Undulatus is a cloud variety in which patches, sheets or layers show parallel wave-like ripples, like a corrugated sky. Sometimes a double system of undulations crosses at an angle. The ripples arise from wave motion in the atmosphere: wind shear between two air layers moving differently sets up gentle billows, with cloud forming in the rising wave crests and thinning or clearing in the sinking troughs.
Worth knowing
Undulatus is one of the few cloud varieties the WMO recognises across many genera — from high cirrostratus right down to low stratus — because it describes a pattern of wave ripples rather than a height or composition. The waves typically run perpendicular to the shear that produces them.
Note
Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.
