Atmospheric pressure is the weight of the air above a specific point on Earth. This pressure is not static; it decreases significantly as altitude increases, following a non-linear pattern. The atmosphere is a compressible fluid, meaning air molecules are packed more tightly at the surface than at higher elevations. This compression causes the pressure to drop exponentially, not linearly, as one ascends from sea level.
Why Pressure Drops as Altitude Increases
The primary physical mechanism driving the drop in pressure is the decreasing weight of the air column above the observer. Air has mass, and gravity pulls this mass toward the Earth’s surface, creating the force we measure as atmospheric pressure. At sea level, a person has the entire column of the atmosphere pressing down on them.
As an individual moves higher, they leave a significant portion of the atmosphere’s mass below them. The column of air remaining above is shorter and less dense, resulting in a lower total weight pushing down. This is analogous to how the pressure at the bottom of a stack of bricks is greater than the pressure on a brick near the top.
The atmosphere is a compressible gas, which significantly affects how rapidly pressure decreases. The weight of the air above compresses air molecules closer together near the surface, making the air denser at lower altitudes. As altitude increases, the overlying weight decreases, allowing the air to expand and become less dense. This density gradient causes air pressure to drop faster near the surface, and the rate of pressure change slows down at higher elevations.
Quantifying the Decrease
Scientists and engineers use standardized models, such as the International Standard Atmosphere (ISA), to predict how pressure changes with altitude. The ISA model provides a common reference point, defining the average sea-level pressure as 1,013.25 hectopascals (hPa) or 29.92 inches of mercury (inHg). This standardization is useful for aviation and other industries.
The decrease in pressure is most dramatic close to the Earth’s surface. Near sea level, the pressure drops by roughly one inch of mercury for every 1,000 feet of ascent. This quick initial drop highlights the non-linear, or exponential, nature of the pressure-altitude relationship.
The pressure at 18,000 feet (about 5,500 meters) is roughly half the pressure at sea level. This means that a person is already above approximately 50% of the Earth’s atmosphere at this moderate altitude. Beyond this point, the pressure continues to decrease, but the rate of change per foot of ascent becomes progressively smaller.
How Low Pressure Affects the Human Body
The human body is adapted to sea level pressure, and a rapid decrease in external pressure can cause immediate physiological effects. One noticeable impact is the expansion of gases trapped within enclosed body spaces. As external pressure drops, the air inside the middle ear and sinuses expands, causing the familiar sensation of “ear popping” or sinus pain.
A more serious consequence of low pressure is the reduced availability of oxygen, known as hypoxia. While the percentage of oxygen in the air remains the same (about 21%), the lower atmospheric pressure means the partial pressure of oxygen is also lower. This makes it harder for the lungs to transfer oxygen into the bloodstream, leading to symptoms like dizziness, fatigue, and impaired judgment.
This low partial pressure of oxygen is the main cause of Acute Mountain Sickness (AMS), which can develop after a rapid ascent to high altitudes. In extreme, rapid depressurization scenarios, decompression sickness (the “bends”) can occur. This happens when dissolved gases, primarily nitrogen, form bubbles in the body’s tissues and blood, potentially causing joint pain, paralysis, or collapse.