Mist pouring over Aran Benllyn

Creiglyn Dyfi, source of the river Dovey (Afon Dyfi)

Aran Benllyn from Foel Hafod-fynydd

Kelvin-Helmholtz (K-H) wave clouds over the Aran Fawddwy ridge.  These wave clouds
form in stable conditions when wind speed and direction are both steady, but if strong
horizontal winds disturb the boundary between a warm air mass and a layer of cooler air
below, the upper air may start to move at a faster rate than the lower, causing the crests
of the waves to advance ahead of the main body of cloud.  The result is a rare short-lived
sequence of distinctive billowing clouds known as a Kelvin-Helmholtz (K-H) wave, after
the 19th century scientists who studied them.  This example lasted about ten minutes.

The Arans ridge from Bwlch Sirddyn