Earth’s atmosphere is mostly four gases by abundance (EK ERT-4.D.1): nitrogen ≈ 78.08%, oxygen ≈ 20.95%, argon ≈ 0.93%, and carbon dioxide ≈ 0.04% (about 410 ppm today, variable). Water vapor isn’t fixed—it ranges from ~0% to ~4% of air by volume depending on humidity and location. The rest are trace gases (neon, helium, methane, ozone, nitrous oxide, CFCs, etc.) that together make up the tiny remainder but can be important greenhouse or reactive species. These compositions and the roles of greenhouse/trace gases are what the CED expects you to know for Topic 4.4 (see EK ERT-4.D.1 and related keywords like greenhouse gases, ozone). For a quick Topic 4.4 review, check Fiveable’s study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9) and try practice problems at (https://library.fiveable.me/practice/ap-environmental-science).

As you go upward the atmosphere’s temperature doesn’t change steadily—it reverses at layer boundaries. Using AP terms (EK ERT-4.D): - Troposphere (surface to ~8–15 km): temperature decreases with altitude (average lapse rate ≈ 6.5°C per km). Weather and most water vapor live here. - Stratosphere (~15–50 km): temperature increases with altitude because the ozone layer absorbs UV radiation, warming this layer. - Mesosphere (~50–85 km): temperature again decreases with altitude; this is the coldest layer. - Thermosphere (~85–600 km): temperature increases dramatically with altitude as sparse gas absorbs high-energy solar radiation (but “temperature” here refers to very energetic particles, not heat felt). - Exosphere (above ~600 km): temperatures are variable; gas is extremely thin and gradually transitions to space. This layering is defined by temperature gradients (CED EK ERT-4.D.2). For a quick topic review, check the Fiveable Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9) and Unit 4 overview (https://library.fiveable.me/ap-environmental-science/unit-4). For extra practice, try problems at (https://library.fiveable.me/practice/ap-environmental-science).

The troposphere (surface to ~10–12 km) is the lowest, densest layer and contains ~75–80% of the atmosphere’s mass, most water vapor, and almost all weather. Temperature generally decreases with altitude there (the temperature lapse rate), which drives convection and vertical mixing. The stratosphere (about 12–50 km) sits above the troposphere, is much more stable, and has temperature increasing with altitude because the ozone layer absorbs UV radiation. That ozone absorption warms the stratosphere and limits vertical mixing, so weather systems don’t cross into it. For the AP exam, be able to link these differences to where weather, greenhouse gases, and the ozone layer act (CED EK ERT-4.D.2; keywords: troposphere, stratosphere, ozone layer, temperature lapse rate). Review this topic in Fiveable’s study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9) and practice questions at (https://library.fiveable.me/practice/ap-environmental-science).

The ozone layer sits in the stratosphere (roughly 10–50 km altitude, peaking ~20–30 km) because of how ozone (O3) is formed and where conditions favor its buildup. High-energy UV from the Sun splits O2 into O atoms; those free O atoms combine with O2 to make O3. That photolysis needs strong UV, which is abundant above the troposphere. Also, the stratosphere has a temperature inversion (it warms with altitude), so vertical mixing is weak compared to the turbulent troposphere. That stability lets ozone accumulate rather than get mixed down and destroyed. Finally, ozone itself absorbs UV and heats the layer, reinforcing the stratospheric temperature profile. All of this explains why the ozone layer forms in the stratosphere, not lower (too much mixing) or much higher (too few O2 molecules). For the APES CED, this links EK ERT-4.D.2 (layers by temperature gradients) and the “ozone layer” keyword—see the Topic 4.4 study guide on Fiveable for more review (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9). For extra practice, try problems at (https://library.fiveable.me/practice/ap-environmental-science).

Temperature changes with altitude because different layers absorb and lose energy in different ways. In the troposphere (where we live) temperature drops with height—the normal lapse rate—because the surface warms the air from below. In the stratosphere temperature rises with height because the ozone layer absorbs UV radiation and heats that layer. In the mesosphere temperature falls again with altitude because there’s little ozone or air to absorb energy. In the thermosphere temperature climbs sharply as very shortwave solar radiation (X-rays, extreme UV) is absorbed by sparse gas and creates the ionosphere; gas molecules are hot even though air density is tiny. These temperature gradients define the layers (troposphere, stratosphere, mesosphere, thermosphere, exosphere) you need to know for EK ERT-4.D.2 on the APES CED. For more review, check the Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9) and try practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Bottom to top: - Troposphere (surface to ~12 km): Contains ~75% of atmospheric mass and most water vapor and greenhouse gases. Temperature generally decreases with altitude (lapse rate). Weather, clouds, and life occur here. - Stratosphere (~12–50 km): Temperature increases with altitude because the ozone layer absorbs UV radiation. Stable layering limits vertical mixing—helpful for high-altitude aircraft and long-lived pollutants. - Mesosphere (~50–85 km): Temperature falls again; this is the coldest layer. Meteors burn up here. - Thermosphere (~85–600 km): Temperature rises strongly as sparse gas absorbs high-energy solar radiation. Contains the ionosphere (charged particles) where auroras and radio signal reflection occur; low-orbit satellites pass through or above this region. - Exosphere (>~600 km): Very thin, transitional zone to space dominated by light gases (H, He); particles can escape to space. This matches EK ERT-4.D.2 in the CED. For a focused AP review, see Fiveable’s Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9) and more unit resources (https://library.fiveable.me/ap-environmental-science/unit-4). For extra practice, try the AP problems page (https://library.fiveable.me/practice/ap-environmental-science).

Scientists use direct sampling, remote sensing, and historical archives to figure out atmospheric composition. For today’s atmosphere we take air samples from ground stations, aircraft, and balloons and analyze them with instruments like gas chromatographs and mass spectrometers to measure major gases (N2 ~78%, O2 ~21%, Ar ~0.93%) and trace gases (CO2 ≈ 0.04% or ~400 ppm, methane, N2O, CFCs). Satellites and ground-based spectrometers measure absorption lines in sunlight to map gases globally and in different layers (troposphere, stratosphere, etc.). For past atmospheres scientists use ice cores and trapped air bubbles to get long-term records. These methods align with EK ERT-4.D.1 (gases and abundances) and EK ERT-4.D.2 (layers by temperature). For a focused review, check the Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9), Unit 4 overview (https://library.fiveable.me/ap-environmental-science/unit-4), and lots of practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Nitrogen (N2) is most abundant (~78%) because of its chemical stability and long-term cycling compared to oxygen. N2 is a very unreactive diatomic gas—so it doesn't get used up quickly by respiration or oxidation—so it accumulates in the atmosphere. Oxygen (O2), although produced in large amounts by photosynthesis, is continuously consumed by respiration, decomposition, and reactions that oxidize rocks and minerals. Over geologic time, some early free oxygen was removed by reacting with Earth's crust; the balance today reflects a steady-state between biological O2 production and many O2 sinks. AP-relevant terms: major gases, trace gases, and biogeochemical cycles (EK ERT-4.D.1). For a quick review of atmospheric composition and layers, see the Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9). For more practice on CED-aligned questions, try the unit practice bank (https://library.fiveable.me/practice/ap-environmental-science).

Order (bottom to top): troposphere → stratosphere → mesosphere → thermosphere → exosphere. Quick tips to remember: - Mnemonic: “The Strong Men Take Exams” (Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere). - Short facts to attach to each layer (use on the test): troposphere = weather, temperature generally decreases with altitude (lapse rate); stratosphere = ozone layer and temperature increases with altitude; mesosphere = coldest layer where meteors burn up; thermosphere = very hot, contains the ionosphere (auroras, absorbs high-energy radiation); exosphere = thin outer layer merging to space. Why this matters for APES: the CED expects you to identify layers based on temperature gradients and key features (ozone, ionosphere, weather) (see Topic 4.4 study guide: https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9). For broader review and extra practice, check the Unit 4 overview (https://library.fiveable.me/ap-environmental-science/unit-4) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

The troposphere has most of our weather because it contains almost all the air mass, water vapor, and aerosols that drive weather processes. It’s the densest layer (about ~75% of the atmosphere’s mass) and temperatures generally decrease with altitude (the temperature lapse rate), which creates vertical instability and convection—warm air rises, cools, condenses, and forms clouds and precipitation. The troposphere sits directly above Earth’s surface, so surface heating and moisture feed storms and wind. Other layers don’t have weather because they’re either too stable or too thin. The stratosphere has a temperature inversion (it warms with altitude due to ozone absorbing UV), which suppresses vertical mixing. The mesosphere and thermosphere are very low density, so they lack the moisture and mass needed for clouds and storms. (See the Topic 4.4 study guide for diagrams: https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9). For more unit review or practice Qs, check the Unit 4 page (https://library.fiveable.me/ap-environmental-science/unit-4) and Fiveable practice problems (https://library.fiveable.me/practice/ap-environmental-science). This ties to AP ERT-4.D (layers & temperature gradients).

Temperature gradients (how temperature changes with altitude) come from how and where the atmosphere gains or loses energy. - Troposphere: temperature generally falls with altitude (average lapse rate ≈ 6.5°C per km) because the ground absorbs sunlight, heats the air by conduction and convection, and warm air rises. Weather and greenhouse gases (CO2, water vapor) trap heat near the surface. - Stratosphere: temperature increases with altitude because ozone absorbs UV radiation, heating that layer and creating a temperature inversion—this defines the ozone layer and protects life. - Mesosphere: temperature falls again with altitude because there’s little ozone or water vapor to absorb solar energy. - Thermosphere: temperature rises sharply with altitude as sparse gas molecules absorb high-energy solar and charged-particle radiation (this region overlaps the ionosphere). - Exosphere: temperatures are complex because gas is extremely thin; molecules rarely collide, so “temperature” is more about individual particle energy. These layer definitions and causes align with CED keywords (troposphere, stratosphere, ozone, greenhouse gases, temperature lapse rate, ionosphere). For a focused review, check the Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9) and practice problems (https://library.fiveable.me/practice/ap-environmental-science) to prep for AP exam questions on atmosphere structure and temperature gradients.

Right now the atmosphere is ~78% nitrogen, ~21% oxygen, ~0.93% argon and ~0.04% CO2 (plus variable water vapor and trace gases). If those proportions changed, you'd get big, predictable effects tied to the CED keywords (greenhouse gases, ozone, troposphere, etc.). Examples: - More CO2 or methane → stronger greenhouse effect → higher global temps, altered precipitation, ocean acidification (CO2 dissolves and lowers pH). This is core to Unit 4 and appears on the exam (see free-response prompts about greenhouse gases). - Less O2 → problems for aerobic organisms (reduced metabolic rates) and less combustion/fire behavior. - More nitrogen in reactive forms (NOx, NH3) → more eutrophication, smog, and altered nutrient cycles. - Less stratospheric ozone → higher UV radiation, more skin cancer and ecosystem damage. For a focused review on composition, layers, ozone and greenhouse gases, check the Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9) and Unit 4 overview (https://library.fiveable.me/ap-environmental-science/unit-4). For practice, use Fiveable’s APES practice problems (https://library.fiveable.me/practice/ap-environmental-science).

The thermosphere has very high measured temperatures (hundreds to ~2,000°C) because the few gas molecules there absorb intense solar UV and X-ray energy and move very fast. But "hot" in temperature isn’t the same as feeling hot—there are so few molecules (extremely low density) that they carry very little total thermal energy. Heat transfer by conduction or convection is almost nonexistent, so you don’t get much energy into your skin or suit. Astronauts stay comfortable because their suits and spacecraft provide insulation and active thermal control, and because collisions with air molecules are too rare to transfer significant heat. The thermosphere also overlaps the ionosphere (ions from UV/X-ray ionization), which affects radio signals but not direct heating of astronauts. This distinction (temperature vs. heat capacity and density) is a key part of EK ERT-4.D about atmospheric layers. For review, check the Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9); practice questions are at (https://library.fiveable.me/practice/ap-environmental-science).

Short answer: yes—know the rough altitudes and why layers are defined (temperature gradients), but you don’t need super-precise boundaries on the exam. Typical thicknesses (approximate): - Troposphere: 0–~12 km (weather, temperature decreases with altitude). - Stratosphere: ~12–50 km (temperature increases with altitude; contains ozone layer). - Mesosphere: ~50–80 km (temperature decreases). - Thermosphere: ~80–~700 km (temperature increases; contains the ionosphere). - Exosphere: ~700–several thousand km (fades into space). For the APES exam focus on: layer order, the reason layers are defined (temperature gradients), key features (troposphere = weather; stratosphere = ozone; thermosphere = ionosphere/aurora), and relevant gases/roles (greenhouse gases, ozone). Exact km cutoffs can vary by source, so memorize the relative positions and functions, not nitpicky numbers. For review, check the Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9) and get extra practice (https://library.fiveable.me/practice/ap-environmental-science).

Short answer: they’re different layers defined by temperature and density. The mesosphere sits above the stratosphere (~50–85 km). Temperature falls with altitude there, making it the coldest layer (down to ~–90°C). It’s where most meteors burn up. The exosphere is the outermost layer (starting around ~600–700 km and gradually fading into space). It has extremely low density, mostly light atoms (H, He), molecules move so far between collisions that they can escape to space. Why this matters for APES: the CED expects you to know layers by temperature gradients and key features (troposphere → stratosphere → mesosphere → thermosphere → exosphere) per EK ERT-4.D. For a quick review, check the Topic 4.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-4/earths-atmosphere/study-guide/7Z9K5q4df3Hvtvuh33x9). Want practice questions on layers and atmosphere topics? Try Fiveable’s practice bank (https://library.fiveable.me/practice/ap-environmental-science).