Pressure-gradient force is the trigger for the movement of air. Gravity causes the air to press down against the surface of the Earth; this is atmospheric pressure. The pressure in two different given areas maybe different, and this pressure difference is called the pressure gradient. When their is a pressure gradient, it acts as a force driving air from an area of high pressure to an area of low pressure. This force is known as the pressure-gradient force, and increases as the difference in pressure increases over a given area. Air temperature is a common cause of air pressure differences, which intern causes differences in air density. Warm air is less dense, and therefore rises creating a low pressure area, where as cold are descends and creates and area of high pressure. The cool high pressure air moves to the warm low pressure area, and this movement of air is manifest as wind. This video illustrates wind, the result of air moving from low high pressure to low pressure.
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