What Causes Barometric Pressure?
By Kelvin O'Donahue
Along with the air temperature and wind speed, the atmospheric pressure is a key measurement used by meteorologists for forecasting the weather. Because the instrument used to make this measurement is a barometer, the reading is also called barometric pressure. Changes in barometric pressure result from changing conditions in our atmosphere. Changes in the atmosphere are what causes changes in the weather, so tracking the barometric pressure helps scientists predict local weather.
The atmospheric pressure at any place is just the weight of the atmosphere above that spot. Though the surrounding air seems like a lot of nothing, the column of air above people's heads is hundreds of miles tall. Because of that height, it weighs enough to exert a pressure of about 1,013.25 millibars (14.7 pounds per square inch) at sea level.
Elevation and Atmospheric Pressure
Because atmospheric pressure is caused by the weight of the column of air above a location, it should make sense that making the column shorter reduces the weight. That's exactly what happens, so barometric pressure gets lower as you go from the seashore to the top of a mountain. Not only does the column of air get shorter, the air is also denser at lower elevation. That's why it is harder to breathe high in the mountains than at the shore.
Meteorologists use a mathematical formula to correct atmospheric pressure measured at high altitude to a standard value, which is what it would be at sea level. If they did not make this calculation, they could not compare pressures between two places at different elevations.
High and Low Pressure Systems
The air in Earth's atmosphere is in constant motion. Not only does the air move around the surface in the form of wind, but the air also slowly rises in some places and falls in others. In areas where the air is falling toward the surface, the downward motion of the air column adds a little to the local air pressure. The result is slightly greater pressure than normal for that elevation, which is called a high pressure. At places where the air is rising, the weight of the air column is reduced just a little, causing a region of low pressure. These regions of high and low pressure move slowly around on the surface.
Meteorologists track barometric pressure because changes help predict changing weather conditions. As air sinks in a high pressure zone, it warms and expands, which slows or stops the formation of clouds. As a general rule, rising barometric pressure means clear weather. The air in a low-pressure area cools as it rises, which has the opposite effect: Cooling the air causes more clouds to form, which tends to bring cloudy and even stormy weather.
When combined with other measurements, changing barometric pressure is a key to helping meteorologists predict the weather.
Kelvin O'Donahue has been writing since 1979, with work published in the "Arizona Geological Society Digest" and "Bulletin of the American Association of Petroleum Geologists," as well as online. O'Donahue holds a Master of Science in geology from the University of Arizona, and has worked in the oil industry since 1982.