WAVE ENERGY, DECAY AND DIRECTION

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.