The altostratus undulatus is a type of altostratus cloud with signature undulations within it. These undulations may be visible (usually as "wavy bases"), but frequently they are indiscernible to the naked eye. These formations will generally appear in the early stages of destabilizing return flows, especially over the southern plains of the United States, when the surface temperature is still relatively cool.^[1] The wavy strips of clouds are generally near an inversion surface.

Altostratus undulatus cloud
Cirrus and Altostratus undulatus clouds
Abbreviation	As
Genus	Alto- (high), -stratus (layered)
Species	undulatus (waves)
Altitude	2400-6100 m (8,000-20,000 ft)
Classification	Family B (Medium-level)
Appearance	Wavy, like ripples in a pond
Precipitation	Usually no

Also referred to as billow clouds, wind row clouds, or wave clouds, variations of the undulatus can be elements that have merged or single elements that have stretched through the sky.