The gradient of pressure in hydrostatics is equal to the body force density (generalised Stevin's Law).
In petroleum geology and the petrochemical sciences pertaining to oil wells, and more specifically within hydrostatics, pressure gradients refer to the gradient of vertical pressure in a column of fluid within a wellbore and are generally expressed in pounds per square inch per foot (psi/ft).
The concept of a pressure gradient is a local characterisation of the air (more generally of the fluid under investigation).
The pressure gradient often has a small but critical horizontal component, which is largely responsible for wind circulation in the atmosphere.
As indicated above, the pressure gradient constitutes one of the main forces acting on the air to make it move as wind.