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Atmospheric pressure is the amount of force being exerted on an area by all the air above that area. Natural variations, or currents, in the atmosphere mean the actual pressure at different points on the Earth's surface can vary. When atmospheric pressure is lower than normal, it's an indication that there are fewer air molecules above an area. If an area has a higher than normal atmospheric pressure, then it indicates there are more air molecules in the atmosphere above. The pressure in the atmosphere can affect the weather, allowing storms to form and fluctuations in temperature to occur independent of the seasons.

Atmospheric pressure can be measured with a device known as a barometer. One common form of a barometer uses liquid mercury in a U-shaped tube. If there are changes in atmospheric pressure, then the mercury inside the glass will react and either move closer to or farther away from an opening in the glass. The result is that the mercury will either rise or fall to indicate a change. Another type of barometer uses a sealed glass capsule that expands or contracts as the pressure outside changes.