What are waves?
Waves are always the first thing you notice
about the ocean.
Except on very calm days, there are always waves skimming across the
surface
of the sea. What exactly are they doing there? We usually find waves in
a place where energy has appeared. A basic law of physics called the
conservation of energy says that energy can't
be created or destroyed; it can
only ever be converted into other forms. When energy suddenly appears,
concentrated in one place, something has to happen to it. Usually,
energy doesn't stay put: it tends to travel out in all directions to
other places that don't have as much.
Picture: Energy likes to travel outwards!
This bubble-shaped explosion of gas and dust is 14 light-years wide and
is expanding at 4 million miles per hour (2,000 kilometers per second).
It's the remnant of a supernova (exploding star).
Photo courtesy of NASA Jet Propulsion
Laboratory (NASA-JPL).
Think about a couple of familiar examples. Suppose you bang a
kettle drum in the middle of a football stadium. As your arm beats the
stick, you make the drum skin vibrate up and down with kinetic energy
(the energy of movement). As the drum skin bounces back and forth, it
produces waves of sound energy. These travel through the air, making
air molecules vibrate in sympathy, carrying sound energy in all
directions until it dissipates and gradually disappears. Something
similar happens if you switch on a lamp in the middle of a dark room.
This time, waves of light energy travel out from the lamp in all
directions. Why can't you see sound and light
travelling in the same way that you can see waves on the ocean? Sound
travels at over 1000 km/h (600 mph)—so it is both quick and invisible.
Light
travels much faster than sound at 300,000 km per second (186,000 miles
per second)—so
it carries electromagnetic energy between
two places virtually
instantaneously. Even though you can see light, you cannot see it
travelling.
The great thing about ocean waves is that you can see them coming.
If you're surfing, even fast-moving waves are slow enough to catch and
carry you along. Try doing that with sound or light! The properties of
an ocean wave are also very easy to see. You can estimate its amplitude
(height) just by looking out to the horizon. Its wavelength
(the distance from one wave crest to the next) and frequency
(the number of waves that travel past in a certain amount of time) are
also very easy to see.
If sound waves come from people beating drums, and light waves come
from people switching on lamps, where do ocean waves come from? If you
live in the northern hemisphere, far from the equator, you've probably
noticed that there are more waves around in the autumn or spring than
in the summer. In the UK, for example, there's most wind in the autumn
and winter—and that's usually the best time of year for surfing in
places like Cornwall. Wind is important because it's what puts energy
into the ocean: it makes ocean waves in more or less exactly the same
way as you make sound waves when you bang the skin of a drum.
Photo: The curling wave behind this surfer provides all the kinetic energy
that drives him forward. No waves, no energy, no surfing!
Photo by very kind permission and courtesy of Mike Baird and
Bairdphotos.com,
published on Flickr under a
Creative Commons licence.
What's the difference between wind swell and groundswell?
The waves that arrive at your beach are not necessarily created
anywhere nearby. Back in the 1950s, an ocean scientist named Walter
Munk conducted an amazing series of experiments with ocean waves. He
managed to prove that some waves travel over 15,000 km (over 9000
miles) across the open ocean before they reach their eventual
destination. That's like travelling across the United States, from New
York City to California, four times!
Generally, the more widely spaced and the cleaner waves
are when they roll up on the shore, the further they have travelled.
Waves like this are known as swell (or groundswell) and
they make the best waves for surfing. Groundswell is the reason you can
have quite large waves washing up on your beach even when there's
little or no wind blowing. Waves generated nearby (by winds blowing in
the local area) are known as wind swell.
They are usually
choppier, smaller, messier, harder to surf, and less interesting to
surfers than groundswell. Often the waves in a particular place are a
mixture of groundswell and wind swell—a random collection of waves that
have travelled from far away mixed with waves that have come a much
shorter distance.
Photo: Small groundswell rolling in.
Note how the waves are almost evenly spaced. The distance between one
wave crest
and the one following is the wavelength of the wave.
These waves have a wavelength of about 15 meters (50 ft).
Why is groundswell cleaner than windswell?
When the wind blows on the sea, it produces all kinds of waves of
different wavelengths, frequencies,
and speeds.
As the waves travel, the faster waves gradually separate out from the
slower waves.
The further the waves go, the more chance they have to sort themselves
into an orderly pattern. Groundswell has more time to get itself into
shape than windswell.
Eventually, the waves form into distinct little groups called sets:
when they finally arrive at their destination, a little group of good
waves will arrive at once.
Then there will be a pause.
Then the next group of waves will arrive a bit later.
When and why do waves break?
Swell is only one of the ingredients for great surfing.
Surfers don't like any old waves: they want waves that peel
(break gradually to the left or right along the wave crest) rather than
close out (where the crest folds over and
smashes to pieces all in one go).
When a wave is peeling, you can ride back and forth across the crest as
it slowly breaks;
with a wave that's closing out, there's nowhere much to go.
In surfing slang, waves that close to the right are called, not
surprisingly, "righthanders",
while left-breaking waves are "lefthanders".
The angle at which the wave peels makes it more or less interesting to
surf.
The steeper the angle, the harder it is to surf and the more
interesting moves you can pull.
What makes a wave break... and peel rather than closing out?
When water flows, in the ocean or in a
river, its upper layers are
travelling faster than its lower layers (indeed, the water is usually
stationary on the ocean floor or on a river bed). Think about waves
arriving at a beach. As they travel from
the open ocean to the shore, they move up a gradual sandy incline, and
start to slow down. The bottom of a wave slows more
quickly than the top. So instead of a wave moving forwards as one, we
have a whole
series of water layers sliding past one another, with the top layers
moving fastest
and the bottom moving slowest.
A wave breaks when the top part of the wave goes so far over the bottom
part that the wave
can no longer support itself—so it completely collapses.
A wave peels when this process happens gradually along the length of
the wave rather than all at once. If you like, a peeling wave is
breaking in two dimensions: along the crest of the wave as the wave
advances up the beech or reef.
Waves can break in many different
ways, and that largely depends on the profile of the seabed
underneath them (known as the bathymetry).
All waves will break eventually, but major features like rock or coral
reefs, ledges,
and sandbars will make one side break before another, causing waves to
peel. Nearby groins (sea fences), piers, and jetties can also make
waves peel. Different shapes of reef produce different breaking
effects.
Over the last decade or so, surf science has become advanced enough for
engineers to
start designing artificial reefs.
Rocks or ballast are buried at a key point offshore to give the waves a
helping hand
in breaking and peeling early in places where they might otherwise
simply close out.
Swell and bathymetry are not the only things that affect the quality
of your
surfing. How the wind is blowing on your beach will make a big
difference too. Waves are obviously always travelling from the open
ocean towards the beach, like scaled-up versions of ripples on a pond,
but the wind can be blowing in any direction. If the wind is blowing
directly out to sea, it is known as an offshore
wind. As it
blows, it will naturally tend to prop up the waves, stopping them from
breaking so quickly, cleaning out some of the smaller choppier
waves, and making the waves finally break with greater intensity in
shallower water.
A combination of strong ground swell and a light offshore wind is
always
best for surfing, especially if the wind has been blowing for a few
days (both to create groundswell and to give it time to travel to your
beach).
If the wind blows in the opposite direction, so it is onshore,
it will make the waves collapse much too soon—spoiling your fun!
Tell me why...
Science (and physics in particular) can explain most of the strange
things you'll notice when you're riding along on your surfboard.
Questions like...
Why do you have to paddle?
Whether you're on a surfboard or a bodyboard, if a great wave is
heading towards you, you have to paddle like mad to be able to catch
it. In other words, you have to be travelling with some speed and momentum
as the wave hits you to stand any chance of riding along with it. Why
is that? To travel with a wave, you have to accelerate to the speed
it's traveling. In other words, you have to gain a certain amount of
kinetic energy very quickly. If you've already got some kinetic energy
to start
with—if you're already moving when the wave catches up with you—it's
much easier for the wave to accelerate you a little bit more. Or in
simple terms, the faster you paddle, the more likely you are to catch
your wave.
Photo: This surfer is paddling like mad with his arms to gain speed.
When the huge wave catches him up, he'll have enough momentum to leap to his feet and surf.
Photo by very kind permission and courtesy of Mike Baird and
Bairdphotos.com,
published on Flickr under a
Creative Commons licence.
Why can small kids ride small waves?
Have you noticed how young kids can ride almost any waves—but older
ones can't?
It's back to momentum again.
To move you forward, a wave has to give you a certain amount of
momentum and energy.
Both of these depend on your mass (how much "stuff" your body is made
from).
The bigger you are, the more energy you need to travel at a certain
speed.
So the older and bigger you are, the bigger the waves you need for
surfing.
Why do waves suck you backwards?
At school you learn about two kinds of wave. There are waves like
sound, which travel by a sort of push-pull process, making patterns of
alternately squeezed up dense air (compressions) and thinned-out,
less-dense air (rarefactions). Sounds waves are called compression
waves; they're also called longitudinal waves,
because the
air molecules that carry sound energy move in the same direction as the
wave travels. Then there are waves like light that travel in a
familiar, up-and-down pattern. These are known as transverse
waves,
because they vibrate at right angles to the direction in which the wave
travels.
But ocean waves are not like the waves you learn about at school.
They move the water surface round in circles as they travel along.
Watch a seagull sitting on the ocean as a wave approaches. The gull is
sucked backwards up the front of the wave, lifted onto the wave's
crest, pushed forward as
the crest passes by, and then lowered down to pretty much the same
place
it started off in. This happens because ocean wave energy is not
travelling purely on the surface of the ocean: it also affects the
layers of water underneath. You'll have noticed this sucking effect if
you've ever caught waves on a bodyboard. As you lie on the sea surface,
you'll feel yourself being pulled backwards as a wave approaches. This
is another reason why you have to be paddling forwards to catch a wave.
If you're not paddling forwards, you're definitely going to get sucked
backwards!
Photo: Layers of water slide past
one another at the shore. At the top of the beach, water is sucking
back down into the sea.
Just offshore, a small wave is breaking inwards and up the beach. In
between, another small wave has just broken and is coming to a halt.
There are at least three layers of water sliding over one another here.
Why does a surfboard have a curved front edge?
Everything from the biggest ocean liner to the smallest surfboard
has a curved front edge. Why? If you push a curved edge over water, the
curve makes water travel more quickly underneath than on top. This
generates an upward force called lift that moves you up and slightly
out of the water—an effect called planing. Because you're partly
out of the water, there's less drag (water resistance) and you go
faster. You can see planing happening on almost any boat as it picks up
a bit of speed. With a hydrofoil,
the planing is so spectacular
that the entire boat lifts out of the water. The same science is at
work on a surfboard, only not quite so dramatic!
What about tides?
Photo: Low tide with little wind or swell.
Even if you could catch waves this small, they have too little energy
to take you anywhere.
This water's definitely not for surfing, but you might be able to
skimboard it.
Tides have nothing to do with waves. Tides are caused by the Moon
and the Sun working together to "pull" the sea back and forth with
their gravity, rather like a giant blanket moving up and down a bed! Tides change the depth
of the water on your beach. When the tide is "in", the waves come in
further and break later; when the tide is "out", the waves break
further out. Depending on the profile of the seabed, a rising tide (one
coming in) or a falling tide (one going out) will make the waves tend
to break somewhat better or somewhat worse than usual,
depending on the local seabed. Some places work well as high tide approaches; some
work best when the tide is going out.
When the Moon and Sun line up, twice a month, they make higher tides than usual
called spring tides, which give deeper water
during high tides
(when the tide is in) and shallower water at low tide (when the tide is
out). In between the spring tides are neap tides,
when the sea moves
back and forth less than usual, high tides are less deep, and low tides
are less shallow. Again, depending on the seabed, high and low tides,
and spring and neap tides, will can make the surfing better or
worse—but it varies wildly from place to place. If you're in a place
that needs deep water to make the waves break properly, the highest
spring tides are going to be better than the lowest neap tides. But
elsewhere, the opposite may be true.
Why wear a wetsuit?
The synthetic rubber traps water next to your body, which provides a
useful layer of insulation to keep you warm.
Read our article about
wetsuits to find out more.
Can science make you a better surfer?
Of course! It won't make you stand on the board any better. But if
you understand what waves are, how they are made, and where they come
from, you'll have a much better idea of when the surf's going to be up.
And if you can predict when the waves are ready to ride,
you're halfway there already. If surfing is a quest for the perfect
wave, science can at least point you in the right direction.
It might not make you a better surfer, but it
certainly won't make you any worse!
Further reading
Books you can read
- Surf Science by Tony Butt and Paul Russell.
Honolulu, Hawaii: University of Hawaii Press, 2002. This is a very clear, simple introduction
to wave and surf science, well illustrated with good colour illustrations and photos.
The authors are clearly scientists who know their surf.
- The Book of Surfing by Michael Fordham. London: Bantam Press, 2008. A great, coffee-table introduction to surf-culture.
Websites
