How a Swell is Generated, and How Surf is Forecasted

Surf forecasting is part science and part art. Open ocean storms generate swells and weather forecasting equipment and buoys register this energy. Then it is up to the surf forecaster to create models for the swell, determine how much the swell will degrade, and finally determine size and quality at a particular surf spot.

To start off, wind is the key ingredient in creating swell for surfing. High pressure is characterized by lighter, warmer packages of air and low pressure is characterized by denser, colder air packages. Air in a high pressure system is attracted to low pressure systems creating wind. A low pressure system will strengthen when it collides with a warm air mass making the air spin faster generating more wind.

Waves are generated by wind blowing over the surface of the water. Wind first creates small waves but the more wind over a greater distance causes larger waves to form. The biggest contributors to wave size and period are wind speed and fetch where fetch is the distance the wind is blowing. The larger a wave grows, the more surface area the wind has to grip the wave and add energy to it making it larger. The only thing preventing swells from growing beyond a certain point is whitecapping which decreases wave size and strength.

As waves spread out from a storm, they begin to disperse and group themselves together. Waves of similar sizes and speed form into sets of waves and travel together in the ocean. Now the waves will degrade as they travel great distances through the ocean. The closer you are to a significant wave generating event, the bigger the surf you will see when it hits land. As the waves travel great distances, you will see a swell clean up as waves spread apart and are not all stacked on top of each other.

Wave height is determined by two primary factors, wave height / amplitude and period of the wave. Wave height is obvious as a swell may be 3 feet high in open water. The period is the time it takes to go from trough to peak to trough of a wave and this is measured in seconds. The higher the period of a wave, the faster the wave will move and the more deep water energy the wave has as well. You will here surfers refer to a long period swell of about 12-14 seconds or greater as a groundswell and shorter period swells as windswell. Groundswells with high periods will generate a larger breaking wave than a swell with a similar height but with a shorter period.

Waves break when they encounter shallow water and the bottom of the wave slows enought that the top of the wave crumbles forward falling over the top of the bottom of the wave. The more rapidly the ocean bottom goes from deep to shallow, the faster and more powerfully the wave will break. Ocean bathymetry refers to structures that change the depth of the water such as sand bars, reefs, and rivermouths and ocean floor bathymetry also plays a critical role in the way a wave breaks.

To determine the surf forecast, complex data is gathered from groups like NOAA and NWS. Models have been created that look at wind speed and direction as well as fetch to determine wave size and swell direction leaving a storm. These models will estimate how the swell will propagate through the ocean to create a surf forecast. Then local individuals from surf reporting services will venture out to breaks in the early morning to capture the surf report looking at wave size, shape, and quality.