What is the air made of, and why does temperature rise and fall as you climb through the atmosphere?
Topic 4.4 Earth's Atmosphere: describe the composition of the atmosphere and the four main layers, and explain how temperature changes with altitude.
A focused answer to APES Topic 4.4, covering atmospheric composition, the four layers (troposphere, stratosphere, mesosphere, thermosphere), the temperature profile, the ozone layer, and the role of the atmosphere in weather and protection, with a worked composition calculation.
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What this topic is asking
The College Board (Topic 4.4) wants you to describe the composition of the atmosphere, name its four layers, and explain the temperature profile with altitude. This sets up the climate and pollution topics later in the course.
Composition
The four layers
The temperature profile
The ozone layer is the key feature: by absorbing most of the Sun's ultraviolet radiation it both warms the stratosphere and protects life at the surface from UV damage (a link to Unit 9 stratospheric ozone depletion).
Why this matters
The atmosphere is the stage for weather, climate and air pollution. Its uneven heating drives the global wind patterns of Topic 4.5; the troposphere's greenhouse gases set up the climate topics of Unit 9; and the stratospheric ozone layer is the focus of the ozone-depletion topic. Understanding which layer holds weather, ozone or the greenhouse effect prevents confusion across the whole back half of the course.
Try this
Q1. Identify the most abundant gas in the atmosphere. [1 point]
- Cue. Nitrogen, at about 78%.
Q2. Explain why temperature increases with altitude in the stratosphere. [2 points]
- Cue. The stratosphere contains the ozone layer, which absorbs the Sun's UV radiation and converts it to heat, so the upper stratosphere is warmer than the lower stratosphere.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)4 marksSection II (FRQ). (a) Identify the two most abundant gases in the atmosphere and their approximate percentages. (b) Identify the atmospheric layer in which weather and most life occur. (c) Describe the role of ozone in the stratosphere. (d) Explain why temperature increases with altitude in the stratosphere.Show worked answer →
A 4-point FRQ on the atmosphere.
(a) Identify (1 point): nitrogen (about 78%) and oxygen (about 21%).
(b) Identify (1 point): the troposphere, the lowest layer, where weather occurs and most organisms live.
(c) Describe (1 point): stratospheric ozone absorbs most of the Sun's harmful ultraviolet (UV) radiation, protecting life at the surface.
(d) Explain (1 point): temperature rises with altitude in the stratosphere because ozone there absorbs UV radiation and converts it to heat, warming the upper stratosphere.
Markers reward the 78% nitrogen and 21% oxygen figures, the troposphere for weather and life, ozone's UV-absorbing role, and UV absorption by ozone as the reason for the stratospheric temperature increase.
AP 2018 (style)1 marksSection I (multiple choice). In which atmospheric layer does nearly all weather occur? (A) Troposphere (B) Stratosphere (C) Mesosphere (D) Thermosphere. Justify your choice.Show worked answer →
A 1-point MCQ on atmospheric layers. The answer is (A).
The troposphere is the lowest layer, where almost all water vapor, clouds and weather occur and where most organisms live. The stratosphere (B) holds the ozone layer but little weather; the mesosphere (C) is where meteors burn up; the thermosphere (D) is the outermost layer. The trap is confusing the stratosphere (ozone) with the troposphere (weather).
Related dot points
- Topic 4.5 Global Wind Patterns: explain how uneven solar heating and the Coriolis effect drive atmospheric circulation cells and global wind belts.
A focused answer to APES Topic 4.5, covering uneven solar heating, convection and the Hadley, Ferrel and polar cells, the Coriolis effect, the trade winds and westerlies, and why deserts and rainforests sit where they do, with a worked latitude-climate question.
- Topic 4.7 Solar Radiation and Earth's Seasons: explain how the tilt of Earth's axis and its orbit produce variations in insolation that cause the seasons.
A focused answer to APES Topic 4.7, covering solar radiation (insolation), the 23.5 degree axial tilt, the solstices and equinoxes, the angle of incidence, why the tilt and not distance causes the seasons, and latitude effects, with a worked insolation question.
- Topic 4.8 Earth's Geography and Climate: explain how geographic features such as mountains and proximity to water shape regional climate, including rain shadows and El Nino and La Nina.
A focused answer to APES Topic 4.8, covering how mountains, latitude, ocean currents and proximity to water shape regional climate, the rain shadow effect, and the El Nino and La Nina (ENSO) cycle, with a worked rain-shadow question.
- Topic 1.4 The Carbon Cycle: describe the major reservoirs and fluxes of the carbon cycle and explain how natural processes and human activities move carbon between them.
A focused answer to APES Topic 1.4, covering carbon reservoirs and fluxes, photosynthesis and respiration, decomposition, combustion, the ocean as a carbon sink, and how fossil fuel burning alters the cycle, with a worked carbon-flux calculation.
- Topic 4.1 Plate Tectonics: explain how convection in the mantle drives plate movement and describe the three types of plate boundary and their landforms and hazards.
A focused answer to APES Topic 4.1, covering mantle convection, the three plate boundary types (divergent, convergent, transform), the landforms and hazards each produces, hot spots, and the link to natural resources, with a worked boundary-identification question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)