Definition
The principle that, when temperature is held constant, the density of a gas is directly proportional to its pressure. Increasing the pressure on a fixed volume of air packs more air molecules into that space, raising its density; decreasing the pressure allows the molecules to spread out, lowering its density.
Plain English
Push harder on air and you squeeze more of it into the same space, so it gets denser. Ease off the pressure and the air spreads out, so it gets less dense. This only holds true if the temperature stays the same.
Context Anchor
Seen in aircraft performance discussions, especially when comparing airport elevation, weather conditions, takeoff distance, and climb performance.
Derivation
Pressure comes from a Latin word meaning “to press.” Density comes from a Latin word meaning “thick” or “closely packed.” Together, the phrase points to how strongly air is being pressed and how closely the air is packed in a given space.
Why Pilots Care
Changes in pressure alter air density and therefore directly affect lift, engine power, propeller efficiency, and required takeoff distance.
Analogy
Think of air like people in a room. Higher pressure is like more people being packed into the same room; lower pressure is like fewer people in that room.
Grounding Statement
Think of a bicycle pump: as you compress the air inside, the same amount of air is forced into a smaller space, so the air inside the pump becomes denser.
Intuition Check
Do not read this as pressure being the only thing that controls density. In this context, it means: if temperature is held the same, higher pressure makes air denser and lower pressure makes air less dense.
Example Sentence 1
Because pressure decreases with altitude, density also decreases, which is why the airplane needs a longer takeoff roll on a high-elevation airport.
Example Sentence 2
The pilot checked the effect of pressure on density before departure because the altimeter setting indicated denser air than forecast.