Meteorologists determine the composition of air by analyzing its various constituents; these are mainly displayed in terms of percent of the atmosphere. The first 40 to 50 miles (64 to 80 kilometers) above the surface contains 99 percent of the total mass of the Earth’s atmosphere. It is generally uniform in composition, except for a high concentration of ozone, known as the ozone layer, at 12 to 30 miles (19 to 50 kilometers) in altitude.

In the lowest part of the atmosphere—the area in which humans, other animals, and plants live—the most common gases are nitrogen (78.09 percent), oxygen (20.95 percent), argon (0.93 percent), carbon dioxide (0.03 percent), and minute traces of such gases as neon, helium, methane, krypton, hydrogen, xenon, and ozone. Water vapor is also present in the lower atmosphere, although variable and at a very low percent. Higher in the atmosphere, the composition and percentages change as the atmosphere thins. (For more about percents, see “Math Basics.”)