As the name suggests, the undulatus variety of clouds is defined by a pattern of undulations. Like other cloud varieties, the pattern can appear on part of one or another of the ten main cloud types. The cloudy billows form due to the shearing effect that occurs when the air above and below the cloud layer moves at distinctly different speeds or in different directions. You can think of them as ripples or waves on the surface of water or furrows in a field. Or they might make you think of fish. This example of the undulatus pattern appearing in the mid-level solid-looking cloud Altocumulus was spotted by Jacinta Hennelly (Member 64,403) over Galway, Ireland. Altocumulus undulatus are sometimes known as ‘mackerel skies’ because the undulating clouds can resemble the patterns found on the backs of mackerel fish.