Pressure Gradient Force

Atmospheric motion is influenced by several forces. For explanation and simplicity, we split those forces into those which affect horizontal flow and those which affect vertical motion. The primary forces which affect horizontal motion are the pressure gradient force, the Corolis force, and friction.

Pressure Gradient

Before we can explain a force, we must understand what causes it. So first, we will explain the pressure gradient. In the atmosphere, there is not uniform pressure everywhere. The change in pressure over a distance is the pressure gradient. The pressure gradient is a vector, i.e. it has a magnitude and a direction. The magnitude is based on the distance between the high and low pressure centers, and how strong the high and low pressure centers are. The direction is simple -- it points from lower pressure toward higher pressure, as shown below.

Pressure Gradient Force (PGF)

Associated with the pressure gradient is a force. It is called the Pressure Gradient Force, or PGF. Its magnitude is directly proportional to the magnitude of the pressure gradient, but its direction is the opposite. Remember the energy transfer method of conduction? Well, the PGF is similar in behavior. In conduction, heat moves from an area of higher temperature to the area of lower temperature. In the atmosphere, pressure behaves similarly. Pressure gradient force is directed from areas of higher pressure toward areas of lower pressure. The pressure here is a measure of air pressure, so the PGF drives air outward from high pressure regions to low pressure regions.

You can experiment with a pressure gradient force calculator to look at the amount of force created by differences between high and low pressures.

If the earth did not rotate, air would flow directly from the high pressure center to the low pressure center. However, in reality, the air spirals out of high pressure areas in a clockwise direction and spirals into low pressure areas in a counter-clockwise direction, in the northern hemisphere. This spiraling is due to the Coriolis force, our next topic.