What is the difference between waves and swells

In the days before the internet, savvy entrepreneurial local surfers were known to set up premium rate phone lines and provide eyeball surf checks for landlocked hopefuls.

Related: A complete guide to tides. Surf forecasting is seen by some as somewhat of a dark art. Taking the time to learn and understand the basics of how to read a surf forecast is a good skill to learn and will help you to put together another piece of the surfing puzzle.

Surf forecasting is a collection of meteorological data combined with complex algorithms and swell models to predict local surf conditions in advance. All designed to assist surfers in understanding what surf conditions can be expected at their local surf spot, displayed in a way that is easily digestible and quick to understand.

Swell is energy that has been transferred into the sea by wind. The longer and stronger the wind blows like a hurricane the more energy that is transferred and so larger the swell.

This energy then propagates from where it is created out into the ocean, much like ripples in a pond. As any surfer will tell you — having advanced information at your fingertips can play a big role in the planning of surf opportunities and allows the modern surfer to manage their time efficiently.

With surfing conditions in constant change from one day to the next, the insights provided through surf forecasting proves invaluable to the surfer whose days in the water are limited. On a national or international level surf forecasting provides data backed guidance with the planning of successful surfing events and competitions. Large scale surfing competitions are big business with events becoming increasingly popular. Well known non surfing brands provide considerable sponsorship deals in exchange for exposure to the ever-growing surfing audience.

Surf contest directors are under more pressure now than ever to hold surfing contests that allow surfers and destinations to showcase their potential. On a philosophical level, surf forecasting provides the average everyday surfer with the means for a higher understanding of their local surf spot, surfers now have the means to learn and understand the favoured weather patterns or swell period and direction needed for that elusive secret spot.

All surf forecasting websites glean at least some of their data from the same source. This is a US government-funded service and provides the data for free, allowing surf forecasting websites to collate what data they wish and display it in a format that is useful for surfers.

The main payers in the surf forecasting world are Surfline and Magicseaweed. Each use data provided by NOAA combined with many other meteorological sources to feed their own unique swell models. Using complex algorithms, mathematics and historical data, these individually unique swell models decipher the information and produce the forecasts we see online. There are a whole host of surf forecasting websites to choose from, a great place to start is to find a surf forecasting site that suits you, your location and the depth of knowledge you wish to gain from it.

How each website displays its data is key to the user understanding what the surf report actually means. Only concentrate on the three main areas which will provide you with the most valuable and important snippets of information. These bits of information only become valuable to you when you can relate what you see on the screen to what you see on the beach.

Once you understand these three basics you can start digging deeper, which we cover later in this article. Not to be confused with swell height. This allows the waves to cover more distance. Where the swell originates describes the swell direction. A North Swell is heading South, and North facing coastlines will receive the swell. Swell direction breaks down as follows:.

The angle of the swell can be used to determine how the surf will be at a particular spot. Depending on the coastline, points, reefs, and sandbars, the surf in one spot could be totally different from somewhere nearby. Figure 9. There are two ways to analyze them: local statistical analysis and spectral analysis.

Figure The total height and period are calculated by decomposing the profile into a succession of elementary waves. There are two possibilities to evaluate the height Figure 10 :. The average values of H d and H u are the same and give an estimate of the total height H.

We do the same for the period with T d and T u to have an estimate of the period T. In practice, we are more interested in what is called the significant height H s which corresponds to the average value of the heights of the third highest waves [3].

The marine meteorology gives H s which is calculated by numerical modelling from the spectrum of the swell, knowing the wind speed and the fetch see 7.

In shallow coastal areas the model also incorporates the bottom topography. Spectral analysis is another way to statistically describe a wave train. It involves representing the distribution of wave energy as a function of frequency. The curves depend on parameters representing the wind speed and the fetch which are at the origin of the waves Figure The graphs show that in the wind action zone, the waves become progressively stronger in energy amplitude , while evolving towards lower frequencies, which corresponds to longer waves.

Sometimes a very large wave two to three times Hs , called a rogue wave , appears in the middle of a wave train. The accounts of this phenomenon observed by various sailors have long been questioned. However, modern recording methods have confirmed this reality. Figure 9 shows the signal of a rogue wave with a height of about 26 metres in a wave train of significant amplitude Hs of A wave is qualified as a rogue wave when it appears in an isolated way in the middle of a wave train with an amplitude of more than 2 times higher than the significant amplitude of this train.

It is estimated that 22 cargo ships sank between and due to rogue waves, causing hundreds of casualties [5]. The origin of a rogue wave is not yet well understood and could be explained by non-linear interactions or by the meeting of wave trains of different origins.

However, scientists have been able to carry out a laboratory experiment that shows a rogue wave resulting from the crossing of two wave trains at an angle of degrees [6]. Is the energy given up by the wind to the wave recoverable? This is a question that has become very important in recent years in the context of renewable energy research.

This has led in recent decades to the development of various techniques to recover this energy. Projects have been completed or are underway to test the cost-effectiveness of these techniques and several hundred patents have been filed. Insertion in coastal protection schemes presents particularly attractive opportunities. Mutriku wave power plant by BiMEP. This type of installation Figure 12 consists of a concrete box immersed in water, open to the sea at the bottom 1 and whose free surface inside is in contact with a volume of air 2.

The reciprocating movement of the sea level causes the air inside the caisson to compress and decompress, thereby activating a turbine 3 which produces electricity. It can be seen thati-the freely running swell has a smoother appearance than the waves in the windy section. The motion of the swell is nearly irrotational and nonturbulent, unless the swell runs into other regions where the water is in turbulent motion.

Turbulence is a property of the fluid rather than of the wave motion. After the waves have travelled a distance from the generating area they have lost some energy due to air resistance, internal friction, and by large scale turbulent scattering if they run into other storm areas, and the rest of the energy has become spread over a larger area due to the dispersive and angular spreading characteristics of water gravity waves.

All of these mechanisms lead to a decrease in energy density. Thus, the waves become lower in height.