Definition
The ratio of the actual density of the air at a given altitude and temperature to the density of air under standard sea-level conditions (1.225 kg/m³, or 0.002377 slugs/ft³). It is expressed as a decimal number, with standard sea-level air having a density ratio of 1.0. The symbol is the Greek letter sigma (σ).
Plain English
A number that compares how thick the air is right now to how thick it would be on a standard day at sea level. A value of 1.0 means the air matches standard sea-level conditions; a smaller number means the air is thinner.
Context Anchor
Seen in aircraft performance calculations, especially when comparing engine power, propeller efficiency, lift, and takeoff or climb performance under different altitude and temperature conditions.
Derivation
From Latin densus, meaning 'thick' or 'crowded,' and ratio, meaning 'a comparison or reckoning.' So a density ratio is literally a comparison of how thick one air sample is against another — in this case, against a fixed standard.
Why Pilots Care
A lower density ratio reduces engine power, propeller thrust, and wing lift, lengthening takeoff rolls and reducing climb performance.
Analogy
Think of standard sea-level air as a full measuring cup. A density ratio of 0.80 means the same-sized cup has only about 80 percent as much air in it.
Grounding Statement
On a hot day or at a high airport, the air is thinner, so the density ratio is lower than it would be at sea level on a standard day.
Intuition Check
Density ratio is not the same thing as density altitude. Density ratio is a comparison number; density altitude is an altitude value that describes how the airplane performs in the existing air.
Example Sentence 1
On a hot summer afternoon at a mountain airport, the density ratio is well below 1.0, which is why the takeoff roll is noticeably longer.
Example Sentence 2
At a density ratio of 0.8 the aircraft required more runway than the performance charts predicted at sea level.