Definition
A long, smooth, rolling wave on the surface of open water, generated by distant winds and persisting after the winds that created it have died down. Swells are characterized by their direction of travel, height (crest to trough), and period (time between successive crests).
Plain English
A wave on the ocean that rolls in steadily from far away, separate from the choppy waves the local wind is making right now.
Context Anchor
Pilots encounter this term in marine weather information, coastal flying, seaplane operations, and overwater planning.
Derivation
From Old English 'swellan,' meaning to grow or expand. The water surface 'swells' upward into a rolling rise — the same root sense of something rising and falling in a smooth, sustained way.
Why Pilots Care
Determines the best landing heading and helps assess whether sea conditions are safe for seaplane operations or emergency ditching.
Grounding Statement
A pilot may see light wind at the coast but still find large rolling waves arriving from a distant storm.
Intuition Check
Swell does not mean “good” here, and it does not simply mean any rough water. In aviation and weather use, swell means longer rolling waves that can come from weather somewhere else.
Example Sentence 1
Before ditching, the pilot studied the water surface and chose to land parallel to the primary swell rather than into the local wind.
Example Sentence 2
During the ditching briefing, the crew planned to touch down parallel to the swell to reduce the chance of flipping.