AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/4/4-4)
Unit 4 · Topic 4.4
4.4 Earth's Atmosphere
Earth's atmosphere is mostly nitrogen and oxygen, with small but important amounts of argon, carbon dioxide and water vapor. It's divided into layers by how temperature changes with height, from the troposphere where weather happens to the stratosphere's ozone layer and beyond.
Key terms
- troposphere
- stratosphere
- ozone layer
- mesosphere
- thermosphere
What the air is made of
Gases present in tiny amounts can still have big effects. Carbon dioxide and water vapor trap heat (greenhouse gases), and ozone (O₃) blocks harmful ultraviolet (UV) radiation. Because CO₂ is so scarce, concentrations are usually given in parts per million (ppm) instead of percent.
The atmosphere's makeup has changed over Earth's history. Early Earth had almost no free oxygen; it built up over billions of years as photosynthesizing bacteria and later plants released O₂. Today, people are changing it again, mainly by adding CO₂, methane and other gases, and the air also carries pollutants and particles that vary from place to place.
| Gas | Approximate share of dry air |
|---|---|
| Nitrogen (N₂) | 78% |
| Oxygen (O₂) | 21% |
| Argon (Ar) | 0.93% |
| Carbon dioxide (CO₂) | About 0.04% (around 425 ppm) |
| Water vapor (H₂O) | Varies from near 0 to about 4% (not counted in dry air) |
The layers
| Layer | Approximate height | Temperature with height | Key facts |
|---|---|---|---|
| Troposphere | 0 to about 8–18 km (thicker at the equator) | Decreases | Holds about 75–80% of the atmosphere's mass and almost all its water vapor; all weather happens here |
| Stratosphere | Top of troposphere to about 50 km | Increases | Contains the ozone layer, which absorbs UV and warms this layer; air is stable with little mixing |
| Mesosphere | About 50 to 85 km | Decreases | Coldest layer; most meteors burn up here |
| Thermosphere | About 85 to several hundred km | Increases sharply | Absorbs high-energy X-rays and UV; auroras occur here; the International Space Station orbits here |
| Exosphere | Outermost | Hard to define; gas is extremely thin | Gradually fades into space |
Why temperature changes with height
The troposphere is heated mainly from below. Sunlight warms the ground, and the ground warms the air above it, so the air gets colder as you go up, on average about 6.5°C per kilometer. That's why mountaintops are cold.
In the stratosphere, ozone absorbs incoming UV radiation and turns its energy into heat, so temperature rises with height. Because warmer air sits on top of cooler air there, the stratosphere doesn't mix much vertically. In the thermosphere, the few gas molecules absorb very high-energy radiation, so they move very fast, which means a very high temperature, even though the air is so thin it wouldn't feel hot.
Good ozone, bad ozone
Ozone in the stratosphere is helpful: it absorbs most of the sun's harmful UV-B and UV-C radiation, protecting living things from skin cancer, cataracts and damage to crops and plankton. Ozone in the troposphere, near the ground, is a harmful pollutant that forms in photochemical smog and damages lungs and plants. Same molecule, different effects depending on where it is.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Identifying layers from a temperature profile
A weather balloon records air temperature falling steadily from 15°C at the ground to about −55°C at 12 km, then rising with height to about 0°C at 50 km. Name the layers the balloon passed through and explain the temperature change in the upper one.
Show the solutionHide the solution
- Step 1: From the ground to 12 km, temperature decreases with height: that's the troposphere.
- Step 2: From 12 km to 50 km, temperature increases with height: that's the stratosphere.
- Step 3: Explain the increase: ozone in the stratosphere absorbs UV radiation from the sun and releases the energy as heat, warming the upper stratosphere.
Answer: Troposphere (0–12 km) and stratosphere (12–50 km); the stratosphere warms with height because ozone absorbs UV radiation.
- Example 2
Converting ppm to percent
The atmosphere contains about 425 ppm of CO₂. Convert this to a percent.
Show the solutionHide the solution
- Step 1: ppm means parts per million, so 425 ppm = 425 ÷ 1,000,000.
- Step 2: 425 ÷ 1,000,000 = 0.000425.
- Step 3: Multiply by 100 to get a percent: 0.000425 × 100 = 0.0425%.
Answer: About 0.04% (0.0425%).
Common mistakes
- Saying oxygen is the most abundant gas. Nitrogen is, at about 78%.
- Putting the ozone layer in the troposphere. It's in the stratosphere; ozone near the ground is a pollutant.
- Saying temperature always drops with height. It drops in the troposphere and mesosphere but rises in the stratosphere and thermosphere.
On the exam
- Know the order of the layers and whether temperature rises or falls in each; questions often describe a temperature-height graph.
- Be ready to explain the difference between stratospheric and tropospheric ozone.
Connected topics
Videos
Check yourself
4 questions on 4.4 Earth's Atmosphere. Pick an answer to see if you got it, and why.
A scientist collects a 500-liter sample of dry air. About how many liters of the sample are nitrogen gas?
Air temperature decreases with altitude in the troposphere but increases with altitude through the stratosphere. Which statement best explains the increase in the stratosphere?
| Altitude (km) | Average temperature (°C) |
|---|---|
| 0 | 15 |
| 10 | −50 |
| 20 | −56 |
| 35 | −36 |
| 50 | −3 |
| 65 | −42 |
| 80 | −77 |
Approximate values based on a standard model of the atmosphere, rounded
Between which altitudes does temperature rise with height, and which layer of the atmosphere is this?
What is the average rate at which temperature falls with height in the lowest 10 km?
0 of 4 answered