 Surf Forecasting (Courtesy Sean Collins and
Surfline.com)
............................................................................................................................. 1
Waves and swell are created by wind. ............................................................ 1
Simply put, waves are created when wind transfers its energy from the air to
the water. ......................................................................................................... 2
Waves decay and get smaller the farther they travel. ..................................... 2
Where the wind or swell is coming from. ....................................................... 3
The most overlooked three-dimensional variable. .......................................... 3
Wave decay and travel. ................................................................................... 3
Conserving energy. ......................................................................................... 4
Wave speed. .................................................................................................... 4
Forerunners. .................................................................................................... 4
Swell period and ocean depth. ........................................................................ 5
Shoaling. ......................................................................................................... 6
Refraction. ....................................................................................................... 7
o
Waves and swell are created by wind.
 Around the earth, we have areas of high air
pressure and areas of low air pressure in the
atmosphere. Think of the air as liquid, as water. The
areas of high pressure are constantly trying to fill
the areas of low pressure. If you have an area of
high water right next to an area of low water with no
barrier between, the high water will flow to fill the
area of low water. The transition of airflow from high
pressure to low pressure is wind. When the wind
blows over the ocean, it creates small ripples on the
surface. As these ripples grow, the wind gets better
friction on the ocean surface. After a period of time,
these ripples grow into small waves or chop on the
water. As the wind increases and continues to blow,
the chop transforms into small waves, then into
larger waves and then, if all goes well, into huge
waves.
o
Simply put, waves are created when wind transfers its
energy from the air to the water.
 Wave generation requires three variables: wind
velocity, wind duration and wind fetch. The harder
the wind blows, the longer the time it blows and the
greater the distance it blows, the bigger the waves.
Limitation of any one of these variables will
severely restrict the development of wave heights
and the transfer of energy into the water.
 As waves grow larger, the distance between waves
will become greater, signifying more and more
energy being transferred deeper into the ocean. As
more energy is transferred deeper into the water,
the waves have better ability to sustain that energy
as they travel great distances across the oceans.
The most common way to measure wavelengths is
by measuring swell period, which is the time
between successive wave crests as they pass a
stationary point on the ocean surface, such as a
buoy.
o
Waves decay and get smaller the farther they travel.
 In the middle of a storm there is a confused mix of
sea state. Various waves of different heights,
directions and swell periods turn the ocean surface
into a chaotic mess. We call this the wave spectrum.
All of these waves are the result of different cycles
of the storm, with theshort-period waves generated
by current winds in the local area and the longer
period waves generated by winds earlier in the
storm's life that have had a longer time to develop.
 As the waves move out of the storm area, they
decrease greatly in size within the first thousand
miles (more than 60 percent) and slowly thereafter.
This is caused by three factors: short-period waves
and chop dissipating rapidly once outside of the
wind-generation area; directional spreading of
waves as they move away from the storm at
different angles and the separation of waves as they
travel forward at different speeds after leaving the
storm area. This initial wave-decay process allows
the underlying long-period waves to move out from
beneath the messy short-period sea state in the
middle of the storm. Once these longer period
waves break free from the storm's confusion, they
are easily identified as a more organized wave train,
which we call swell
o Where the wind or swell is coming from.
 In the marine community, directions are always
identified as the direction the swell or the wind is
"coming from," not the direction it's headed.
Degrees used are true degrees with north at 0 or
360 degrees (and then moving clockwise), east at 90
degrees, south at 180 degrees and west at 270
degrees. Northeast may be anywhere between 0 and
90 degrees, southeast between 90 and 180 degrees,
southwest between 180 and 270 degrees and
northwest between 270 and 360 degrees.
o
The most overlooked three-dimensional variable.
 Most surfers look at waves from a two-dimensional
perspective: wave height and direction. But waves
need to be analyzed from a three-dimensional
perspective, which also includes the swell period.
The swell period variable is the X-factor. It's the
make or break variable and plays a huge role in the
eventual size of a swell. This is why:
o Wave decay and travel.
 The longer the swell period, the more energy the
wind has transferred into the ocean. Long-period
swells are able to sustain more energy as they
travel great distances across the ocean. Shortperiod swells (less than 14 seconds between wave
crests) are steeper as they travel across the ocean
and, therefore, are more susceptible to decay from
opposing winds and seas. Long-period swells
(greater than 14 seconds) travel with more energy
below the ocean surface and are less steep so they
can easily pass through opposing winds and seas
with very little affect.
o
Conserving energy.
 Swells travel as a group of waves or a "wave train."
As the swell moves forward, the wave in the front of
the wave train will slow down and drop back to the
rear of the group while the other waves move
forward by one position. Then the next wave in front
moves back and another takes its place -- much like
a rotating conveyor belt that is also moving forward.
It's a process somewhat similar to the "drafting"
technique used by bicycle racers and car racers,
and it enables wave trains to conserve their energy
as they travel great distances across the oceans.
Working together to sustain energy.
o
Wave speed.
 The speed of a swell or a wave train can be
calculated by multiplying the swell period times 1.5.
For example, a swell or a wave train with a period of
20 seconds will be traveling at 30 knots in deep
water. (Knots are nautical miles per hour. One knot
equals 1.2 mph on land.) A swell with a period of 10
seconds will travel at 15 knots. The individual
waves actually move twice as fast as the wave train
or the swell, and a single wave's speed can be
calculated by multiplying the swell period times
three. So <I>individual</I> waves with a period of 20
seconds travel at 60 knots in deep water. Again,
think of the wave train like a rotating conveyor belt
that is also moving forward.
o
Forerunners.
 Long-period waves move faster than short-period
waves, so they will be the first to arrive.
Forerunners are the initial long-period waves that
travel faster than the main body of the swell.
Usually, forerunners are pulses of energy with
periods of 18 to 20 seconds or more. A wave train's
peak energy will usually follow in the 15- to 17second range. The swell period will steadily drop
during the life cycle of the swell as it arrives on the
coast. The farther a swell travels, the greater the
separation of arrival time between the forerunners
and the peak of the swell. Often the forerunners will
only be inches high but can be measured by buoys
and other sensitive oceanographic instruments. To
the naked eye, forerunners are very hard to see;
sometimes you can pick them out as slight bumps
on a jetty or other rocks. Surfers with a sharp eye
can often sense forerunners as the "ocean seems to
be moving" with extra surging and currents. Even
though forerunners may only be inches high, they
constitute a large amount of energy. LOLA uses
real-time buoy data to separate these tiny
forerunners from the rest of the swell in the water
so we can identify the first signs of a new swell -before we can see it at the beach.
o Swell period and ocean depth.
 The depth at which the waves begin to feel the
ocean floor is one-half the wavelength between
wave crests. Wavelength and swell period are
directly relative, so we can use the swell period to
calculate the exact depth at which the waves will
begin to feel the ocean floor. The formula is simple:
take the number of seconds between swells, square
it, and then multiply by 2.56. The result will equal
the depth the waves begin to feel the ocean floor. A
20-second swell will begin to feel the ocean floor at
1,024 feet of water (20 x 20 = 400. And then 400 x
2.56 = 1,024 feet deep). In some areas along
California, that's almost 10 miles offshore. An 18second wave will feel the bottom at 829 feet deep; a
16-second wave at 656 feet; a 14-second wave at
502 feet; a 12-second wave at 367 feet; a 10-second
wave at 256 feet; an eight-second wave at 164 feet;
a six-second wave at 92 feet and so on. As noted
above, longer period swells are affected by the
ocean floor much more than short-period swells.
For that reason, we call long-period swells ground
swells (generally 12 seconds or more). We call
short-period swells wind swells (11 seconds or less)
because they are always generated by local winds
and usually can't travel more than a few hundred
miles before they decay. Long-period ground swells
(especially 16 seconds or greater) have the ability to
wrap much more into a surf spot, sometimes 180
degrees, while short-period wind swells wrap very
little because they can't feel the bottom until it's too
late.
o
Shoaling.
 When waves approach shallower water near shore,
their lower reaches begin to drag across the ocean
floor, and the friction slows them down. The wave
energy below the surface of the ocean is pushed
upward, causing the waves to increase in wave
height. The longer the swell period, the more energy
that is under the water. This means that long-period
waves will grow much more than short-period
waves. A 3-foot wave with a 10-second swell period
may only grow to be a 4-foot breaking wave, while a
3-foot wave with a 20-second swell period can grow
to be a 15-foot breaking wave (more than five times
its deep-water height depending on the ocean floor
bathymetry). As the waves pass into shallower
water, they become steeper and unstable as more
and more energy is pushed upward, finally to a
point where the waves break in water depth at about
1.3 times the wave height. A 6-foot wave will break
in about 8 feet of water. A 20-foot wave in about 26
feet of water. A wave traveling over a gradual
sloping ocean floor will become a crumbly, slow
breaking wave. While a wave traveling over a steep
ocean floor, such as a reef, will result in a faster,
hollower breaking wave. As the waves move into
shallower water, the speed and the wavelength
decrease (the waves get slower and move closer
together), but the swell period remains the same.
o
Refraction.
 Waves focus most of their energy toward shallower
water. When a wave drags its bottom over an
uneven ocean floor, the portion of the wave
dragging over shallower water slows down while
the portion wave passing over deeper water
maintains its speed. The part of the wave over
deeper water begins to wrap or bend in toward the
shallower water -- much the same as how waves
wrap and bend around a point like Rincon or Malibu.
This process is called refraction. Deep-water
canyons can greatly increase the size of waves as
the portion of the swell moving faster over deep
water bends in and converges with the portion of
the swell over shallower water. This multiplies the
energy in that part of the wave, causing it to grow
into a larger breaking wave as it nears shore. The
effects of a deep-water canyon just offshore is often
why we see huge waves along one stretch of beach,
while maybe just a few hundred yards down the
beach the waves are considerably smaller. This
happens at spots such as Black's and El Porto in
Southern California, and Maverick's in Northern
California. Remember, the longer the swell period,
the more the waves will be affected by the ocean
floor bathymetry, the more they will wrap into a spot
and the more the waves will grow out of deep water.