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Elementary Explanation of Air Density

Air density, by definition, is the total mass of air in a unit volume of atmosphere. It has great importance in the field of aeronautics and other disciplines of science, including marine science and oceanography. There are two main factors that determine the density of air: temperature and barometric pressure. Normally, air density is taken as 100 percent in a temperature and pressure condition of 85 degrees F and 29.2 inches of barometric pressure.

  1. Altitude

    • Air density is less at higher altitudes and more at lower altitudes. High barometric pressure draws air molecules close together, resulting in more mass per unit volume of air. Therefore, with an increase in pressure, air density increases as well. This is the reason mountaineers are recommended to take some additional oxygen cylinders with them while climbing mountains. As air density decreases with altitude, the concentration of oxygen also starts dropping at higher altitudes.

    Temperature

    • Higher temperature makes the molecules of air move at a much faster rate than normal, which results in the separation of molecules from each other. This is why a higher temperature forces the air density to decrease quickly and a lower temperature causes air density to increase.

    Humidity

    • Although the effect of humidity on air density is not as much as that of temperature and pressure, it does affect air density. Air density decreases with an increase in the moisture content of the air. Regardless of the types of molecules present in the air, the total number of molecules is always constant; therefore, an increase in humidity causes water molecules (H20) to increase at the expense of oxygen and nitrogen. But the molecular weight of oxygen and nitrogen is greater than the molecular weight of water, and this is the reason why an increase in humidity forces the air density to decrease.

    Ideal Gas Law

    • Ideal gas law is used to measure the density of dry gas. Ideal gas law is based on the hypothetical ideal gas and has its own equation of state. Although the ideal gas law is very useful in correctly determining the air density in many different conditions, it has some limitations as well. Ideal gas law implies that three major factors are largely responsible for changes in air density: temperature, pressure, and humidity.

    Significance

    • Air density plays an important role in aeronautics and space science. For example, it is crucial to measure air density exactly before launching a rocket into outer space. A wrong calculation can even lead to a wrong trajectory or an explosion. Air density is also important for airplanes. Even automobile and motor bike racers take account of air density before starting their races.

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