of the Tides
by Paul Doherty
Surfers consult tide tables
as often as they consult weather and wave predictions. While predicting
tides may be more systematic than predicting wave heights, knowing
the tide's effect on the waves is another matter. There are no rules,
except one: You have to know something about your beach to know
how it will be affected by the tides.
All good surfing beaches
have what's called a break, a place where the beach drops
off under the water. The break is where the waves crest, and therefore
is where surfers catch their rides. The depth of the water over
the break changes with the tides: The water is deeper when the tide
is high, shallower when it's low. At some point in the tide cycle,
the water will be at its best depth for making good surfing waves.
Problem is, that depth and that time in the tide cycle depend on
factors unique to your beachwhere the break is, the slope
of the beach, etc. The
bottom line is that tide affects surf, but in order to make the
most of it, you need to know your beach.
tide table for San Francisco for Nov. 12-14, 2002. Is the moon
waxing or waning?
(view entire image)
Spend a month on your
beach, and you'll start to get a good sense of its tidal cycles.
Each day, there are two high tides, and each day they arrive about
an hour later than the day before. As the moon waxes from first
quarter to full, the high tides get higher. They fall back as the
moon wanes toward third quarter then rise again as the moon approaches
The motions of the tides
have been known for centuries. Long ago, people noticed that there
is a connection between the moon and the tides. The moon rises about
an hour later each day, just as the tides do, and the heights of
the tides follow the phases of the moon. The largest tidal range,
the difference between the height of the low and high tides, comes
at new moon and full moon, while the smallest range comes at first-quarter
and third-quarter moons.
Although the details
of tidal motion are complex, the basic ideas were worked out in
the seventeenth century by Isaac Newton. We've cooked up a few activities
to help you investigate the tides. But first, a little background
information to consider: