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.
Energy
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.
Wave Decay
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 the short-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.
Direction
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.
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