Earth’s atmosphere is divided into five main layers, stacked by altitude and separated by shifts in temperature. Starting from the ground, these layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Each has distinct characteristics that affect weather, protect life from radiation, or define where space begins. The whole system is held together by gravity and thins dramatically with altitude: air pressure drops by about 63% for every 8.5 kilometers you go up.
What the Atmosphere Is Made Of
Dry air has a remarkably consistent recipe. Nitrogen makes up 78.08% of the atmosphere, oxygen accounts for 20.95%, and argon fills most of the remaining 0.93%. Carbon dioxide, despite its outsized role in climate, represents just 0.042% of the total volume. The global average hit a record 422.8 parts per million in 2024, measured by NOAA’s Global Monitoring Lab.
Water vapor is the wild card. It isn’t included in that dry-air breakdown because its concentration swings from nearly zero over deserts to around 4% in humid tropical air. Despite being variable, water vapor is the most important greenhouse gas and the raw material for clouds, rain, and storms.
Troposphere: Where Weather Happens
The troposphere is the layer you live in. It stretches from the surface to roughly 12 kilometers (7.5 miles) up, though it’s thinner at the poles (about 8 km) and thicker at the equator (up to 16 km). This layer contains about 80% of the atmosphere’s total mass, which is why nearly all weather, from thunderstorms to hurricanes, plays out here.
Temperature drops steadily as you climb through the troposphere, cooling by about 6.5°C for every kilometer of altitude. That cooling is what drives convection: warm air rises, cools, and sinks back down, creating the circulation patterns behind wind and precipitation. At the top of this layer sits the tropopause, a thin zone where the temperature stops falling and levels off. The tropopause acts as a ceiling that traps most weather below it.
Stratosphere: Home of the Ozone Layer
Above the tropopause, the stratosphere extends from about 12 km to 50 km (31 miles). Here, something unusual happens: temperature increases with altitude instead of decreasing. The reason is ozone. Ozone molecules concentrated in this layer absorb high-energy ultraviolet light from the Sun and convert that energy into heat. This warming effect creates a stable, layered environment with very little vertical mixing, which is why commercial jets cruise near the bottom of the stratosphere for smoother flights.
The ozone layer is critical for life on the surface. It filters out the most damaging wavelengths of UV radiation, the kind that causes skin cancer and damages plant DNA. After decades of damage from industrial chemicals, the ozone hole over Antarctica is slowly healing. NASA and NOAA ranked the 2025 ozone hole as the fifth smallest since 1992, and projections show a full recovery to 1980s levels around the late 2060s. At the top of the stratosphere, the stratopause marks another temperature plateau before conditions shift again.
Mesosphere: The Coldest Layer
The mesosphere spans from 50 km to about 85 km (53 miles) above the surface. Temperature reverses direction again here, dropping with altitude until it reaches the coldest point anywhere in Earth’s atmosphere: roughly -90°C (-130°F) at the mesopause, the boundary at the top of this layer.
This layer is where most meteors meet their end. Space rocks entering the atmosphere at tens of thousands of kilometers per hour slam into enough air molecules in the mesosphere to generate extreme friction, causing them to vaporize and produce the streaks of light we call shooting stars. Near the top of the mesosphere, at altitudes between 76 and 85 km, thin, eerie clouds called noctilucent clouds sometimes form. These “night-shining” clouds are visible only at twilight, when the Sun is below the horizon for an observer on the ground but still illuminates ice crystals at that extreme altitude.
Thermosphere: Extreme Heat, Thin Air
The thermosphere begins at about 90 km and extends to somewhere between 500 and 1,000 km (311 to 621 miles), depending on solar activity. Temperatures here soar to between 500°C and 2,000°C or higher. That sounds lethal, but you wouldn’t feel warm standing in it. The air is so thin that individual molecules carry a lot of energy yet there are far too few of them to transfer meaningful heat to a solid object. Temperature at this altitude is really a measure of how fast individual particles are moving, not how hot a surface would get.
High-energy solar radiation tears electrons away from gas particles in the thermosphere, creating electrically charged ions. This region of charged particles is called the ionosphere, and it overlaps with much of the thermosphere. The ionosphere is why AM radio signals can bounce around the curve of the Earth at night, and it’s where the aurora borealis and aurora australis light up the sky as charged particles from the Sun interact with those ions.
The International Space Station orbits within the thermosphere, at roughly 400 km altitude. The legal edge of space is defined differently depending on the authority: the United States sets it at 80 km, while the international Kármán line is placed at 100 km. Both boundaries fall near the bottom of the thermosphere.
Exosphere: The Fade Into Space
Beyond the thermosphere lies the exosphere, beginning at the thermopause (500 to 1,000 km up). This is less a traditional layer and more a gradual fade. Hydrogen atoms here are so spread out that they follow independent paths, like individual balls thrown into the air, rather than behaving collectively the way gas does lower down. There’s no hard outer edge. The exosphere technically extends to about 200,000 km from Earth’s surface, roughly halfway to the Moon, where the last traces of Earth-bound particles finally give way to the solar wind.
How Pressure and Density Change With Altitude
The atmosphere doesn’t end at a sharp boundary because air pressure and density decrease exponentially with altitude rather than dropping to zero at some fixed line. The concept that describes this is called scale height: approximately every 8.5 km you ascend, air pressure and density fall to about 37% of their previous value. At sea level, atmospheric pressure is about 1,013 millibars. By the time you reach the cruising altitude of a commercial jet (around 10 to 12 km), pressure has dropped to roughly a quarter of its surface value. By 30 km, it’s about 1% of what you feel at the beach.
This rapid thinning explains why the vast majority of the atmosphere’s mass is packed into the lowest layers. The troposphere and stratosphere together account for about 99.9% of total atmospheric mass, even though the exosphere stretches hundreds of thousands of kilometers into space.
The Boundaries Between Layers
Each major layer is separated by a transition zone named with the suffix “-pause.” The tropopause (around 12 km), stratopause (around 50 km), mesopause (around 85 km), and thermopause (500 to 1,000 km) all share a common trait: temperature briefly stops changing with altitude before the trend reverses in the next layer. These pauses aren’t physical barriers. Air and particles can still move across them. But they mark real shifts in how the atmosphere behaves, driven by different heating and cooling mechanisms in each layer.
The alternating temperature pattern, cooling then warming then cooling then warming, is one of the defining features of Earth’s atmosphere. It results from which energy source dominates at each altitude: the ground heats the troposphere from below, ozone heats the stratosphere from within, and solar radiation heats the thermosphere from above. The mesosphere, caught between two heated layers with no strong local heat source, simply cools.