Altostratus undulatus (As und) is a sheet, layer, or patch of altostrati clouds, composed of merged or separate elements that are elongated and parallel, either suggestive of ocean waves or arranged in ranks and files. Sometimes two distinct wave systems are apparent (biundulatus). The nearly parallel lines of cloud are oriented at right-angles to the wind direction, with cloud bases near an inversion surface. Altostratus undulatus may assume a very striking appearance when illuminated by low-angle illumination at sunrise or sunset.