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Swell is an ocean wave system not raised by the local wind blowing at the time of observation, but raised at some distance away due to winds blowing there. Swell waves travel out of a stormy or windy area that have travelled from their fetch and continue on in the direction of the winds that originally formed them as sea waves. The swell may travel for thousands of miles before dying away. As the swell wave advances, its crest becomes flattened and rounded and its surface smooth. Swell waves are characterized by a relatively long and regular period.

Swell waves normally come from a direction different from the direction of the prevailing wind and sea waves at the time of observation. However, sea and swell waves may occasionally be seen coming from essentially the same direction, thus making it more difficult to distinguish the two systems, especially if the sea waves are high.

Sea waves and one or more systems of swell waves are frequently present at the same time, forming "cross seas." Again, sea waves may be absent (as would occur under conditions of very light winds), but one or more systems of swell waves may still be present.

A long, high ocean swell whose height rises dramatically as it passes through water that is shallower than one-tenth wavelength is known as groundswell or ground swell. An international scale of sea disturbance and swell (Douglas Scale) ranging from 0 to 9 with one figure for disturbance and one for swell is named named after Sir Henry Douglas (1876-1939), former director of the British Naval Meteorological Service.

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