Barometric pressure, also known as atmospheric pressure, is the force exerted by the weight of the air column above a specific point on Earth’s surface. Its fluctuations are responsible for nearly all changes in local weather. Understanding these shifts is fundamental to meteorology and provides the earliest warning signs of changing conditions.
Defining Atmospheric Pressure and Measurement
Atmospheric pressure is defined as the force per unit area exerted by the gas molecules that make up the air. At any location, this pressure is the cumulative weight of all air molecules in the column stretching from the ground up to the edge of space.
The standard average atmospheric pressure at sea level is 1,013.25 millibars (mb), equivalent to 1,013.25 hectopascals (hPa). In the United States, pressure is often reported in inches of Mercury (inHg), where the sea-level standard is 29.92 inches of Mercury.
The classic mercury barometer operates by balancing the weight of the atmosphere against a column of mercury inside a sealed tube. Modern instruments rely on the aneroid barometer, which uses a small, flexible, vacuum-sealed metal box. Changes in pressure cause the capsule to expand or contract, and a system of levers translates this movement to a dial or digital readout.
How Pressure Systems Drive Weather
Differences in barometric pressure create pressure gradients, which drive all atmospheric movement. Air naturally flows from areas of higher pressure to lower pressure, which we experience as wind. This flow attempts to equalize pressure imbalances, generating distinct weather patterns.
High-pressure systems (anticyclones) form where air sinks toward the surface. As air descends, it warms, causing moisture to evaporate rather than condense. This subsidence suppresses cloud formation and precipitation, leading to stable weather with clear skies.
Conversely, low-pressure systems (cyclones) develop where air rises from the surface. As the air ascends, it cools, causing water vapor to condense and form clouds, often resulting in precipitation. This upward motion draws in surrounding air, associating low-pressure systems with stronger winds and unsettled weather.
In the Northern Hemisphere, the Coriolis effect causes winds to circulate around these centers. Air flows outward and clockwise around a high-pressure center, and inward and counterclockwise around a low-pressure center. This rotational motion reinforces the vertical air movement, maintaining fair weather in highs and generating storms in lows.
Interpreting Pressure Changes and Trends
Pressure Trends
For weather forecasting, the rate and direction of pressure change are more important than the absolute reading. A rising barometer, especially quickly, indicates a high-pressure system is moving in, suggesting improving or continuing fair conditions. A slow, steady rise points toward prolonged clear weather.
When the barometer begins to fall, it signals the approach of a low-pressure system and the likelihood of deteriorating weather, such as clouds, wind, and precipitation. A rapid drop, such as a decrease of 3 to 4 millibars over a few hours, is the most reliable predictor of an imminent storm or a significant shift to severe weather.
Sea Level Correction and Isobars
To compare readings across different geographical areas, meteorologists adjust the raw measurement for altitude, a process called correction to mean sea level pressure. Since air pressure decreases naturally as elevation increases, this correction standardizes all readings to the pressure value they would have at sea level, allowing for accurate mapping of pressure centers.
On a weather map, lines connecting points of equal, sea-level-corrected pressure are called isobars. The spacing of these isobars provides a visual representation of the pressure gradient. Closely spaced isobars indicate a steep pressure change over a short distance, which translates directly to stronger winds, while widely spaced isobars signify a weak pressure gradient and calmer conditions.