The force that causes acceleration from higher to lower pressure in a fluid such as the air. It arises because of a pressure gradient or change between higher and lower pressure. As an analogy, consider the pressure gradient that exists at the nozzle of a vacuum cleaner when it is turned on. A motor inside creates a region of relatively low pressure contrasted with the region of higher pressure outside. The dust and air molecules outside the vacuum, being at a higher pressure, feel a force pulling toward the low pressure inside the vacuum. This is a pressure gradient force. In the atmosphere, regions of high and low pressure form ultimately as a result of unequal heating. In moving from a high pressure region to a low pressure region, the motion is said to be along the pressure gradient. The pressure gradient force itself points in the direction of the lower pressure. In the vacuum cleaner example, this is evidenced by the fact that air is sucked into the lower pressure inside. The magnitude of the pressure gradient force is proportional to the magnitude of the pressure gradient itself. The lower the pressure inside the vacuum cleaner, the more powerful the vacuum will be. In the atmosphere, pressure gradient is related to a quantity called geopotential height. The pressure gradient force is proportional to the geopotential height gradient.