Earth's Protective Layer
What you commonly call air, scientists call the atmosphere. The atmosphere (AT muh sfeer) is the layer of gases that surrounds Earth. 
The atmosphere forms a protective boundary between Earth and space and provides conditions that are suitable for life. The atmosphere protects Earth's surface from continual pounding by meteoroids, chunks of rock and metal from space. The atmosphere also protects Earth from much of the high-energy radiation from space.
The atmosphere holds in heat and helps to moderate Earth's temperatures. Without the atmosphere, Earth's surface would be similar to the moon's, boiling hot during the day and freezing cold at night. Earth's relatively constant temperatures allow life to flourish.
The atmosphere also provides the gases that are essential for life. Carbon dioxide in the atmosphere is essential for plants and some other organisms to carry out photosynthesis. Photosynthesis is the process of capturing the sun's energy to make food. Oxygen is produced during photosynthesis and released into the atmosphere. Your body uses oxygen to carry out many life functions.
Composition of the Atmosphere
The atmosphere is a mixture of different gases. The composition of the atmosphere is fairly uniform up to an altitude of about 80 kilometers. Earth's atmosphere is a mixture of nitrogen, oxygen, water vapor, and many other gases, in which tiny solid and liquid particles are suspended. As Figure 2 shows, two gases—nitrogen and oxygen—make up more than 99 percent of clean, dry air. The amount of water vapor in air varies from 0.02 percent in cold, dry air to more than 4.0 percent in warm, moist air.
Various amounts of water droplets and solid particles are suspended in the atmosphere. Some solid particles can be seen as floating dust, but most particles are microscopic. These solid particles come from various sources, including smoke from fires, ash and dust from volcanic eruptions, and salt from ocean spray.
Figure 2 Dry air is a mixture of many gases. Air also contains water vapor, which makes up between 0.02 percent and 4.0 percent of air's volume. Using Tables What is the most abundant gas in dry air?
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