[Ref. Wiki, Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 4-4]
3H , low(er) air density –
Air density changes with variation in actual height (altitude), temperature and humidity. At 1013.25 hPa and 15°C, air has a density of approximately 1.225 kg/m³ according to ISA (International Standard Atmosphere).
Effect of Temperature on Density
Temperature ↑ , Air density ↓ – Air density varies inversely with temperature. This statement is true only at a constant pressure.
Effect of Altitude on Density
Altitude↑ , Air density ↓ – Both temperature and pressure decrease with altitude and have conflicting effects upon density. However, a fairly rapid drop in pressure as altitude increases usually has a dominating effect. Hence, pilots can expect the density to decrease with altitude.
Effect of Humidity (Moisture) on Density
Humidity ↑ , Air density ↓ – Water vapor is lighter than air; consequently, moist air is lighter than dry air. Therefore, as the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance.
Effect of Pressure on Density
Pressure x2 ↑ , Air density x2 ↑ (if temperature constant)
Pressure x2 ↑ , Air density x2 ↑ (if temperature constant) – If the pressure is doubled, the density is doubled; if the pressure is lowered, the density is lowered. This statement is true only at a constant temperature.
When the air density is half, the IAS is half as well.