What is the molecular weight of air typically taken to be for density calculations?

In atmospheric science and related fields, the molecular weight of air is commonly accepted to be approximately 28.7 kg/kmol for density calculations. This value is derived from the average composition of air, which consists mainly of nitrogen (about 78% with a molecular weight of 28 kg/kmol) and oxygen (about 21% with a molecular weight of 32 kg/kmol), along with trace amounts of other gases. The molecular weights of these gases contribute to the overall average molecular weight of air, resulting in the figure of 28.7 kg/kmol.

Using this value allows for consistent calculations when determining the density of air under various conditions, particularly in engineering applications such as evaluating airflow and pollutant dispersion. The precision of the value makes it useful for a wide range of atmospheric and environmental analysis without adding unnecessary complexity.