Atmospheric pressure is the force exerted by the weight of the air column above a specific point on the Earth’s surface. Air molecules collectively exert significant pressure due to gravity pulling the mass of the atmosphere downward. This pressure constantly changes as the atmosphere moves and is a foundational factor in weather patterns. The instrument used to measure this force is called a barometer.
Understanding Barometric Units and Measurement
Barometric pressure is measured using several units. The two most common units used globally are inches of mercury (inHg) and hectopascals (hPa). The unit “inches of mercury” originated from the earliest barometers, which used a column of mercury to physically balance the weight of the atmosphere.
The height of the mercury column, measured in inches, directly indicated the atmospheric pressure. In the metric system, the standard unit of pressure is the Pascal (Pa), but in meteorology, the hectopascal (hPa) is preferred. A hectopascal is equivalent to 100 Pascals.
The millibar (mb) is an older metric unit that is numerically identical to the hectopascal (1 mb = 1 hPa). Meteorologists often use hPa or mb for weather maps, while in the United States, inHg is still frequently used for public weather reports. All these units measure the same physical phenomenon: the weight of the air above the point of measurement.
What Defines a “Normal” Sea-Level Reading
The concept of a “normal” or standard barometric pressure is defined under specific, controlled conditions. This standard atmospheric pressure is set at sea level, where the average temperature is 15°C (59°F). This value serves as a baseline for comparing readings taken around the world.
The internationally accepted standard atmospheric pressure is 1,013.25 hPa (or millibars). This figure translates to approximately 29.92 inHg. It represents the average pressure experienced at sea level, though actual pressure rarely matches this value due to dynamic weather conditions.
Why Readings Fluctuate: Altitude and Temperature Effects
Barometric readings fluctuate constantly, primarily due to altitude and temperature. Barometric pressure decreases predictably as altitude increases because there is less air mass remaining above the point of measurement. For example, at an altitude of about 5.6 km (18,000 ft), the pressure drops by roughly 50% compared to sea level.
To make pressure readings from different elevations comparable for weather forecasting, meteorologists apply a correction called “pressure reduction to sea level.” This calculation estimates what the pressure would be if the barometer were located at sea level, removing the static effect of elevation. Temperature also affects air density, which influences pressure. Warm air is less dense than cool air, which can cause the same pressure level to be found at a higher altitude in warm air than in cold air.
Interpreting Changes: Barometric Pressure and Weather
The most practical application of a barometer is predicting short-term weather changes. A high barometric pressure reading (typically above 1,013.25 hPa or 29.92 inHg) is associated with sinking air. This sinking motion compresses the air, inhibits cloud formation, and generally leads to fair, clear skies and calm weather.
Conversely, a low barometric pressure reading (below the standard value) is linked to rising air. As air rises, it expands and cools, allowing moisture to condense and form clouds, which often results in precipitation, wind, and stormy weather. The absolute value of the pressure is less important than the rate and direction of its change.
A rapidly falling pressure suggests that a strong weather system, such as a storm, is approaching. A slow, steady drop might only indicate a gradual shift toward general rain or cloudy conditions. When pressure is rising or remaining steady, it is a sign that the current weather conditions are likely to continue or improve.