glassy patch or streak on a relatively undisturbed ocean or lake surface, formed where surface tension is reduced by a monomolecular layer of organic matter produced by plankton or by man; closer to shore most of the material is man-made hydrocarbon pollutant. Slicks are patchy when the wind velocity is less than about 13 kilometres per hour (7 knots). Winds with higher velocities break slicks into narrow, closely spaced windrows aligned parallel to the wind direction. Elongate parallel slicks may also form over and migrate with the trailing slopes of internal waves.