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 in evidence.
This term applies mainly to Cirrocumulus, Cirrostratus, Altocumulus, Altostratus, Stratocumulus and Stratus.
The text enclosed in grey-shaded boxes, like this example, comprises Annex I to the Technical Regulations (WMO-No. 49) and has the legal status of standard practices and procedures.