Definition
The ratio of the actual air density at a given altitude and condition to the standard air density at sea level (0.002377 slugs per cubic foot at 59°F / 15°C). It is a dimensionless number, normally written as the Greek letter sigma (σ).
Plain English
A simple number that tells you how thick the air is right now compared to how thick it would be on a standard day at sea level. A value of 1.0 means the air is exactly standard; less than 1.0 means thinner air; more than 1.0 means denser air.
Context Anchor
Seen in aircraft performance, airframe, engine, propeller, and high-altitude discussions where air density affects how the aircraft behaves.
Derivation
From Latin densitas (thickness, compactness) and ratio (a calculated relation between two values). The word literally points to a comparison of how thick one mass of air is against a fixed reference.
Why Pilots Care
Lower values reduce lift, engine power, and propeller efficiency, directly affecting required runway length and climb performance.
Analogy
Think of sea-level air as a full basket of air molecules. If the basket is only three-quarters as full, the density ratio is about 0.75.
Grounding Statement
As altitude goes up or temperature rises, the air usually becomes less dense, so the density ratio goes down.
Intuition Check
Density ratio is not a percentage of fuel, weight, or aircraft loading. It is a comparison of air density to standard sea-level air density.
Example Sentence 1
On a hot day at a high-elevation airport, the density ratio drops well below 1.0, which is why takeoff distances are noticeably longer.
Example Sentence 2
As altitude increased, the falling density ratio reduced available horsepower and lengthened the climb.