Barometric pressure represents the weight of the air above us. This atmospheric phenomenon, also known as air pressure, constantly presses down on every surface of the Earth.
Understanding Barometric Pressure
Barometric pressure is the force exerted by the air molecules in Earth’s atmosphere. This pressure is measured using an instrument called a barometer. Scientists typically measure barometric pressure in units such as millibars (mb) or hectopascals (hPa), and also in inches of mercury (inHg) in some regions like the United States. This measurement helps differentiate between high and low pressure systems, which are fundamental to weather analysis.
Defining “Good” Barometric Pressure
There is no single numerical value that defines “good” barometric pressure, as it varies with factors like altitude. Instead, “good” barometric pressure for weather generally refers to a stable, moderate pressure, which is typically associated with fair weather conditions. The average barometric pressure at sea level is approximately 1013.25 millibars (29.92 inHg). A reading in the range of 29.80 to 30.20 inHg (1009 to 1022 mb) is often considered normal at sea level, indicating steady weather. This stability, rather than a specific high or low value, is generally what constitutes favorable weather.
Why Barometric Pressure is Important
Barometric pressure plays a significant role in weather forecasting. Rising barometric pressure often signals improving weather, leading to clear skies and calm conditions. Conversely, falling pressure suggests approaching storms, cloud formation, and precipitation. Meteorologists monitor these pressure trends to predict short-term weather changes, as a rapid drop in pressure can indicate an impending storm.
What Causes Barometric Pressure to Change
Barometric pressure fluctuates due to several atmospheric factors. Altitude is a primary influence, as pressure decreases significantly with increasing elevation because there are fewer air molecules above to exert weight. For example, at 18,000 feet, the pressure is roughly half that at sea level. Large-scale weather systems also drive pressure changes; high-pressure systems are characterized by sinking, denser air that typically brings clear, stable weather, while low-pressure systems involve rising, less dense air associated with clouds and precipitation. Temperature also contributes, with warmer air tending to have lower pressure because its molecules are more spread out, making it less dense, while colder air is denser and tends to have higher pressure.