Atmospheric pressure is the force exerted by the blanket of air surrounding the Earth, pushing down on every surface. This pressure is the result of countless air molecules constantly moving and colliding with objects and surfaces. The observed phenomenon is that this pressure consistently decreases as one moves higher in altitude, a change that can be felt in the ears during airplane travel or mountain ascents.
Atmospheric Pressure Explained as Weight
The fundamental reason atmospheric pressure decreases with altitude is directly related to the mass of the air above the point of measurement. Atmospheric pressure at any level is essentially the total weight of the entire column of air stretching from that level all the way up to the vacuum of space. The force responsible for giving the atmosphere this weight is gravity, which pulls the air molecules toward the planet’s surface.
When a measurement is taken at sea level, the instrument is supporting the weight of the complete atmospheric column above that point. This considerable downward force creates the standard atmospheric pressure of 101,325 Pascals, or one atmosphere.
As an individual ascends, they are physically moving above a portion of the atmosphere. Consequently, the length and mass of the air column remaining above them are reduced. Since pressure is generated by the weight of the air mass, having less air above means less weight pushing down, which results in a measurable decrease in atmospheric pressure. The higher the altitude, the smaller the remaining overhead air column becomes.
The Role of Air Density and Compression
The decrease in pressure is also heavily influenced by how tightly the air molecules are packed together, a property known as density. Air is a gas and is therefore highly compressible, meaning its volume can be reduced by applying external force. The massive weight of the overlying atmosphere presses down on the air at lower altitudes.
This continuous downward push forces the air molecules near the surface closer together, which significantly increases the density of the air at sea level. This higher density ensures a greater number of molecular collisions per unit of time, which is the physical mechanism by which pressure is exerted on a surface.
Conversely, as altitude increases, the weight of the air column pressing down becomes progressively smaller. This reduction in compressive force allows the air molecules to spread out into a larger volume. The resulting lower density means there are fewer air molecules in a given space, leading to a corresponding drop in pressure. This relationship between the weight of the air column and the compressibility of the gas is responsible for the drop in density and pressure with ascent.
The Exponential Rate of Pressure Drop
While the drop in pressure is continuous, the rate at which it decreases is not linear; instead, it follows an exponential curve. This means that the pressure drops much more steeply near the Earth’s surface and slows down considerably at higher altitudes. This non-linear gradient is a direct consequence of the air’s compressibility and the way the atmosphere’s mass is distributed.
A large fraction of the atmosphere’s total mass is concentrated very close to the ground. For instance, roughly half of the entire mass of the Earth’s atmosphere is located below an altitude of approximately 5.5 kilometers (about 18,000 feet). This statistic illustrates how quickly the atmospheric density and pressure fall off initially. Because the air is so compressed at the bottom, one must climb only a relatively short distance to move above 50% of the total air mass, resulting in a rapid drop in pressure.