A swell, in the context of an ocean, sea or lake, is the formation of long wavelength waves on the surface of the sea. These series of surface gravity waves are not generated by the local wind. Swell waves often have a long wavelength but this varies with the size of the water body, e.g. wavelengths are rarely more than 150 m in the Mediterranean. Swell wavelength, also, varies from event to event. Occasionally, swells which are longer than 700 m occur as a result of the most severe storms. Swells have a narrower range of frequencies and directions than the wind sea, because swell waves have dispersed from their generation area and have been dissipated.The dissipation of swell energy is much stronger for short waves, which is why swells from distant storms are only long waves. The dissipation of waves with periods larger than 13 s is very weak but still significant at the scale of the Pacific Ocean.[1] These long swells lose half of their energy over a distance that varies from over 20000 km (half the distance round the globe) to just over 2000 km. This variation was found to be a systematic function of the swell steepness: the ratio of the swell height to the wavelength. The reason for this behaviour is still unclear but it is possible that this dissipation is due to the friction at the air-sea interface Swells are often created by storms thousands of nautical miles away from the beach where they break, and the propagation of the longest swells is only limited by shorelines. For example swells generated in the Indian Ocean have been recorded in California after more than half a round-the-world trip.[2] This distance allows the waves comprising the swells to be better sorted and free of chop as they travel toward the coast. Waves generated by storm winds have the same speed and will group together and travel with each other, while others moving at even a fraction of a metre per second slower will lag behind, ultimately arriving many