As air flows over the top of an aerofoil the pressure drops. Compared to the high / normal pressures on the underside, a pressure gradient is formed with higher pressure further above the wing. You get:
less pressure = less particles = less density
This sketch summarizes what I know so far:
My questions are:
Does lower air density above the wing mean the air weighs less compared to the weight of the air underneath the wing? If the difference is negligible then it's OK, we can forget it.
What forces act on the air particles causing them to speed up?