Barometric pressure, also known as atmospheric pressure, is the weight of the air pressing down on a specific point on the Earth’s surface. This pressure is created by the cumulative weight of air molecules in the atmosphere above us. Because air masses are constantly moving, the pressure fluctuates, and these fluctuations are directly linked to changes in weather. Monitoring these changes with a barometer is a powerful tool for predicting weather shifts, often with more localized accuracy than a general forecast.
How Barometric Pressure is Measured
The standard measurement of barometric pressure is often expressed in two primary units: inches of mercury (inHg) and millibars (mb) or hectopascals (hPa). The average sea-level pressure, which meteorologists use as a baseline for comparison, is approximately 29.92 inHg, which is equivalent to 1013.25 mb or hPa. A traditional aneroid barometer uses a sealed metal cell that expands or contracts with pressure changes, moving a needle across a dial.
Digital barometers often display two values: station pressure and sea-level corrected pressure. Station pressure is the absolute reading at your specific location and altitude. To ensure readings are comparable across different elevations, meteorologists apply a correction to calculate the pressure as if the measurement were taken at mean sea level. This process, called sea-level correction, standardizes the data. The corrected reading is used for weather map analysis and most consumer weather reports.
What Sustained High and Low Readings Indicate
Sustained pressure values indicate the nature of the current air mass. A sustained high reading, generally considered to be above 30.20 inHg (1023 mb), is characteristic of a high-pressure system. High pressure is associated with sinking air, which warms and dries as it descends. This sinking motion suppresses cloud formation, resulting in conditions like clear skies, light winds, and fair weather.
Conversely, a sustained low reading, typically below 29.50 inHg (999 mb), signals the presence of a low-pressure system. Low pressure encourages air to rise, which cools and allows water vapor to condense, leading to cloud development and potential precipitation. Low-pressure areas are often linked to unsettled, cloudy, and sometimes stormy weather.
Predicting Weather Using Pressure Changes
While the absolute pressure value indicates the current weather system, the real power of the barometer lies in interpreting the rate and direction of pressure change. Observing the trend over several hours is often more predictive than the current reading itself. When the pressure is steady, it means the current weather pattern is likely to continue without significant change in the immediate future.
A rapidly falling pressure is the strongest indicator that a storm or significant weather event is imminent, often within the next 12 to 24 hours. The faster the pressure drops, the more severe and immediate the weather change is likely to be, pointing to an approaching low-pressure center or weather front.
A slow or gradual fall, however, often suggests a period of worsening weather, such as increasing cloudiness or light, prolonged rain. When the pressure begins to rise, it generally signals improving weather conditions.
A slow rise suggests the current system is stabilizing or a fair-weather high-pressure system is gradually moving in. A rapid rise in pressure indicates that a low-pressure system is passing and clearer, often cooler, high-pressure conditions are quickly replacing it. This rapid rise frequently brings strong, gusty winds as the system shifts.
Local Factors That Influence Pressure Readings
Altitude is the most significant factor influencing local pressure readings, as air pressure naturally decreases by approximately 0.11 kilopascals for every 10 meters of elevation increase. This is why sea-level correction is necessary to compare data regionally, though the actual station pressure is the true measure of the air’s weight at your specific spot.
Temperature also plays a role because warmer air is less dense than colder air, which can cause local pressure to drop slightly. Similarly, air with high moisture content is less dense than dry air, meaning high humidity can also contribute to a subtly lower pressure reading. While barometric pressure is a powerful forecasting tool, it should be used alongside other observations like wind direction and temperature to create a more comprehensive and accurate local prediction.