What is meant by the density of air?

• A. The weight of a given volume of air.
• B. The mass of air in a given volume.
• C. The speed of airflow over a wing.
• D. The pressure of air at sea level.

Answer: (A)

Density tells you how much matter is in a given volume. For air, this is usually described as the weight of the air occupying that volume, because that weight directly relates to the forces we deal with in flight and how the air behaves around wings and engines. In physics terms, density is mass per unit volume, and weight per unit volume (weight density) is that same quantity scaled by gravity. Since gravity on the Earth’s surface is roughly constant, using weight per volume is a practical, common aviation convention. That’s why describing density as the weight of a given volume of air best captures what density conveys for aeronautical purposes. The other options describe speed of flow or pressure, which are different properties.