The atmosphere is composed of molecules that have mass and are affected by gravity. Atmospheric pressure is the force exerted on a surface by the weight of the air column directly above it. Since the amount of air above any point changes, atmospheric pressure is not constant. However, there is a specific value defined as the average at a universal reference point. This standardized value allows scientists and meteorologists to compare measurements globally and is the foundation for weather forecasting.
Defining Barometric Pressure and Standard Sea Level
Barometric pressure is the measurement of atmospheric force, named after the barometer, the instrument used to measure it. Pressure decreases rapidly as altitude increases because the column of air weighing down becomes shorter. For instance, at 18,000 feet, the pressure is roughly half that at sea level.
To make pressure readings from different elevations comparable, a universal reference point, known as mean sea level (MSL), is used. MSL is the average height of the ocean’s surface over a long period, serving as the zero-elevation datum for standardized measurement. A raw pressure reading, known as station pressure, is only the actual pressure at that specific location and altitude. By correcting the station pressure to what it would be if measured at MSL, meteorologists can accurately track the movement of pressure systems across the globe.
Standard Atmospheric Pressure: The Numerical Value
The standard atmospheric pressure at mean sea level is a fixed value used in physics and meteorology as a benchmark. This value is formally defined as one standard atmosphere (atm). In the International System of Units, this standard is precisely 101,325 Pascals (Pa).
The most common units used by meteorologists are based on this standard. The standard value is 1013.25 hectopascals (hPa) or 1013.25 millibars (mb). In the United States and aviation, the pressure is often expressed in inches of mercury (inHg), where the standard value is 29.92 inches of mercury. These conversion equivalents all represent the average weight of the atmosphere pressing down on a square area at the ocean’s surface.
Measuring Atmospheric Pressure
The measurement of atmospheric pressure relies on instruments called barometers, which have evolved significantly from their original design. The historical standard, the mercury barometer, measures pressure by balancing the weight of the atmosphere against a column of mercury within a tube. The height of the mercury column directly indicates the pressure, which is the basis for the inches of mercury unit.
Modern weather stations primarily use aneroid barometers, which are mechanical instruments that measure pressure changes using a sealed metal chamber containing a partial vacuum. As the external air pressure fluctuates, the chamber expands or contracts, moving a needle across a calibrated dial. Digital barometers use highly sensitive electronic sensors to detect minute pressure changes and convert them into a digital reading.
Factors Influencing Pressure Deviation
The actual barometric pressure at any location is almost always fluctuating due to dynamic atmospheric conditions, making these deviations a powerful tool for weather forecasting. The most significant factors influencing pressure are large-scale weather systems and air temperature.
Weather systems are categorized based on their pressure relative to the surrounding area. High-pressure systems, marked by a higher-than-average reading, are associated with sinking air. This sinking motion compresses the air, leading to higher surface pressure and generally resulting in clear skies and fair weather.
Conversely, low-pressure systems are characterized by lower-than-average pressure readings and rising air. As air rises, it cools and moisture condenses, leading to cloud formation and precipitation. A rapidly dropping barometric pressure reading indicates an approaching storm or worsening weather.
Temperature also plays a direct role in pressure deviation because it affects air density. Warmer air is less dense than cooler air, meaning a column of warm air weighs less and results in a lower pressure reading. Conversely, cold air is denser, leading to a heavier column of air and a higher-pressure reading. This relationship is a major factor in the formation and movement of the high- and low-pressure systems that dictate regional weather patterns.