Layers & Composition of the Atmosphere
Learn the four main layers of Earth's atmosphere, their altitudes, temperatures, and the gases that make up air.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Layers & Composition of the Atmosphere, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Earth's atmosphere is a blanket of air that surrounds our planet and makes life possible. But this blanket is not uniform — it has distinct layers, each with its own temperature, density, and characteristics. Understanding these layers helps explain why airplanes fly at certain altitudes, why meteors burn up before reaching the ground, and how weather happens. In this lesson, you will identify each atmospheric layer, learn how temperature changes with altitude, and discover what gases make up the air we breathe and the air high above us.
Composition of the Atmosphere
The atmosphere is a mixture of gases. By volume, nitrogen (N₂) makes up about 78 percent of the air, and oxygen (O₂) makes up about 21 percent. Together, these two gases account for 99 percent of the air we breathe. The remaining 1 percent includes argon (about 0.9 percent), carbon dioxide (about 0.04 percent), and trace amounts of other gases like neon, helium, and methane.
Oxygen is essential for most living things on Earth, while nitrogen cycles through soil and water to support plant growth. Carbon dioxide, though a tiny fraction of the atmosphere, plays a crucial role in photosynthesis and the greenhouse effect. Water vapor also floats in the air in varying amounts depending on temperature and location — it is not usually counted as part of the "dry air" composition, but it is always present to some degree.
Understanding atmospheric composition matters because different gases absorb different amounts of solar radiation and trap heat in different ways. This knowledge helps explain why the temperature of each layer changes the way it does.
Oxygen is essential for most living things on Earth, while nitrogen cycles through soil and water to support plant growth. Carbon dioxide, though a tiny fraction of the atmosphere, plays a crucial role in photosynthesis and the greenhouse effect. Water vapor also floats in the air in varying amounts depending on temperature and location — it is not usually counted as part of the "dry air" composition, but it is always present to some degree.
Understanding atmospheric composition matters because different gases absorb different amounts of solar radiation and trap heat in different ways. This knowledge helps explain why the temperature of each layer changes the way it does.
The Troposphere: Where Weather Happens
The troposphere is the layer closest to Earth's surface, extending from ground level to approximately 10 to 18 kilometers (about 6 to 11 miles) above sea level. Its thickness varies with latitude — it is thicker at the equator and thinner at the poles.
In the troposphere, temperature decreases with altitude at an average rate of about 6.5 degrees Celsius per kilometer of elevation. This means the lower you are (closer to Earth), the warmer it is, and the higher you go, the cooler it becomes. This happens because the troposphere is heated from below: solar radiation passes through the atmosphere and warms Earth's surface, and then Earth's surface radiates heat upward, warming the air above it.
All weather occurs in the troposphere — clouds, rain, snow, thunderstorms, and wind. This is also the layer where most airplanes fly. By the time you reach the top of the troposphere (the tropopause), temperatures have dropped to about .
In the troposphere, temperature decreases with altitude at an average rate of about 6.5 degrees Celsius per kilometer of elevation. This means the lower you are (closer to Earth), the warmer it is, and the higher you go, the cooler it becomes. This happens because the troposphere is heated from below: solar radiation passes through the atmosphere and warms Earth's surface, and then Earth's surface radiates heat upward, warming the air above it.
All weather occurs in the troposphere — clouds, rain, snow, thunderstorms, and wind. This is also the layer where most airplanes fly. By the time you reach the top of the troposphere (the tropopause), temperatures have dropped to about .
The Stratosphere: The Ozone Layer
Above the troposphere lies the stratosphere, extending from approximately 18 kilometers to 50 kilometers (11 to 31 miles) above sea level. Unlike the troposphere, temperature increases with altitude in the stratosphere. This is the opposite trend, and it happens because the stratosphere contains ozone.
Ozone (O₃) is a form of oxygen made of three oxygen atoms bonded together. It is concentrated in a region called the ozone layer, roughly 20 to 30 kilometers above sea level. Ozone absorbs ultraviolet (UV) radiation from the Sun — radiation that would be harmful to most life on Earth. As ozone absorbs UV radiation, it converts that energy into heat, warming the stratosphere. This is why temperature rises instead of falls as you climb through this layer.
The stratosphere is very dry and stable, so it is an ideal place for aircraft that need to fly at very high altitudes for long periods. Weather does not occur here because there is insufficient moisture and convection to produce clouds and precipitation.
Ozone (O₃) is a form of oxygen made of three oxygen atoms bonded together. It is concentrated in a region called the ozone layer, roughly 20 to 30 kilometers above sea level. Ozone absorbs ultraviolet (UV) radiation from the Sun — radiation that would be harmful to most life on Earth. As ozone absorbs UV radiation, it converts that energy into heat, warming the stratosphere. This is why temperature rises instead of falls as you climb through this layer.
The stratosphere is very dry and stable, so it is an ideal place for aircraft that need to fly at very high altitudes for long periods. Weather does not occur here because there is insufficient moisture and convection to produce clouds and precipitation.
The Mesosphere: The Coldest Layer
The mesosphere extends from approximately 50 kilometers to 85 kilometers (31 to 53 miles) above sea level. In this layer, temperature decreases again with altitude, and the mesosphere is the coldest layer of the atmosphere, with temperatures dropping to about at its upper boundary.
Why does temperature drop here? The mesosphere receives very little direct heating from ozone absorption (ozone is concentrated lower, in the stratosphere) and is too high to be heated much by radiation from Earth's surface. Solar radiation passes through, but little energy is converted to heat in this thin layer.
The mesosphere is important for one visible reason: most meteors burn up and disintegrate in the mesosphere as they enter Earth's atmosphere at high speed. The friction between the meteor and the air molecules creates the bright streak you see — a meteor or "shooting star." The mesosphere is also home to noctilucent clouds, thin, silvery clouds that form at very high altitudes and are visible only at twilight.
Why does temperature drop here? The mesosphere receives very little direct heating from ozone absorption (ozone is concentrated lower, in the stratosphere) and is too high to be heated much by radiation from Earth's surface. Solar radiation passes through, but little energy is converted to heat in this thin layer.
The mesosphere is important for one visible reason: most meteors burn up and disintegrate in the mesosphere as they enter Earth's atmosphere at high speed. The friction between the meteor and the air molecules creates the bright streak you see — a meteor or "shooting star." The mesosphere is also home to noctilucent clouds, thin, silvery clouds that form at very high altitudes and are visible only at twilight.
The Thermosphere: An Extremely Thin and Hot Layer
The thermosphere extends from approximately 85 kilometers (53 miles) upward to about 500 kilometers (310 miles) or more. Temperature increases dramatically with altitude in this layer, reaching thousands of degrees Celsius at the top.
However, "hot" in the thermosphere is misleading. Temperature measures how fast particles are moving, but the thermosphere is so thin — with very few air molecules — that there is little actual heat energy. If an astronaut were in the thermosphere, they would actually feel cold because there are so few molecules to transfer heat to their suit.
The thermosphere is where auroras (the northern and southern lights) occur. Solar wind — streams of charged particles from the Sun — interacts with oxygen and nitrogen molecules in this layer, causing them to glow in brilliant colors of green, red, purple, and blue. The International Space Station orbits in the thermosphere, just below the outer edge of the exosphere, which is the uppermost boundary of Earth's atmosphere.
However, "hot" in the thermosphere is misleading. Temperature measures how fast particles are moving, but the thermosphere is so thin — with very few air molecules — that there is little actual heat energy. If an astronaut were in the thermosphere, they would actually feel cold because there are so few molecules to transfer heat to their suit.
The thermosphere is where auroras (the northern and southern lights) occur. Solar wind — streams of charged particles from the Sun — interacts with oxygen and nitrogen molecules in this layer, causing them to glow in brilliant colors of green, red, purple, and blue. The International Space Station orbits in the thermosphere, just below the outer edge of the exosphere, which is the uppermost boundary of Earth's atmosphere.
Key terms
- Troposphere.
- The lowest layer of Earth's atmosphere, extending from ground level to about 10 to 18 kilometers altitude, where all weather occurs and temperature decreases with altitude.
- Stratosphere.
- The second layer of the atmosphere, extending from about 18 to 50 kilometers altitude, containing the ozone layer and having temperature that increases with altitude.
- Mesosphere.
- The third layer of the atmosphere, extending from about 50 to 85 kilometers altitude, where temperature decreases with altitude and most meteors burn up.
- Thermosphere.
- The fourth and outermost layer of the atmosphere, extending from about 85 kilometers to 500 kilometers altitude, where temperature increases dramatically with altitude and auroras occur.
- Ozone.
- A form of oxygen (O₃) composed of three oxygen atoms, concentrated in the stratosphere where it absorbs ultraviolet radiation and protects life on Earth.
- Atmospheric composition.
- The mixture of gases that make up air; nitrogen comprises about 78 percent, oxygen about 21 percent, and trace gases make up the remaining 1 percent by volume.
- Tropopause.
- The boundary between the troposphere and stratosphere, where temperature stops decreasing and begins to increase with altitude.
- Aurora.
- A natural light display in the thermosphere caused by solar wind particles colliding with oxygen and nitrogen molecules, seen as the northern or southern lights.
Worked example
A scientist measures temperature at different altitudes in Earth's atmosphere. At sea level, the temperature is 15 degrees Celsius. At 5 kilometers altitude, the temperature is 10 degrees Celsius. At 25 kilometers altitude, the temperature is 5 degrees Celsius. At 60 kilometers altitude, the temperature is 0 degrees Celsius. At 100 kilometers altitude, the temperature is 100 degrees Celsius. Identify which atmospheric layers the scientist sampled and describe the temperature trend in each layer.
Let's work through this by matching the altitudes to the layers and identifying the temperature trends.
From sea level to 5 km: We start at 15°C and drop to 10°C as altitude increases. This is the troposphere, where temperature decreases with altitude at about 6.5°C per km. The measurement shows roughly 1°C per km, which is reasonable given real-world variation.
From 5 km to 25 km: Temperature continues to drop from 10°C to 5°C, still in the troposphere and entering the stratosphere. At 25 km, we are in the stratosphere, where temperature should begin to increase, but this point shows only a small decrease. This is near the tropopause (around 18 km), so we are transitioning between the two layers.
From 25 km to 60 km: Temperature holds steady around 5°C to 0°C. At 60 km, we are well into the mesosphere. In the stratosphere (up to 50 km), temperature should increase, but in the mesosphere (50 to 85 km), temperature decreases again. This measurement shows a slight trend, indicating we are transitioning from the stratosphere into the mesosphere where temperatures begin dropping again.
From 60 km to 100 km: Temperature climbs dramatically from 0°C to 100°C. At 100 km, we are in the thermosphere, where temperature increases sharply with altitude.
Summary: The scientist sampled the troposphere (sea level to ~5 km, temperature decreases), stratosphere (~18 to 25 km, temperature increases slightly), mesosphere (25 to 60 km, temperature decreases), and thermosphere (60 to 100 km, temperature increases dramatically). Each layer shows its characteristic temperature trend with altitude.
From sea level to 5 km: We start at 15°C and drop to 10°C as altitude increases. This is the troposphere, where temperature decreases with altitude at about 6.5°C per km. The measurement shows roughly 1°C per km, which is reasonable given real-world variation.
From 5 km to 25 km: Temperature continues to drop from 10°C to 5°C, still in the troposphere and entering the stratosphere. At 25 km, we are in the stratosphere, where temperature should begin to increase, but this point shows only a small decrease. This is near the tropopause (around 18 km), so we are transitioning between the two layers.
From 25 km to 60 km: Temperature holds steady around 5°C to 0°C. At 60 km, we are well into the mesosphere. In the stratosphere (up to 50 km), temperature should increase, but in the mesosphere (50 to 85 km), temperature decreases again. This measurement shows a slight trend, indicating we are transitioning from the stratosphere into the mesosphere where temperatures begin dropping again.
From 60 km to 100 km: Temperature climbs dramatically from 0°C to 100°C. At 100 km, we are in the thermosphere, where temperature increases sharply with altitude.
Summary: The scientist sampled the troposphere (sea level to ~5 km, temperature decreases), stratosphere (~18 to 25 km, temperature increases slightly), mesosphere (25 to 60 km, temperature decreases), and thermosphere (60 to 100 km, temperature increases dramatically). Each layer shows its characteristic temperature trend with altitude.
Practice questions
The atmosphere is made up of different gases. Which two gases make up approximately 99 percent of the air by volume?
- Nitrogen and oxygen
- Oxygen and argon
- Nitrogen and carbon dioxide
- Argon and neon
Answer: Nitrogen and oxygen
Nitrogen makes up about 78 percent of the atmosphere and oxygen makes up about 21 percent. Together, 78 + 21 = 99 percent. Argon is only about 0.9 percent, carbon dioxide is about 0.04 percent, and neon and other gases are trace amounts. Understanding the major gases in air is fundamental to understanding atmospheric processes.
Why does temperature decrease as you go higher in the troposphere?
Answer: Temperature decreases in the troposphere because the troposphere is heated from below. Solar radiation passes through the atmosphere and warms Earth's surface first. Earth's surface then radiates heat (infrared radiation) upward, which warms the air above it. The closer you are to the heat source — Earth's surface — the warmer it is. As altitude increases and you move away from Earth, there is less direct heat source, so temperature drops.
This is a key concept that students often misunderstand. Many think the Sun directly heats the air, but actually most solar radiation passes through the atmosphere. The atmosphere is largely transparent to visible light. What heats the troposphere most is the infrared radiation from Earth's warm surface. This is also why the upper troposphere is much colder than the lower troposphere, even though it is closer to the Sun.
Look at this table showing temperature at different altitudes:Altitude (km) Temperature (°C) 0 20 10 –30 30 –5 70 50
Based on these measurements, which two layers of the atmosphere are represented in this table, and what is the main reason the temperature trend changes between them?
Answer: The troposphere (0 to ~18 km) and the stratosphere (18 to ~50 km) are represented. Between 0 km and 10 km, temperature decreases, which is typical of the troposphere. Between 10 km and 30 km, temperature increases, which is typical of the stratosphere. The temperature trend changes because the stratosphere contains ozone, which absorbs ultraviolet (UV) radiation from the Sun. This UV absorption converts solar energy into heat, warming the stratosphere instead of cooling it. In the troposphere, there is very little ozone, so heating comes only from Earth's surface radiation, which decreases with altitude.
This question tests whether you understand not just the layers and their temperature trends, but also the reason why each trend happens. The presence or absence of ozone is the key difference. Without understanding the mechanism (ozone absorption of UV radiation), students cannot fully grasp why the stratosphere warms instead of cools. Notice also that the data in the table are realistic: tropospheric temperatures drop roughly 6 to 7°C per km, and stratospheric temperatures turn around and increase, just as the theory predicts.
FAQ
- If the thermosphere is so hot, why don't astronauts burn up in it?
- Temperature and heat are not the same thing. Temperature measures how fast gas molecules are moving, but the thermosphere is extremely thin — there are very few molecules present. Heat requires energy transfer between objects. Even though individual molecules are moving very fast (high temperature), there are so few of them that they transfer very little heat to an astronaut or spacecraft. It is like the difference between one very fast bullet and a blizzard of slow snowflakes. One bullet can cause harm; millions of snowflakes move slowly but cause much more heat transfer. In the thermosphere, you have very few fast particles, so little net heat transfer occurs.
- Why is the ozone layer important?
- Ozone is important because it absorbs ultraviolet (UV) radiation from the Sun. UV radiation can damage DNA and cause skin cancer and cataracts in humans. It also harms many organisms and plants. The ozone layer acts as a shield, blocking most of this harmful radiation before it reaches Earth's surface. Without the ozone layer, life as we know it could not exist on Earth's surface. This is why the loss of ozone due to chemicals like CFCs (chlorofluorocarbons) in the 20th century was such a serious environmental concern.
- Do planes fly in the troposphere or the stratosphere?
- Most commercial airplanes fly in the troposphere, usually at altitudes between 10 and 12 kilometers. A few high-altitude aircraft, like the U-2 spy plane, can fly at altitudes above 20 kilometers and enter the lower stratosphere. Planes prefer these high altitudes because there is less air resistance (thinner air) at higher altitudes, which makes flying more fuel-efficient. However, commercial planes cannot go too high because they need oxygen to breathe for passengers and crew, and oxygen is much rarer in the stratosphere and above.
- What causes the auroras we see in the sky?
- Auroras (the northern and southern lights) occur in the thermosphere when solar wind — streams of charged particles from the Sun — collides with oxygen and nitrogen molecules in the atmosphere. These collisions transfer energy to the molecules, causing them to glow. Oxygen typically glows green or red, while nitrogen glows blue or purple. Auroras are most visible near the North and South Poles, where Earth's magnetic field channels solar wind particles down toward the poles. Auroras usually appear in the night sky as shimmering curtains or arcs of colored light.
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The Crimsora tutor teaches Layers & Composition of the Atmosphere live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.