Barometric pressure, or atmospheric pressure, refers to the weight of the air column pressing down on the Earth’s surface. This measurement constantly fluctuates due to the movement of air masses, which drive all weather changes. Falling pressure is commonly associated with the arrival of wet weather. Monitoring these shifts provides a reliable method for anticipating precipitation, as rain is typically a feature of low-pressure systems, meaning the pressure drop precedes the actual rainfall.
Understanding Atmospheric Pressure
Atmospheric pressure is the force exerted by the cumulative weight of all air molecules above a specific location. Since air has mass, gravity pulls these molecules downward, creating measurable pressure. This pressure is tracked using a barometer. The standard unit in meteorology is the hectopascal (hPa) or millibar (mb). At sea level, the average pressure is 1013.25 millibars (29.92 inches of mercury). Readings below this value indicate low pressure, while readings above it indicate high pressure. These measurements reflect whether the air above is heavy or light, which relates directly to the type of weather expected.
Why Pressure Drops Before Precipitation
The drop in barometric pressure before rainfall is caused by the approach of a large-scale weather system called a low-pressure area or depression, not by the water itself. These systems are characterized by air that is warmer and less dense than the surrounding air masses. Because this air is lighter, it exerts less downward force on the surface, causing the barometric reading to fall.
This lower density air begins to rise, a process known as atmospheric lifting or convection. As the air ascends into the upper atmosphere, it encounters lower pressure, causing it to expand. This expansion requires energy, leading to a significant cooling effect. The air cools to its dew point, the temperature at which the water vapor condenses into liquid droplets.
This continuous cycle of rising, cooling, and condensation forms clouds, which are the precursor to rain. The initial pressure drop signals that a rain-producing system is moving into the area. A rapid drop in pressure over a short period suggests a more intense weather system is approaching, potentially bringing stronger winds and heavier precipitation.
Pressure Trends During and After Rain
Once the low-pressure system has fully arrived, the barometric pressure typically stabilizes at its lowest point or continues a slow decline as the core of the system passes over. During this period, the mechanisms for cloud formation and precipitation are fully active, and rainfall is usually at its heaviest. This lowest pressure reading often marks the geographical center of the storm or the passing of an associated weather front.
The rainfall event concludes when the low-pressure system begins to move away and is replaced by a different air mass. This transition is signaled by a distinct and sustained rise in barometric pressure. The rising pressure indicates that a high-pressure system is arriving, characterized by cooler, denser air.
In a high-pressure system, the air mass is sinking instead of rising. As the air descends, it compresses and warms, which causes any suspended water droplets to evaporate. This process inhibits cloud formation, pushing the storm system out and leading to clear skies, lower humidity, and generally fair weather. The return of higher pressure confirms the passage of the rain-producing system.