The rate of change in atmospheric pressure between two areas, providing a force which moves air from high to low in an effort to even up the unequal mass distribution of the air. On a global scale, the most powerful pressure gradients are in a meridional direction, caused by meridional disparities in insolation. See Bacon and Carter (1993) Int. J. Climat. 13 on the connection between mean wave height and atmospheric pressure gradient in the North Atlantic, and McPherson (2007) PPG 31, 3. The pressure gradient wind is the movement of air in response to pressure differences, blowing from high to low. It is modified, however, by the action of the Coriolis force. The term is also used in fluvial geomorphology; see Powell (1998) PPG 22, 1.