Air is set in motion by the Pressure Gradient Force (PGF). This, as you well know, is a flow from high pressure to low pressure. In nature, regions of excess move toward regions of deficit. This comes from the second law of thermodynamics that states higher energy states move toward lower energy states. Nature is always trying to balance. The result of trying to balance and equalize pressure results in wind. The pressure gradient force acts on a horizontal plane.

What causes the pressure changes in the atmosphere? The primary reason is through the warming and cooling of air. The pressure exerted by a gas changes as it becomes more dense or less dense. Since a cold air mass is denser and has a lower thickness than warm air, pressure decreases more rapidly with height in cold air than in warm air. When cold dense air is placed next to warm less dense air, wind results by nature trying to balance the pressure differences at each level in the atmosphere between the two air masses. This can occur on all scales of motion. Examples: On the planetary scale, temperature gradients create the polar jet stream; on the synoptic scale it creates jet streaks; on the mesoscale it creates sea breeze circulations and the low level jet; on the microscale it creates lake breezes.

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