Swell consists of waves that have left their generation area and travel long distances with periods of 10–20 seconds and a regular, orderly appearance. It can originate from storms thousands of kilometres away and arrive at coasts with completely clear weather.
Difference from wind sea
Unlike wind sea (local, chaotic, short-period), swell is stored energy that propagates efficiently. On Spain's Atlantic coasts, North Atlantic storm swell generates 3–6 m waves even on calm days. When combined with local wind sea, dangerous cross seas result.