. But what is the height of a tide and
how do we figure it out? The tide height is computed
from a standard tide height called the zero tide
height. The zero tide height is the average height
of all the low tides or lowest tides in a given
period of time for a given location. Without going
into all the details, suffice it to say that there
is a zero tide height from which all the other tide
heights can be gauged.
In
the figure above, the zero tide height is indicated
by a white line. Tides above this line have positive
values for their heights and tides below this line
have negative vales for their height. For example,
what is the height of the low low tide for this day?
Because the dark blue graph touches the zero line at
the low low tide, the height of this tide is zero
feet. Using the same process, calculate the height
of the lo hi, hi lo and hi hi tides. You should be
able to see that they are slightly less than 4 feet,
a little more than 2.5 feet and a little more than 5
feet, respectively.
Note
also that the times of the 4 tides are given at the
top of the graph. For example, what is the time of
the high low tide? It is 1:12 PM.
Another useful way to determine the times of tides
is a tide table. The time and height of the tides
are listed in columns for each date. What is the
time of the hi hi tide on March 11? What is the
height of the lo hi tide? What is the time and
height of the lo lo tide? When is the hi lo tide?
Did
you note that the times given are in GMT? What are
the local times for the tides you just identified?
Make sure you can identify these
correctly. Use the links to tide tables and make
sure you can figure out the different tides, their
heights and their times. Many people think this is
easy but when I give exam questions on it, too many
people miss the questions.
Note
that the low low tide on March 16 (at 22:36) is a
minus tide, i.e. -0.7 feet. That means that sea
level at the low low tide is 0.7 feet below the zero
tide height.
Calculate the tidal range for this day. Recall that
the tidal range is the difference between the
highest and lowest tide of the day. We've already
said that the low low tide is -0.7 feet. Looking at
the tide table, we can see that the high high tide
(at 16:01) is 5.8 feet. Setting up the calculation:
Height (hi hi) - Height (lo lo) = Tidal Range, or
5.8 feet - (-0.7) feet = ????
If you did not get 6.5 feet, then check again.
Recall that when you subtract a negative, you add.
Thus,
5.8 + 0.7 = 6.5 feet.
Here's an easy way to think about it. Imagine you
are standing on the beach. Towards the land, another
person is standing 5.8 feet away. Below you, towards
the ocean, another person is standing 0.7 feet. What
is the distance between the two people? Take
yourself away and measure and you find that 6.5 feet
is the distance between them. Try it with pencils
and different positive and negative values.
Knowing the time and height of tides can make your
beach-going experience much more pleasurable. If you
want to see the many fascinating organisms living on
rocky shores, you will want to visit during the
lowest tides. If you are hiking along the coasts
(see the article in Backpacker this month), you will
need to calculate the times and heights of the tides
to gain access around the headlands. If you own a
boat, you will want to know the time and height of
the tides to make sure you stay afloat in whatever
location you are boating. If you are a surfer, the
time and height of the tides are vitally important
for the type of waves you will surf at any given
location.
There are many practical reasons for knowing
something about the tides. With a little practice,
it can be quite fun reading a tide calendar. |
