Earth's atmosphere is composed of a large array of gases. Some of these gases are much more abundant than others. In fact, in the absence of water vapor just two gases make up about 99% of the air by volume. Nitrogen takes up about 78% of the air by volume and Oxygen takes up about 21% of the air by volume. Argon takes up about 1% by volume. The most significant variable gas in the atmosphere is water vapor. It generally ranges from a trace to around 4% depending on how much moisture is in the air. Even when the air is extremely humid, water vapor will take up no more than about 4% of the air by volume. All other gases in the atmosphere occupy a percent by volume of well less than 1%. Some of the most significant trace gases include carbon dioxide, the noble gases, ozone, and hydrogen.

Density is the mass divided by the volume. To find the density of air the mass of a sample of air is measured and compared to the volume it occupies. A problem with giving a value for the density of air is that there is no set value. The density of air will change with height and with a change in the weather. The density of air is going to depend on the air pressure, the temperature of the air and how much moisture is in the air. Meteorologists assign a standard temperature and pressure for air. A common value you will run across is the average density of air at sea level using the standard atmosphere. The standard atmosphere has a sea level pressure of 1013.25 millibars and a temperature of 15 C.

At standard temperature and pressure the density of air is 1.275 kilograms per meter cubed. This density as you would expect is much less than liquids or solids. A meter cubed of water weights about 1000 kilograms while a meter cubed of dirt weights about 2 to 3 times as much as water.