Earth’s atmosphere is a mix of gases, with nitrogen making up about 78% by volume and oxygen about 21%. The remaining 1% is a combination of argon, carbon dioxide, and a handful of other trace gases, plus variable amounts of water vapor. Beyond its chemical ingredients, the atmosphere is organized into distinct layers that stretch from the ground to the edge of space.
The Main Gases
Dry air (meaning air with water vapor removed from the calculation) breaks down like this: 78.08% nitrogen, 20.95% oxygen, and 0.93% argon. Those three gases alone account for 99.96% of the atmosphere. The final sliver, roughly 0.04%, is a mix of trace gases including carbon dioxide, methane, nitrous oxide, and ozone.
Nitrogen is by far the dominant gas, but it’s mostly inert. You breathe it in and breathe it right back out. Oxygen is the gas your body actually uses for energy, and it’s also the gas that allows fire to burn. Argon, the third most abundant gas, is a noble gas with no color, smell, or chemical reactivity. It’s essentially just along for the ride.
Trace Gases and Why They Matter
The trace gases make up a tiny fraction of the atmosphere, but they have an outsized effect on climate and life. Carbon dioxide sits at about 424 parts per million (ppm), which is 0.04% of the atmosphere. That sounds negligible, but CO2 traps heat radiating from Earth’s surface, and even small changes in its concentration shift the planet’s energy balance. Unlike water vapor, which is concentrated near the ground, CO2 mixes evenly through the atmosphere up to about 50 kilometers in altitude, giving it a broad warming influence.
Methane is present at roughly 1,942 parts per billion (ppb), more than double its pre-industrial level. It’s a far more potent heat-trapping gas than CO2, molecule for molecule, though it breaks down faster. Nitrous oxide, at about 338 ppb, is 25% above pre-industrial levels and also contributes to warming while playing a role in depleting the ozone layer.
Other trace gases include neon, helium, krypton, and hydrogen, all present in tiny amounts. Ozone is rare overall but concentrated in a specific band of the upper atmosphere where it absorbs ultraviolet radiation from the sun.
Water Vapor: The Wild Card
All the percentages above describe “dry” air because water vapor is the one atmospheric gas that fluctuates wildly. It can range from nearly 0% in cold, arid regions like deserts and polar ice sheets to about 4% in hot, humid tropical air. On a global average, it hovers around 1% of the atmosphere by volume.
Water vapor is also a greenhouse gas, and in fact, it’s the most abundant one. But its concentration is driven by temperature: warmer air holds more moisture. This creates a feedback loop. As other greenhouse gases warm the planet, more water evaporates, which traps more heat, which drives more evaporation. Water vapor amplifies warming that other gases initiate, but it doesn’t stay in the atmosphere long enough to be the primary driver on its own.
The Ozone Layer
Ninety percent of the atmosphere’s ozone sits in the stratosphere, between about 10 and 50 kilometers above the surface. Peak concentration occurs around 32 kilometers up, where ozone reaches roughly 8 molecules per million molecules of air. That’s still vanishingly thin, but it’s enough to absorb most of the sun’s harmful ultraviolet radiation before it reaches the ground. Without this layer, surface life would face dramatically higher rates of DNA damage.
The formation of ozone in the stratosphere actually generates heat, which is why temperatures in that layer rise with altitude rather than falling as they do closer to the ground.
Particles Floating in the Air
The atmosphere isn’t just gases. It also contains particulate matter: a mix of solid particles and liquid droplets suspended in the air. Dust, dirt, soot, smoke, sea salt crystals, pollen, and volcanic ash all contribute. Some particles are large or dark enough to see with the naked eye, while others are microscopic.
These particles affect the atmosphere in several ways. They serve as seeds for cloud formation, since water vapor needs a surface to condense onto. They scatter and absorb sunlight, which can cool or warm the air depending on the particle type. And at ground level, fine particulate matter is a major air quality concern because particles small enough to inhale deeply can affect the lungs and cardiovascular system.
Five Layers From Ground to Space
The atmosphere is organized into layers defined mainly by how temperature changes with altitude. Each layer has different characteristics and plays a different role.
Troposphere
This is the layer you live in. It starts at the surface and extends up to about 6 kilometers at the poles and 18 to 20 kilometers at the equator. Nearly all weather happens here because this layer contains the bulk of the atmosphere’s mass and moisture. Temperature drops steadily as you climb, from an average of about 17°C (62°F) at sea level to around -51°C (-60°F) at the top.
Stratosphere
Above the troposphere, the stratosphere stretches from roughly 6 to 20 kilometers (depending on latitude) up to about 50 kilometers. This is where the ozone layer sits, and because ozone formation releases heat, temperatures actually rise with altitude here, reaching about -15°C (5°F) near the top. Commercial jets sometimes cruise in the lower stratosphere because the air is calm and dry, with almost no weather to contend with.
Mesosphere
From about 50 to 85 kilometers, the mesosphere is where temperatures drop again, making it the coldest layer of the atmosphere. This is also where most meteors burn up as they hit thickening air on their way down. The bottom of this layer sits around -15°C, but temperatures plunge much colder toward the top.
Thermosphere
Extending from about 85 to 600 kilometers, the thermosphere is where solar radiation begins to interact directly with individual gas molecules. Ultraviolet and X-ray energy from the sun gets absorbed here, driving temperatures as high as 2,000°C (3,600°F) near the top. That number is misleading, though. The air is so thin at this altitude that there are very few molecules to carry that heat, so a thermometer wouldn’t register a burn. The International Space Station orbits within this layer. The thermosphere also contains the ionosphere, a zone of electrically charged particles that reflects certain radio waves back to Earth’s surface, making long-distance radio communication possible.
Exosphere
The outermost layer begins around 600 kilometers and fades into space at roughly 10,000 kilometers. Gas molecules here are so widely spaced that they rarely collide with one another. Hydrogen and helium are the dominant elements at this altitude, and the fastest-moving molecules occasionally escape Earth’s gravity entirely. There’s no sharp boundary between atmosphere and space, but the internationally recognized Kármán line at about 100 kilometers is the most commonly used marker. NASA awards astronaut status for flights above 80 kilometers.
How Much Atmosphere Is There
Despite stretching thousands of kilometers outward, the atmosphere is heavily concentrated near the surface. About half of its total mass is packed into the lowest 5.5 kilometers, and roughly 99% lies below 30 kilometers. The higher you go, the thinner the air becomes, until the exosphere is barely distinguishable from the vacuum of space. This concentration near the ground is why even small changes in the composition of the lower atmosphere, where greenhouse gases trap heat and particulate matter affects breathing, have such significant consequences for life on the surface.