http://www.metoffice.gov.uk/learning/clouds/other-clouds/undulatus-asperatus

Undulatus Asperatus

Height of base: 4,000 - 10,000 ft

Shape: Undulating waves

Latin: undulus - waves; asperatus - rough

Whilst not yet an official cloud type, Undulatus Asperatus is currently being
assessed by the World Meteorological Organisation to decide whether it should
become a supplementary feature of mamma clouds in the International Cloud Atlas.
If this goes ahead it will be the first new cloud type in over 50 years.

These wave-like structures on the underside of the cloud makes it look like a
rough sea surface viewed from below. We are not yet certain how they form but
there have been many sited on the Plains of the United States.