Is the weight of a given volume of gas, usually expressed in lb/cu ft or kg/m3 at STP (Standard Temperature and Pressure) conditions.
Dry air has a density of 1.29 gram per litre at 0° Celsius (32° Fahrenheit) at average sea level. Also, the density of air is not constant. Its value varies at different levels. For instance,
- The density of air is about 1.225 kilogram per cubic meter (kg/m3) at 15° Celsius and at sea level. Also, this is the value according to the International Standard Atmosphere (ISA). If we talk about other units then it is 1225.0 gram per cubic meter (g/m3), 0.0765 lb./ (cu ft.), or 0.0023769 slug/ (cu ft.)
- Additionally, the IUPAC standard of temperature and pressure uses dry air density of 1.2754 kg/cubic meter (at 0° C and 100 kPa).
- Furthermore, the density of dry air at 20° Celsius and 101.325 kPa is 1.2041 kg/cubic meter.
- Also, at 70° F (Fahrenheit) and 14.696 psi (per square inch), the density of dry air is about 0.74887 lbs/ft.
Factors that affect the density of air
- At higher altitudes, the density of air is less. For example, the density of air is less in hilly areas than in tropical area. In other words, as the altitude increases the density decreases.
- At higher temperatures, the density of air is less. In other words, the density decreases as the temperature increases as the volume of gas changes with it. Therefore, the air would be denser on a cold day of winter rather than a hot day of summer, provided that the other factors remain the same. Another example of this could be rising of a hot air balloon.
The easiest way to calculate the density of dry air is by applying the ideal gas law. The ideal gas law expresses density as a function of temperature and pressure. Also, just like all gas laws, it is an estimate. But is very good at ordinary temperatures and pressures. Besides, higher pressure and temperature adds more errors in the calculation.
The equation is: ρ = p / RT
Here:
- ρ is the density of air in kg/cubic meter
- p is the absolute pressure in Pa
- T is the absolute temperature in K
- R is the specific gas constant for dry air in J/ (kg. K)
The specific gas constant for dry air is 287.058 J/ (kg. K).