Because the island sits in the trade winds, damp air hits the northeast of the island and has to rise, where it turns into clouds. Because La Palma has a north-south spine called the Cumbre Nueva, the cloud quite often reaches up to the ridge and then tumbles over as the cloud waterfall. This is extremely pretty, and best viewed from around the western side of the tunnel. You can also look down on it from the Los Andennes mirador on the road to the observatory.
Sometimes, when the winds blow from the west, the waterfall tumbles down the eastern side of the ridge.
Occasionally, it keeps going after sunset, and you can see it lit by the full moon, which makes you wonder if you’ve slipped through to Narnia or Middle Earth.
And once, just once, the clouds were reflecting a spectacular sunset which turned them candyfloss-pink.
I didn’t have a camera.