 Let's go through a quick calculation of the pressure gradient force for a low pressure system in the past over Pennsylvania on June 26, 2015. Note that the pressure increases as x increases, but because the pressure gradient force is minus 1 over the density times the pressure gradient, the pressure gradient force, really the pressure gradient acceleration, is negative. This makes sense, since the pressure gradient force would move air from high pressure to low pressure, which is to the west in this case. To find the pressure gradient, we note that the height of Pennsylvania is about 250 kilometers, which is slightly smaller than the distance between the 1008 millibar and the 1016 millibar isobars, which is about 300 kilometers is the distance. So the air density is about 1.2 kilograms per meter cubed. When we put all these numbers together, that is 1 over the density times the change in pressure over the change in distance, we get that the pressure gradient force in this case is 2.2 times 10 to the minus 3 meters per second squared and is directed to 180 degrees or due west.