full text of the classic FAA guide
Air is a mixture of several gases. When completely dry, it is about 78% nitrogen and 21% oxygen. The remaining 1% is other gases such as Argon, Carbon Dioxide, Neon, Helium, and others. Figure 1 graphs these proportions. However, in nature, air is never completely dry. It always contains some water vapor in amounts varying from almost zero to about 5% by volume. As water vapor content increases, the other gases decrease proportionately.
FIGURE 1. Composition of a dry atmosphere. Nitrogen comprises about 78%; oxygen, about 21%; and other gases, about 1%. When water vapor is added, the percentages decrease proportionately. Water vapor varies from almost none to about 5% by volume.