Air is something that everyone is familiar with. It is one of the basic necessity that supports life on earth. So let us learn more about it. Let us find out what is the density of air and how to calculate the density of air.

**Air**

Air is an envelope that surrounds us. It is a mixture of many gases that make the atmosphere of the earth. In general, this mixture of air primarily contains nitrogen, oxygen, carbon dioxide, water vapor, ozone and inert gases such as argon, helium, krypton, xenon etc. It also contains dust, pollen, and spores.

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**Density of air**

The density of air refers to the mass per unit volume of atmospheric gases. The Greek letter ρ (rho) is used to denote it. The density of air depends upon the atmospheric temperature, pressure and humidity. Usually, the units in which the density of air is measured is the Standard Temperature and Pressure (STP).

Furthermore, STP is one atmosphere of pressure at 0 degree Celsius. Also, at sea level, this is a freezing temperature. And so, dry air is less dense than the cited value most of the time. Furthermore, most of the air contains a lot of water vapor which makes it denser than cited value.

**Value of density of air**

Dry air has a density of 1.29 gram per litre at 0 degree Celsius (32 degrees 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/m
^{3}) at 15 degree 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/m^{3}), 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 degrees C and 100 kPa).
- Furthermore, the density of dry air at 20-degree celsius and 101.325 kPa is 1.2041 kg/cubic meter.
- Also, at 70 degree F (Fahrenheit) and 14.696 psi (per square inch), the density of dry air is about 0.74887 lbm/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.

**Calculation of density of air**

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).

**Solved question on density of air**

**Question.** Which of the following instrument is used to measure the density of air?

**A.** Densimeter

**B.** Anemometer

**C.** Barometer

**D.** Bolometer

**Answer.** The correct answer is option A. Besides, we use an anemometer to measure wind speed, Barometer to measure air pressure, and bolometer to measure electromagnetic radiation.

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Low resistance to what type of load, impact or tensile load. With change in low type, property of steel may very. And, also, Will stainless steel have high yield strength?