Turbulence
Why is it bumpy when we fly over mountains on a totally clear day?
Short answer: Wind rolling over the mountain peaks creates invisible 'waves'.
When wind hits a massive mountain range, it rolls over the top and creates ripples in the air on the other side, just like water flowing over a rock in a fast stream. This is called 'Mountain Wave' turbulence. It is completely normal, expected by the pilots, and the plane's flexible wings absorb the bumps easily.