An increase in barometric pressure is a reliable, short-term indicator of changing local weather conditions. Observing whether the pressure is steadily rising or falling provides a simplified method for anticipating the immediate forecast. A sustained rise in pressure signals the gradual approach or strengthening of a high-pressure system, which is strongly associated with settled, fair weather conditions.
What Barometric Pressure Measures
Barometric pressure, or atmospheric pressure, measures the force exerted by the weight of the air column above a specific point on Earth’s surface. It reflects the total mass of the atmosphere situated directly over that location. Measured using a barometer, it is commonly reported in millibars (mb) or hectopascals (hPa) for meteorological purposes, though inches of mercury (inHg) is also used. Standard sea-level pressure is 1013.25 millibars, and readings above this value are considered high pressure.
The Role of High Pressure Systems
A sustained increase in barometric pressure signals the arrival or intensification of a high-pressure system, also known as an anticyclone. These systems are characterized by subsidence, where air slowly descends from the upper atmosphere toward the surface. As this air sinks, the increasing pressure compresses the air mass, causing it to warm slightly. This warming dries out the air, actively suppressing the formation of clouds and precipitation.
The descending air prevents the upward motion, or convection, necessary for moisture to cool and condense into towering clouds. This downward force creates a stable atmosphere, linking high-pressure systems to settled weather conditions. The calmest and sunniest conditions are typically found near the center of the system, where the air subsides most efficiently.
The Expected Fair Weather Forecast
When barometric pressure rises steadily, the forecast is for improving, stable conditions. This sustained rise indicates the high-pressure system is establishing itself, resulting in visible, fair weather. The primary expectation is for clear skies or a noticeable clearing of existing clouds, as the subsiding air evaporates moisture. This clearing often becomes more pronounced as the high-pressure ridge dominates the local weather pattern.
Clear skies are typically accompanied by light winds and tranquility, as the pressure difference between areas is relatively weak within a stable high. A rising barometer often signals the end of recent unsettled or stormy weather associated with a departing low-pressure system. The most reliable observation is the absence of precipitation and cloud development. However, in winter, clear nights due to high pressure can lead to significant radiational cooling, resulting in much colder overnight temperatures or the formation of fog in valleys.
Interpreting the Rate of Pressure Change
Knowing the rate at which pressure rises provides a deeper understanding of the weather system’s characteristics and longevity. A slow, steady increase in barometric pressure over many hours suggests a large, stable high-pressure system is approaching. This slow rise indicates that the fair weather is likely to persist for several days, offering a prolonged period of settled conditions associated with strong, broad anticyclones.
Conversely, a rapid, sharp increase in pressure, such as a quick jump of several millibars, signals a more transient weather event. This fast rise is frequently observed immediately following the passage of a strong, fast-moving cold front. While the weather becomes fair immediately, the system is moving quickly, meaning the period of stable weather may be brief before the next weather system arrives.