(Also called billow clouds, windrow cloud, wave cloud.) A cloud variety composed of merged or separate elements which are elongated and parallel, either suggestive of ocean waves, or arranged in rank and files. Undulatus are principally found in cirrocumulus, altocumulus, altostratus, stratocumulus, and stratus.

Clouds in patches, sheets or layers, showing undulations. These undulations may be observed in fairly uniform cloud layers or in clouds composed of elements, separate or merged. Sometimes a double system of undulations is evident.

(Also called billow cloud, windrow cloud, wave cloud.) A cloud variety 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 formation is by gravity waves that exhibit broad, nearly parallel lines of cloud oriented normal to the wind direction, with cloud bases near an inversion surface. See cloud classification; compare cloud streets.