Barometric pressure, often called atmospheric pressure, is the measure of the force exerted by the weight of the air column above a specific point on the Earth’s surface. This column extends from the ground to the outer edge of the atmosphere. Since air has mass, it exerts a measurable weight, averaging about 14.7 pounds per square inch at sea level. Changes in this atmospheric weight are fundamental indicators of atmospheric activity and impending weather shifts.
The Role of Air Density and Temperature
The physical cause of a barometric pressure drop begins with the heating of an air mass. When air is warmed, gas molecules gain kinetic energy, move faster, and spread farther apart. This thermal expansion causes the air to become less dense, making warm air lighter than cooler air.
This reduction in density causes the warm air to become buoyant and rise through convection. As this lighter air rises, it removes mass from the atmospheric column above the surface location. Since barometric pressure measures the total weight of the air column, the removal of this air mass causes the localized surface pressure reading to decrease.
This mechanism of heating, expansion, and rising air explains why pressure drops, resulting in a reduction of the downward force exerted by the atmosphere.
How Low-Pressure Systems Drive Pressure Drops
Large-scale, sustained pressure reductions are driven by the formation and movement of low-pressure systems, also known as cyclones. These systems are areas where air flow is dominated by convergence at the surface, causing air from surrounding higher-pressure regions to flow inward toward the center of the low.
This converging air is forced to rise in a process known as atmospheric uplift. This continuous upward movement acts like a siphon, effectively removing air mass from the entire atmospheric column above the region. The sustained evacuation of air aloft maintains the lower pressure reading on the ground.
The Coriolis effect, caused by the Earth’s rotation, imparts rotation to these converging air masses. This creates the familiar swirling pattern of a cyclonic system. The intensity and speed of the pressure drop relate directly to the strength of this uplift and the rate of air mass removal.
The approach of an atmospheric front, particularly a warm front, is often associated with falling pressure. A warm front occurs when a lighter, warmer air mass glides up and over a retreating, denser cold air mass. This displacement contributes to the reduction in surface barometric pressure as heavier air is replaced by lighter, rising air.
Predicting Weather Changes from Falling Pressure
A falling barometric pressure reading signals impending atmospheric instability and a change in weather conditions. The pressure drop confirms that air is rising rapidly above the location, which is the precursor to the formation of clouds and precipitation.
As an air parcel rises, it moves into an environment of lower pressure and expands. This expansion requires energy drawn from the parcel itself, causing its temperature to decrease—a process called adiabatic cooling. This cooling drives the formation of weather.
When the rising air cools to its dew point, water vapor condenses around microscopic particles, forming clouds. If pressure continues to fall, indicating sustained uplift, clouds grow vertically, leading to precipitation like rain or snow. A rapidly falling barometer often warns of an approaching storm or severe weather system.
The rate at which the pressure falls indicates the speed and intensity of the approaching weather disturbance. A slow, steady drop over 12 to 24 hours suggests a large, non-severe low-pressure system is approaching. Conversely, a rapid drop of several millibars in a few hours signals the imminent arrival of a strong storm or dynamic front.