What is causing the "waterfall" of cloud?

It was a cloudy start after the stormy weekend but by lunchtime the sky had cleared over Birks. However there was cloud tumbling over the top of Hart Crag and down into Deepdale like a waterfall. How does that work? For there to be cloud, the humidity needs to be at or higher than the saturation level for air of that temperature. Today I suspect that the cloud on the west side of the hills reflects moist air coming in from the sea which would increase the humidity. Where I was standing the sun has increased the temperature so that the air is no longer saturated. My hypothesis is that the cloud is tumbling over the shaded side of the hill where the sun is not reaching so the air there will be colder and become saturated more easily. I'll need to remember to look next time to see if it happens over sunny slopes.