4.2: Air Pressure

Pressure variations across the Earth

Air 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 \(\PageIndex{1}\). 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 \(\PageIndex{1}\): Air pressure within a sealed container.

Figure \(\PageIndex{2}\): A mercury barometer.

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. 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 Though other, outdated, units exist as well, meteorologists use millibars as the units of measurement for air pressure. Under average sea level conditions the atmospheric pressure is 1013.2 millibars. 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.

Air pressure decreases as one moves upward through the atmosphere because the length of the column of air shortens and hence there is less mass above a given location. The rate of decrease also changes with elevation (Figure \(\PageIndex{3}\)). Because air is highly compressible, the air is closely packed together near the surface (high density) and less densely packed aloft. In fact, over 90% of the molecules in the atmosphere are found below 10 miles. As a result, air pressure decreases more rapidly between two elevations close to the surface than between two points separated by the same distance aloft.

Figure \(\PageIndex{3}\): Vertical variation in atmospheric pressure.

As explained previously, we use an average value of sea level pressure to differentiate between high pressure and low pressure. Because air pressure decreases aloft, the average pressure at any point above the surface is less. At 5600 meters the average pressure is 500 mb. If on any given day the pressure is larger than 500 mb we consider it high pressure at this elevation. If we measure a smaller amount then it is low pressure at this elevation. Do not confuse high pressure with higher pressure and low pressure with lower pressure. Adding the "er" to the end of the word indicates a change relative to a previous state. For instance, lets say you measure the air pressure to be 1015 mb at the surface and 516 mb at 5600 meters. Both cases are under the influence of high pressure at their respective locations. However, the pressure is higher at the surface than at 5600 meters. For comparisons between places that are far apart and at different elevations, meteorologists often correct for the drop in air pressure with elevation by calculating what the air pressure would have been at sea level for each location.

Analyzing air pressure patterns

At the highest peaks in the Himalayas, air pressure is only about one-third of the air pressure at sea level! That is why weakly adapted climbers choose to use bottled oxygen as a support. Meteorologists have a variety of ways to then visualize weather data, a map being the most common. To analyze pressure patterns, a constant height map is often used. A constant height map shows this distribution of pressure at sea level (i.e. corrected for elevation). Isobars, lines connecting points of equal air pressure, are used to show pressure patterns on such constant height maps. Most of the maps of air pressure in this book are constant height maps.

Figure \(\PageIndex{4}\): Constant height map with isobars