<P> The pressure - gradient force is the force which results when there is a difference in pressure across a surface . In general, a pressure is a force per unit area, across a surface . A difference in pressure across a surface then implies a difference in force, which can result in an acceleration according to Newton's second law of motion, if there is no additional force to balance it . The resulting force is always directed from the region of higher - pressure to the region of lower - pressure . When a fluid is in an equilibrium state (i.e. there are no net forces, and no acceleration), the system is referred to as being in hydrostatic equilibrium . In the case of atmospheres, the pressure gradient force is balanced by the gravitational force, maintaining hydrostatic equilibrium . In Earth's atmosphere, for example, air pressure decreases at altitudes above Earth's surface, thus providing a pressure gradient force which counteracts the force of gravity on the atmosphere . </P> <P> Consider a cubic parcel of fluid with a density ρ (\ displaystyle \ rho), a height d z (\ displaystyle dz), and a surface area d A (\ displaystyle dA). The mass of the parcel can be expressed as, m = ρ ⋅ d A ⋅ d z (\ displaystyle m = \ rho \ cdot dA \ cdot dz). Using Newton's second law, F = m ⋅ a (\ displaystyle F = m \ cdot a), we can then examine a pressure difference d P (\ displaystyle dP) (assumed to be only in the z (\ displaystyle z) - direction) to find the resulting force, F = − d P ⋅ d A = ρ ⋅ d A ⋅ d z ⋅ a (\ displaystyle F = - dP \ cdot dA = \ rho \ cdot dA \ cdot dz \ cdot a). </P>

With regard to the pressure gradient force air will always move from low to high pressure