More good weather saw another chance to get back into Snowdonia and climb another mountain. This time, we tackled the great Cadair Idris. The whole hike up from the southern side was in thick fog, with a strong wind. Even the summit was shrouded in thick fog. We presumed it was just the general cloud cover, and it wasn't till we got down the north side of the mountain that we realised it was essentially a local cloud stuck on Cadair Idris, in fact rolling down Cadair Idris!
The 'mist' we were hiking through was actually Orographic Fog. Orographic fog is a really interesting phenomenon, it is caused by a light wind hitting a mountain and being forced up and over it. As the air rises, it cools adiabatically due to the drop in air pressure, which forces the water vapour to condense and form clouds. These clouds are carried with the wind, which is gentle enough that it follows the contours of the mountain and drops back down the opposite flank, where the air pressure increases again, allowing the air to warm back up and the water vapour to evaporate again, so the cloud disappears. If the wind is too strong, it won't follow the contours of the mountain back down again but will blow the cloud away from the mountain, creating a 'banner cloud'. Which looks like a bit like a flag.
In the brief time-lapse I recorded with a handheld camera whilst the group analysed a map, you can clearly see this movement down the northern flank of the mountain! Of course, the sight made for some spectacular video footage, like the one where you see my good friend Hector walking past the camera with the orographic fog in the background. :)
Comments