How Wind Whips Up the Perfect Swell
by Robin Marks
The scene: aboard a
ship in the North Sea during a violent storm. The wind is pummeling
the crew on deck as they struggle to control the boat amid the gigantic
waves tossing it about like so much driftwood. After a bit, the
storm blows over. The wind subsides, and the gigantic, threatening
waves are reduced to ripples. Weeks later and a few thousand miles
away, happy surfers are riding the very waves that rocked our North
This is the time between two
wave crests at a given point. It's the equivalent of wave
frequency. The longer the period, the faster the waves are
moving. Good surfing swells often have a period of 13 to 15
seconds, but the real cruisers can have a period of up to
The faster a wave is moving, the more energy it has. More
energy means that the wave will have a deeper effect, and
will actually move more water with it. The important result
for surfers is that a faster wave will be larger when it breaks.
This seems like a straightforward
concept, but it's not. Surfers in various regions measure
wave height differently, but in all cases, they're measuring
something that's equivalent to wave amplitude.
Most measure the face of the wave, from crest to trough. But
Hawaiians are famous for measuring the back side of a wave,
which results in a shorter height. Californians often estimate
wave height relative to a body; shoulder-high means 5 feet.
no clear right or wrong way to measure, but its important
to know what system is used where youll be surfing.
Otherwise you might find yourself in less friendly waters
than you anticipated.
From the shore, it may seem like a wave is water moving toward you.
But in fact, a wave is motion moving through a body of water; most
of the water where it is. When wind and water meet, the result is
wavesthat may seem obvious. But why, exactly, does it happen
that way? We can start to answer that question in your bathtub.
Tension and friction
Fill a tub with water and blow gently across the surface. The waves
you create are probably smaller than your finger. The smallest waves,
ones about that size, arise from the surface tension of water. The
molecules on the water's surface hold together and form a sort of
"skin." You may have seen water bugs skirting around the
surface of a pond. These bugs stay afloat because of surface tension.
This phenomenom even lets you float a paper clip on the top of a
full glass of water! (Read
about surface tension
This "skin" makes the surface of water somewhat stretchy.
What happens to the smooth surface of a rubber band when you stretch
it? It becomes sticky. The same thing happens with water. Blow on
your bath water again. As the air passes over the "sticky"
surface of the water, it grabs some molecules and pushes them into
the ones ahead. Those molecules push on the ones in front of them,
and those push on the ones in front of them, and so on, as the wave
travels to the opposite side of the tub. If you watch closely, you'll
see that the water stays mostly in the same area; it's the disturbance
caused by your breath that's moving across the water.
It's a Long Way from a Bathtub to the Beach
small waves that you make in your bathtub only mimic the very first
stage of big-wave generation. Let's move from your bathtub to a
small lake. If the wind is blowing, you'll see waves moving across
the lake in the same direction as the wind. If there's a strong
wind, the waves become textured, or choppy, and the stronger the
wind, the larger the waves. That's because as the waves move, they
run into each other and merge, combining their energy to get bigger
and move faster. These waves will merge and become bigger still,
as long as they have distance to cover and a way to sustain the
energy that keeps them moving.
Now we're ready to move to the open ocean, where storms churn up
miles of choppy, erratic waves. This is the birthplace of the glassy,
fast-moving swells that delight surfers. If you could watch the
"wind waves" caused during a storm, you'd notice that
they're very choppy, textured, and somewhat haphazard. They may
be big and energetic, but they're not good for surfingyet.
like this one traveling toward a beach near Santa Cruz, California
are created by storms thousands of miles out in the ocean.
Big wind waves move away
from the storm that created them, just like the waves in your bathtub
move away from you as you blow on the water. As waves move through
the water, friction causes them to lose energy. If the storm that
caused the waves didn't impart enough energy or create enough waves,
then friction may take all the energy, and the waves will flatten
out and dissipate before they get very far.
On the other hand, if a storm is strong enough, it will whip up
many waves that will slam into each other, combining their energy.
If enough waves come together, their energy will create a swell
that can travel fast enough and far enough to survive a trip around
the globe. Along the way, the less energetic, choppy elements of
the wave will be lost to friction, and a smooth, glassy face will
present itself. These are the waves that send surfers running for
What's the Recipe for a Swell Swell?
Storms that whip up what will become sweet surfing swells
must be able to create lots of energetic waves. There are three
factors that contribute to the formation of good surfing swells:
how fast the wind is blowing, the surface area of ocean that's affected
by the storm, and the amount of time those winds blow over a given
spot on the ocean. Weather forecasters call these three factors
"wind velocity," "fetch" and "duration,"
respectively, terms worth knowing if you want to read weather images
and make your own surf predictions.
In the best of all possible surfer worlds, the wind would blow extremely
hard for days and days over thousands of miles of ocean. That sort
of thing rarely happens, but storms that bring good waves aren't
all that uncommon. Surfers can watch for developing storms by following
them over the open seas. As Rebecca Roberts describes in
, there are lots of on-line data banks and tools that
can help surfers track storms and waves until they arrive on the
nearest beach. With a little effort and a surf of the Internet,
riders can even make their own surf