When the surface of a cloud layer, or the arrangement of its cloudlets, develops an undulating appearance that suggests waves, it’s defined as the undulatus variety. Undulatus usually forms when the winds above and below the cloud layer are moving at differing speeds and/or in different directions. It is the shearing effect of the two airstreams that gives rise to the cloud billows, which resemble ripples on a sandy beach caused by the movement of water. Wave formations in clouds are so common that the undulatus variety can be observed in six of the ten main cloud types. This is the undulatus species of the mid-level cloud called Altocumulus. The presence of wavelike formations like this is a reminder that the atmosphere around us, though it consists of gases rather than liquids, is just as much an ocean as is the sea below.