Atmospheric pressure represents the weight of the column of air resting above a specific location on the Earth’s surface. This pressure constantly fluctuates based on local weather conditions and the movement of air masses. The instrument used to measure these changes is the barometer, which typically reports readings in units like inches of mercury (inHg) or millibars (mb).
The Specific Pressure Change Indicating Storms
The most reliable indicator of impending stormy weather is a rapidly falling atmospheric pressure. It is not the absolute value of the pressure reading that signals a storm, but rather the speed at which the pressure drops. This rapid decrease signifies that a low-pressure system, or cyclone, is quickly approaching the area.
For practical forecasting, a pressure drop exceeding 1 millibar per hour is considered a rapid fall. In the US system, a drop of 0.06 inches of mercury over a three-hour period can also be a significant warning sign. The faster and greater the drop in pressure, the more intense the approaching weather system is likely to be, potentially bringing high winds, heavy rain, or severe storms.
A slow, gradual fall in pressure, such as a few millibars over a 12-hour period, indicates the approach of a weaker system, perhaps just general cloudiness and light rain. A sustained and large decrease signals a long period of poor weather, while a sudden, sharp decrease suggests a more localized and immediate disturbance.
Why Lower Pressure Leads to Unstable Weather
A drop in atmospheric pressure results from a reduction in the mass of air above a location, which is characteristic of a low-pressure system. Within these systems, the air near the surface is less dense than the surrounding air and begins to rise. This upward movement of air is a process called atmospheric lift, which is the primary mechanism for generating clouds and precipitation.
As the air rises, it expands and cools down, which reduces its capacity to hold water vapor. This cooling causes the moisture within the air to condense, forming water droplets and ice crystals that aggregate into clouds. This leads to increased atmospheric instability and the eventual release of precipitation. The lower the pressure at the center of the system, the stronger the upward motion and the resulting storm.
Interpreting Steady and Rising Pressure Trends
In contrast to a falling barometer, a steady or rising pressure trend indicates stable atmospheric conditions. When pressure is high, it is associated with a high-pressure system, or anticyclone, where air is sinking. This descending air warms up and becomes drier, which actively suppresses the formation of clouds and precipitation.
A sustained high-pressure reading forecasts fair weather, clear skies, and calm winds. When the pressure is actively rising, it signals improving conditions, as the high-pressure system is building or moving into the area. If pressure remains steady, the current weather conditions are likely to persist, whether that means continued clear skies or ongoing light precipitation.