Changes in weather are reliably preceded by measurable atmospheric shifts. Understanding how the atmosphere behaves offers a powerful tool for basic weather prediction. The movement of air masses is directly tied to the force exerted by the air above us. By observing changes in this force, particularly a drop in pressure, a person can anticipate the approach of a storm system long before the first rain begins to fall.
What Is Atmospheric Pressure?
Atmospheric pressure, also known as barometric pressure, is the force exerted on a surface by the weight of the air column extending up to the top of the atmosphere. Air, like all matter, has mass, and gravity pulls this mass toward the Earth’s surface, creating pressure. We do not notice this force, but it is substantial, averaging about 14.7 pounds per square inch at sea level.
This atmospheric force is measured using a device called a barometer. Meteorologists express pressure readings in several units, most commonly in millibars (mb) or hectopascals (hPa), and sometimes in inches of mercury (inHg). Standard sea-level pressure on a calm day is around 1013.25 millibars or 29.92 inches of mercury. A rising barometer indicates increasing pressure, while a falling barometer signals decreasing pressure.
The Pressure Drop: How Low-Pressure Systems Form Storms
The approach of a storm is characterized by a distinct and measurable drop in atmospheric pressure. This drop signals the formation of a low-pressure system, which is the engine for inclement weather. In a low-pressure area, the air is less dense and lighter than the surrounding atmosphere. This difference causes the less dense air to rise rapidly off the surface.
As air flows toward the low-pressure center, it converges and is forced upward into the atmosphere. This upward movement is what creates clouds and precipitation. The rising air expands and cools, which causes the water vapor it contains to condense into liquid droplets, forming clouds. Continued rising and condensation lead to the development of rain, snow, or thunderstorms.
The rate and magnitude of the pressure drop are linked to the storm’s potential intensity. A rapid fall in barometric pressure predicts a more severe weather event. For instance, a “bomb cyclone” is defined by a pressure drop of at least 24 millibars within 24 hours. In powerful storms like hurricanes, extremely low central pressure correlates directly with higher wind speeds and storm strength.
Tracking the Weather Cycle: Pressure After the Storm
Once a storm passes, the weather cycle completes with the return of an area of high pressure. This shift is signaled by a steady or rapid rise in the barometric pressure reading. A high-pressure system is the opposite of a low-pressure system, meaning the air in the column is heavier and denser.
The heavier, denser air in a high-pressure system is forced downward toward the surface, a process called divergence. This sinking air warms as it descends, increasing its capacity to hold moisture. Because the air is warming and drying, it inhibits the formation of clouds and precipitation. The result is clear skies, lighter winds, and calmer, fair weather.
A home barometer offers practical insight into these weather cycles. Steady pressure indicates stable conditions, while a continuous drop forecasts the approach of a storm. Conversely, a rapid rise confirms the low-pressure system has moved on and fair weather is returning.