Atmospheric pressure is the force exerted by the weight of the air above a specific point. This force is typically measured in units like Pascals or millibars. At sea level, the entire column of air from the ground to the edge of space presses down, resulting in the highest pressure.
The Physics Behind Decreasing Pressure
The fundamental reason for the pressure drop is gravity, which pulls air molecules toward the Earth’s surface. This concentrates the vast majority of atmospheric mass at lower altitudes, making the air significantly denser at sea level.
As you ascend, the column of air above you becomes progressively shorter and lighter. With fewer air molecules pressing down, the pressure naturally decreases. At higher elevations, molecules are more spread out, leading to lower air density. This reduced density explains why the air is “thinner” the higher you climb.
How Scientists Model the Rate of Change
The rate at which pressure decreases is not linear but follows an exponential pattern. Air is compressible, meaning the air at the bottom is compressed by the weight above it, making lower layers denser. Consequently, the greatest pressure decrease occurs closer to the Earth’s surface.
To quantify this relationship, scientists rely on models like the International Standard Atmosphere (ISA). The ISA provides a standardized distribution of atmospheric properties and establishes a baseline sea-level pressure of 101,325 Pascals (1013.25 millibars).
Using the ISA model, pressure drops by half at an altitude of approximately 5.5 kilometers (18,000 feet). Pilots and meteorologists use this exponential model to convert pressure measurements into “pressure altitude,” which standardizes flight planning and weather forecasting.
Impacts on Everyday Life and Health
The reduction in atmospheric pressure affects daily life and human physiology. A common phenomenon is the change in the boiling point of water, which is directly tied to external pressure. Water boils when its vapor pressure equals the surrounding atmospheric pressure.
Since pressure is lower at high altitudes, less heat is required. For example, at sea level, water boils at 100°C (212°F). In Denver (1,600 meters), water boils closer to 95°C (203°F). This lower boiling temperature means food takes longer to cook, requiring recipe adjustments.
From a health perspective, the decrease in total pressure also means a decrease in the partial pressure of oxygen. While the percentage of oxygen remains constant at about 21%, the total number of available oxygen molecules drops significantly. This reduced availability can lead to hypoxia, or altitude sickness, affecting people who ascend too quickly above 2,500 meters (8,200 feet).
Technology utilizes this pressure-altitude relationship. An aircraft altimeter is an aneroid barometer calibrated to display elevation based on surrounding air pressure. Weather barometers measure pressure changes to predict weather, as dropping pressure often signals an approaching storm.