Nantasket Beach

The Swell Analysis feature allows you to have a better understanding of the different factors at play which determine the overall size and quality of the surf. Those primary factors are swell size, period, and direction, as well as wind velocity and the submarine topography (bathymetry) and composition at a particular spot.

Swell Height
This is a measure of the significant wave height of swells in the open ocean. Significant wave height is a fundamental metric used to describe the average height of the highest one-third of waves in a given swell group. Note that swell height can be (and often is) different from the size of the surfable waves, due to the impact of the angle at which the swells arrive at a spot, as well as the decay in energy as they reach shallow water (a process called wave shoaling).

Swell Period
Swell period refers to the time interval between two successive wave crests as they pass a fixed point in the water. It is the time it takes for one complete wave cycle to occur, and is typically measured in seconds. Longer swell periods (typically in the range of 12 to 20+ seconds) are indicative of waves that have traveled over great distances without losing much energy.These swells are typically generated by distant storms or weather systems that have a significant fetch (distance over which the wind blows). Longer period swells tend to maintain their energy as they approach the coast. This can lead to larger, more powerful waves breaking in a more organized manner. The surfable waves generated by longer period swells are often cleaner and more powerful. Conversely, shorter period swells (typically in the range of 5 to 10 seconds) indicate waves that have traveled over shorter distances and have been generated locally by nearby wind conditions. These swells have less time to organize into predictable wave trains as they move towards the shore. When shorter period swells encounter shallower water, they tend to steepen and break more abruptly. As a result, the waves produced by shorter period swells can arrive with more frequency, but may be less organized, choppier, and generally less powerful.

Swell Direction
The angle at which an open ocean swell arrives relative to the coast is a critical factor that influences the size and power of the surf. This angle affects how the wave energy is distributed and how the waves interact with the local bathymetry. When a swell approaches the coast at a direct onshore angle, meaning it is perpendicular to the coastline, the waves tend to break more abruptly. This can result in shorter, steeper waves that lack the smooth and long rides surfers prefer. Depending on the bathymetry and composition of a particular spot, waves may also have a tendency to close out, meaning they break all at once without offering a rideable face (typically at beach breaks). When a swell approaches the coast at a favorable oblique angle, meaning it is at an angle between direct onshore and direct offshore, the wave energy is refracted, or bent, by the coastal contours. This refraction causes the waves to bend and wrap around headlands, points, and shallow areas. Certain areas may experience wave focusing, where the wave energy is concentrated, resulting in larger and more powerful waves. Conversely, other areas might experience wave shadowing, where the waves lose energy, leading to smaller waves. In the case of swells which are arriving at a very steep angle relative to a spot, you can generally expect to see a significant loss in the size and power of those swells, if they show up at all.

Wave Energy (kJ)
Wave energy, measured in kilojoules (kJ), is an essential metric shown on the swell table. The values shown by DeepSwell indicate the power of the predicted swells and serves as a guide to assess the potential strength of the surf. Wave energy is a function of the size, period and direction of the waves. While it may seem obvious that larger waves carry more energy, it should be noted that smaller ocean waves with long periods can yield the same surf size and carry similar overall energy as larger waves with short periods. It is this interplay between the amplitude and period of a given swell that allows us to use wave energy as a more accurate descriptor of the surf. As a general reference, a wave energy level of around 100-200 kJ can result in rideable surf. As wave energy reaches the 200 to 1000 kJ zone, we can expect increasingly powerful and larger surf.