Mount Everest represents the ultimate terrestrial environment, a place where the physical conditions push the limits of human survival. The mountain’s immense altitude creates an atmosphere so thin that climbing without supplemental oxygen is a feat reserved only for the most elite athletes. A core factor determining this extreme environment is the atmospheric pressure, the force exerted by the air column above. Understanding the precise measurement of this pressure is the first step in appreciating the biological and physical challenges of the “roof of the world.”
The Specific Answer: Air Pressure at the Summit
The air pressure at the 8,848-meter summit of Mount Everest averages approximately 337 millibars (mbar). This figure is derived from scientific expeditions and meteorological data, though the actual pressure is variable depending on weather conditions. For comparison, the standard atmospheric pressure at sea level is about 1013.25 millibars, meaning the air pressure at the peak is roughly one-third of what people experience at the coast.
The most accepted precise measurement from a 1981 scientific expedition recorded a barometric pressure of 253 Torr, which converts directly to 337 millibars. This pressure can fluctuate slightly, ranging from around 330 to 340 millibars, influenced by the presence of high or low-pressure weather systems that temporarily alter the weight of the air above the mountain.
Understanding Atmospheric Pressure and Millibars
Atmospheric pressure is the force exerted on a surface by the weight of the air molecules above it. It is the cumulative weight of the entire column of air from the top of the atmosphere down to the measurement point. This pressure is why our bodies are not crushed, as internal pressure balances the external force.
The millibar (mbar) is the unit of pressure most commonly used in meteorology for measuring atmospheric changes. One millibar is equivalent to one-thousandth of a bar, a larger unit of pressure. One millibar is exactly equal to one hectopascal (hPa), which is the metric unit often found on weather charts.
The Physics of Low Pressure at High Altitude
The dramatic reduction in pressure at Everest’s summit is a direct consequence of physical laws governing the atmosphere. The weight of the air column decreases exponentially as altitude increases. At sea level, a person has the full weight of the atmosphere pressing down, but at the summit, the air column above is significantly shorter.
Gravity is the force responsible for this distribution, pulling the vast majority of air molecules close to the Earth’s surface. This gravitational pull causes the air near sea level to be compressed and highly dense. As elevation rises, the air becomes less dense, because fewer molecules are packed into the same volume.
This decrease in air density means that with every breath taken at high altitude, a person inhales fewer total gas molecules than they would at sea level. The pressure drop is not linear; the bulk of the air mass is contained in the lower levels of the atmosphere. This explains why the pressure falls off rapidly as one climbs, resulting in the extremely thin air at Everest’s elevation.
Physiological Impact: Why Low Pressure Affects Breathing
The low total atmospheric pressure at the summit presents a profound biological challenge, even though the air’s composition remains unchanged. Air everywhere contains approximately 21% oxygen. However, the total atmospheric pressure determines the partial pressure of oxygen (\(P\text{O}_2\)), which is the factor for human respiration.
\(P\text{O}_2\) is calculated by multiplying the total barometric pressure by the oxygen percentage. At sea level, the \(P\text{O}_2\) is high enough to drive oxygen efficiently across the lung membranes into the bloodstream. At Everest’s summit, the reduced total pressure translates to a low ambient \(P\text{O}_2\), which is approximately 53 mmHg.
This low partial pressure creates an insufficient pressure gradient to move oxygen into the blood effectively. The body struggles to diffuse enough oxygen into the arterial blood because the driving force is weak. Measurements taken from climbers at 8,400 meters showed the mean arterial \(P\text{O}_2\) was low, averaging only 24.6 mmHg.
The region above 8,000 meters is known as the “Death Zone” because oxygen saturation levels fall below the threshold of human tolerance. Without supplemental oxygen, the brain and vital organs are starved, leading to exhaustion, impaired judgment, and eventually, death. The low atmospheric pressure is the limiting factor for human survival at this altitude.