Part 2:
Vertical Motion
Air does not just flow horizontally -- it also flows up and down. The motion is not separated into only horizontal and only vertical flow, we simply break it up that way for simplicity. There are three major forces that influence vertical motion -- the vertical PGF, gravity, and friction. There are also three other factors influencing vertical motion -- convergence, buoyancy, and terrain.
Vertical Pressure Gradient
A pressure gradient is a change in pressure over a distance, directed toward increasing values. We must remember that pressure changes dramatically with height, forming a pressure gradient directed toward higher values (down). This in turn produces a very strong pressure gradient force directed upward.
This pressure gradient is about four times larger in magnitude than the horizontal PGF. However, it is also balanced by a strong force -- gravity.
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