Air
parcel's Upward
Directed Vertical Downwards Upwards
Directed

Vertical
due
to Pressure Gradient and
Directed and Buoyancy

Accelaration Acceleration Gravity
Accel Acceleration

At synoptic
scale, buoyancy acceleration is one order of magnitude smaller than the
vertical pressure gradient acceleration, which is about the same magnitude as
the acceleration of gravity. Thus,
at the synoptic scale, buoyancy acceleration can be dropped on an order of
magnitude basis. Since the upward
directed pressure gradient is then balanced by the downward acceleration of
gravity, the equation above becomes the hydrostatic equation.