Cold-air-runoff winds that are produced when air in contact with terrain surfaces is cooled and flows downslope and/or downvalley. This generic term is often used to indicate aggregate downslope (katabatic) and downvalley flows, when it is difficult to distinguish between the two. This happens frequently in basins, at the upper end of valleys, in complicated topography where the downslope and downvalley directions are not perpendicular, and in simple valleys when the weaker and shallower downslope flows are masked or overwhelmed by the stronger downvalley flow. Over even gently sloping topography, drainage winds also refer to gravity winds that drain cold air into frost hollows, river valleys, and other lower-lying terrain. See downslope wind.