Summary
Highlights
We live on the solid surface of Earth but also at the bottom of a giant ocean of gas called the atmosphere. This atmospheric ocean consists of a mixture of gases, water vapor, and dust particles. Nitrogen makes up over 78% of the atmosphere, followed by oxygen at 21%, and argon at just under 1%. These three gases constitute over 99.9% of the atmosphere, with trace gases like carbon dioxide, hydrogen, methane, and ozone making up the rest. Water vapor content is highly variable, ranging from near zero to 4% depending on location and time.
Atmospheric gases have mass and are pulled towards Earth by gravity, creating atmospheric pressure. At sea level, this pressure is about 10 Newtons per square centimeter, or over 14.5 pounds per square inch. Atmospheric pressure varies with elevation, temperature, and the composition of gases above. Warm, wet air is lighter than cool, dry air, which is why low pressure often signals stormy weather. The atmosphere is also thickest near the equator and thins towards the poles due to Earth's rotation.
The atmosphere has a layered structure, with the lowest and densest layer being the troposphere, where weather occurs. Its height ranges from 8 kilometers at the poles to 16 kilometers at the equator. Gas density and atmospheric pressure decrease rapidly with altitude. Half of all atmospheric gas is within the lowest 5.5 kilometers, 75% within 11 kilometers, and over 90% within the first 20 kilometers. Even in the upper atmosphere, sparse gases are gravitationally locked to Earth.
The upper atmosphere filters out damaging high-energy electromagnetic radiation from the sun, making Earth's surface habitable. Energy from the sun that reaches the ground is absorbed and re-radiated, warming the lower atmosphere and creating a climate conducive to life. This energy flow drives winds and is responsible for global weather patterns. The high concentration of oxygen and low concentration of carbon dioxide in Earth's atmosphere is unusual among solar system planets and is a key indicator of the presence of life, as oxygen is highly reactive and requires continuous production by living organisms, primarily through photosynthesis.