Waves formed on the WeatherRadar yesterday, not in the ocean but in the sky over the Pennines and Cambrian mountains.

Monday afternoon, as cloud cover stretched across much of the UK, cool, moist air in the north-easterly flow was also forced to ascent the mountain barriers of the Pennines and Snowdonia.

This causes so-called gravity waves to develop, an oscillating up and down motion of the flow as the airstream interacts with the terrain. The cloud lines are at right angles to the wind direction and appear upon hitting mountainous, or hilly, areas.

Air ascending up a mountain range happens all the time if the wind direction is right. What is crucial during a lee wave event is an inversion, a stable layer of air that acts as a "lid" above the cool, moist air mass below.

Every time the air rises, it cools and forms a cloud that sits at each crest of the wave. When this airflow hits the inversion it subsides and dries again.

On the WeatherRadar, you can see all the ripples flowing over the mountains, continuing southwest.

Have you seen any near you? If so, send us a picture, we would love to see them. Upload yours here!