What is meant by air density?

• A. The weight of air in a given volume.
• B. The volume of air per unit mass.
• C. The mass of air in a given volume.
• D. The height of the air column.

Answer: (C)

Air density is the mass of air contained in a given volume. In other words, it’s how much matter is packed into a specific space, described by rho = m/V. This is distinct from the air’s weight in that volume; weight density would be weight per volume (mass times gravity), which is a related but separate measure.

Conditions like temperature, pressure, and humidity determine density. Warmer air expands, so it becomes less dense; higher pressure compresses it, making it denser; and humidity lowers density because water vapor has a lower average molecular weight than the gases that compose dry air.

For context, at sea level under standard conditions, air density is about 1.225 kg/m^3. Density decreases with altitude, which is why air becomes thinner and affects engine performance and aerodynamics.