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.
The air that comprises the atmosphere of the Earth is made of several types of molecules. These molecules are like any other, except they are not visible to the human eye and are in a gaseous form. The gravity of the Earth still acts on the molecules and draws them toward the surface. This pulling of the air creates pressure on every surface on Earth.
The higher a person moves into the atmosphere, the lower the pressure becomes. This is because, as an object moves upward, the amount of air molecules above it decreases because the distance between the object and the edge of the atmosphere is shorter. Similarly, if an object is taken below the surface of the Earth, below sea level, then the pressure that the object is experiencing will begin to increase, because more air will be on top of it.
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 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.
Weather patterns across the planet are affected by atmospheric pressure, and the weather itself also can affect the pressure. Tornadoes and other violent cyclonic disturbances are often centralized in an area of incredibly low pressure because of the large-scale movements of the air. High-pressure areas, which have more mass than normal, cause clear weather and calm conditions. The temperature in an area, as well as the humidity, also can affect the pressure by expanding or contracting air or saturating it with tiny water particles.