A swell is the motion caused by a meteorological disturbance, which persists after the disturbance has died down or moved away. Swells often travel for considerable distances out of their generating area, maintaining a constant direction as long as it stays in deep water. As the swell travels away from its generating area, its height decreases thought its length and speed remains constant, giving rise to the long, low regular undulations so characteristic of swell. Two or three swells from different generating areas are often present and these may be partially obscured by the waves also present.

To the human senses, the most obvious patterns of the surface water are indicated by color. The deep blue water of the open sea far from land is the color of emptiness, while the green water of the coastal areas with all its varying hues is the color of life. The sea is blue because sunlight is reflected back to our eyes from water molecules or from very minute particles suspended in the sea. In the journey of the light rays into deep water all the red rays and most of the yellow rays of the spectrum have been absorbed, so when light returns to our eyes it is chiefly the cool blue rays that we see.