Air density is not uniform - it is fluidly dynamic - in a constant state of flux.
Air is constantly moving with high density/pressure areas pushing air into lower density areas (the wind). This moves air around in the horizontal plane.
This gets more complicated when you introduce convection currents with warm air rising and cold air falling. This moves air around in the vertical plane. Much of this is generated by heat (or lack of it) from the sun but it can also be influenced by airborn moisture (humidity).
Then you have the rotation of the planet and its topography (mountains jutting up into the atmosphere) which stirs it all up like a big blender.
The air is turbulent all the time - it is a chaotic system - and you do not have to be flying to experience it, just step outside on a windy day.