Air pressure is the force exerted by the column of air resting above a specific point on the Earth’s surface, caused by the cumulative weight of all air molecules pressing down due to gravity. As altitude increases, the total mass of the air column above decreases, which directly results in a decrease in air pressure. This inverse relationship means that the higher you travel, the less air is pushing down on you.
The Inverse Relationship Between Altitude and Pressure
The magnitude of this pressure change is quite significant, particularly in the lower atmosphere. At sea level, the average atmospheric pressure is approximately 101.3 kilopascals (kPa), often referred to as one atmosphere. For comparison, the pressure at the summit of Mount Everest, at nearly 8,848 meters, is only about 33.7 kPa, which is roughly one-third of the sea-level value.
This relationship is not linear, but exponential, meaning the pressure drops most rapidly close to the surface. Air pressure decreases by approximately 50% for every 5,500 meters (18,000 feet) of elevation gain. Standard atmospheric measurements are often expressed in units like hectopascals (hPa) or millibars (mbar).
The Physics of Why Pressure Drops
The primary reason for the pressure drop is the lessening weight of the overlying air column. At the ocean surface, we are beneath the entire depth of the atmosphere, experiencing maximum atmospheric weight. When a person ascends, they leave a substantial portion of the atmosphere’s mass below them, immediately reducing the downward force.
Another physical factor is the change in air density. Gravity attracts air molecules toward the Earth’s surface, and the weight of the atmosphere compresses the air closest to the ground. This compression makes the air at sea level denser, packing more molecules into the same volume.
As altitude increases, the reduced weight from above means the air is less compressed and becomes significantly less dense. This lower density means there are fewer air molecules per breath, which is the direct cause of the pressure drop. Air pressure and air density decline almost identically as one moves higher.
Consequences for Human Respiration
The drop in total air pressure affects human physiology, especially breathing. While air contains about 21% oxygen at all altitudes, the reduction in total pressure means the partial pressure of oxygen is much lower. This partial pressure drives oxygen across the membranes in the lungs into the bloodstream.
At an altitude of 3,000 meters, which is common for many ski resorts, the partial pressure of oxygen is already only about 70% of the sea-level value. This decrease makes it harder for the body to transfer sufficient oxygen to tissues, leading to a condition called hypoxemia. Unacclimatized individuals rapidly ascending above 2,500 meters often develop Acute Mountain Sickness (AMS).
Symptoms of AMS include headache, nausea, fatigue, and dizziness, often setting in within hours of arrival. To cope with reduced oxygen availability, the body increases the rate and depth of breathing, a process called the hypoxic ventilatory response. For extreme elevations, supplemental oxygen is required because the partial pressure is too low to sustain life.
Effects on Everyday Physics
The decrease in air pressure alters the physical properties of matter, most notably the boiling point of water. Water boils when its internal vapor pressure equals the surrounding atmospheric pressure. Since atmospheric pressure is lower at high altitudes, the water’s vapor pressure does not need to be as high to escape, resulting in a lower boiling temperature.
At sea level, water boils at 100°C (212°F), but for every 152-meter (500-foot) increase in elevation, the boiling point drops by nearly 0.5°C (1°F). For example, at 2,300 meters (about 7,500 feet), water boils at approximately 92°C (198°F). This lower temperature means food cooked by boiling or simmering requires significantly longer cooking times.
In aviation, the pressure difference necessitates pressurized cabins in commercial aircraft to protect passengers from the low external pressure. Sealed containers and bags, such as chip bags, often noticeably swell when transported to high altitudes. The air sealed inside maintains the higher starting pressure, and the reduced external pressure allows the container walls to bulge outward.