A barometer measures atmospheric pressure, also known as barometric pressure. This instrument quantifies the force exerted by the air above a given point. Tracking these pressure readings provides insights into impending weather changes, making the barometer a useful tool for short-term weather forecasting.
Understanding Atmospheric Pressure
Atmospheric pressure is the force exerted by the weight of the air molecules above a specific area on Earth’s surface. Think of it as a column of air extending from the ground up to the outer reaches of the atmosphere, pressing down due to gravity. This pressure is not constant; it fluctuates due to various factors including temperature, altitude, and the movement of air masses.
Air pressure generally decreases with increasing altitude because fewer air molecules press down from above. Temperature also influences atmospheric pressure: warmer air is less dense and rises, leading to lower pressure, while cooler air is denser and sinks, resulting in higher pressure. Pressure is measured in units like inches of mercury (inHg) in the United States, and hectopascals (hPa) or millibars (mb) globally. At sea level, standard atmospheric pressure is approximately 29.92 inHg, or 1013.25 hPa/mb.
Daily fluctuations in pressure occur, typically with two highs and two lows within a 24-hour period, influenced by solar heating. Beyond these daily cycles, larger pressure changes are driven by migrating weather systems. These shifts form the basis for weather prediction.
Interpreting Barometric Readings for Weather
Changes in barometric pressure offer clues about upcoming weather conditions. A rising trend indicates increasing atmospheric pressure, often signaling improving weather. This rise is associated with the approach of a high-pressure system, bringing clear skies, fair conditions, and lighter winds. A slow, steady rise suggests good weather will persist or gradually set in.
Conversely, a falling barometer indicates decreasing atmospheric pressure, suggesting worsening weather. This drop is linked to the approach of a low-pressure system, often bringing clouds, precipitation, and stronger winds. A slow fall might suggest impending rain, while a rapid drop often signals a more severe weather event, such as a storm. For instance, pressure below 29.80 inHg (1009.144 mb) is considered low, often associated with rainstorms.
When barometric pressure remains steady, current weather conditions will continue without significant immediate change. This stability can mean continued fair weather if pressure is high, or ongoing unsettled weather if pressure is low. Understanding these trends is fundamental to using a barometer for short-term weather forecasting.