Air PressurePressure variations across the EarthAir pressure is the force exerted by the weight of a column of air above a particular location. To conceptualize the notion of air pressure, imagine a sealed container full of air as shown in Figure 6.1. When the molecules of air collide with the inside surfaces of the container they exert a pressure. The amount of pressure they exert depends on the number of collisions that occur between the molecules and the inside surface of the container. We can change the pressure in two ways. First, we can increase the density of the air by either putting more air molecules into the container or reducing the volume of the container. Secondly, we can increase the temperature of the air to make the molecules move faster and thus collide with the sides more often. Therefore, changes in air pressure can come about by changes in air density or temperature. In nature, pressure variations across the surface of the Earth are created by mechanical or thermal means. Mechanical changes in pressure occur when air flow is impeded causing a mass of air to build up over a particular location thus increasing air pressure. Heating and cooling the air (thermal mechanisms) also create variations in air pressure. When air is heated it rises, and if pushed away aloft, surface air pressure decreases. Conversely if air is cooled, it subsides toward the surface causing air pressure to increase.
Figure 6.1 Air pressure within a sealed container.
Air pressure is measured using a barometer. Several different barometers exist, two of the most common are the mercury barometer and the aneroid barometer. The mercury barometer is a tube with a reservoir of mercury at one end. Under average sea level conditions, the atmosphere exerts enough pressure to push a column of mercury up to the height of 29.92 inches. The aneroid barometer uses an aneroid or sylphon cell to measure pressure. The aneroid cell is a metal chamber that expands and contracts with changing air pressure. Though inches of mercury are often reported on your daily weather forecast (especially in the United States), meteorologists use millibars as the units of measurement for air pressure. Under average sea level conditions the atmospheric pressure is 1013.2 millibars (29.92 inches of mercury). Average sea level pressure serves as the division between what we call "high pressure" and "low pressure" at the surface. High pressure is defined as values greater than 1013.2 mb and low pressure is below 1013.2 mb. |
