This interesting cloud formation is caused by air forced to rise due to the terrain. As the air cools down it condenses and a cloud appears, like here in an otherwise blue sky. Mountains can give other fascinating clouds as rotor clouds, lenticularis and more.

Another example where the forced movement caused by the mountain creates a topcloud. Note the cable-car cables going to the top on the left, and if you look close enough you might be able to visualize the cable-car station. Maybe you recognize this big rock, being the world famous Pão de Azucar in Rio.

Sometimes the winds make the cloud look like a flag blowing from the top, it's then called a 'banner cloud'. Note that if the mountain wouldn't have been there, there would be no clouds in the sky.

The cloud formation starts just at the top of the mountain. Coincidence?