Crashing waves are usually spotted off the coast on windy days, but every so often the skies above can bring an altogether different type of waves.

Spotted over Swansea, Wales, wave clouds or Kelvin-Helmholtz clouds are quick to form and quick to disappear without a trace soon after.

These clouds appear when the air is very unstable. Forming when faster-moving, warm air flows over slower-moving, colder, and denser air. If the speed at which the warm air travels over the colder air is fast enough, the undulating wave-like formation in the cloud appears.

This phenomenon can be spotted anywhere - just be quick at getting your picture!