FORCES  ON  AN  AIR  PARCEL
Force	Direction	Cause
(1) Weight	Vertical (downward)	Gravity
(2) Pressure Gradient Force (PGF)	Both horizontal (2a) and vertical (2b, buoyancy)	Pressure imbalance
(3) Coriolis (a fictitious force)	Horizontal and centripetal, to the right (left) of velocity in the northern (southern) hemisphere	Earth's rotation
(4) Friction	Primarily horizontal  (only significant near surface), opposite the direction of the velocity	Velocity gradient, drag