Layers of the atmosphere

The correct answer is A, the exosphere. It is the outermost atmospheric layer, where particles are extremely far apart. Its outer boundary is gradual rather than a sharp edge, so it merges into outer space.

A, the mesosphere, is correct. In this layer, temperature generally decreases with altitude, and most meteors heat up and burn as they pass through it. The stratosphere and thermosphere generally have temperatures that rise with altitude, so they do not fit both clues.

A, the troposphere, is correct. The tropopause is the boundary at the top of the troposphere and separates it from the stratosphere above. The stratosphere’s upper boundary is called the stratopause.

A, the lower troposphere, is correct. Gravity keeps most atmospheric gas close to Earth’s surface, so air density generally decreases as altitude increases. The thermosphere can have very high temperatures, but its air is extremely sparse; high temperature does not mean high density.

A is correct. Gravity keeps most air close to Earth’s surface. At higher altitudes, less air lies above a location, so the weight of the air above it—and therefore the pressure—is lower. Air density also generally decreases with altitude.

A is correct. Stratospheric ozone absorbs much of the Sun’s harmful ultraviolet radiation, particularly UV-B and UV-C, reducing the amount that reaches Earth’s surface and helping protect living things.

A is correct. The thermosphere absorbs high-energy solar radiation, including ultraviolet and X-ray radiation, so its temperature can increase greatly with altitude. Its air is extremely thin, so this high temperature does not mean it would feel hot in the same way as dense air near the surface. Most weather and water vapour are in the troposphere.

A is correct. The mesopause is the boundary between the mesosphere below and the thermosphere above. The tropopause separates the troposphere from the stratosphere, while the stratopause separates the stratosphere from the mesosphere.

Answer: A, the troposphere. Most atmospheric water vapour is found in this lowest layer. It can rise, cool, and condense to form clouds and precipitation, so it is central to humidity and everyday weather. The higher layers contain far less water vapour.

Answer: A. Atmospheric layers are easier to distinguish when you connect each layer’s position or approximate height with its temperature trend and characteristic feature. For example, the troposphere is associated with weather, while the stratosphere contains much of the atmosphere’s ozone. The other choices either concern unrelated facts or ignore real differences between layers.

D is correct. Many meteoroids become heated and ablate as they pass through the mesosphere. Temperature generally decreases with height in this layer and is very low near the mesopause. Heavy rain and fog are associated mainly with the troposphere, ozone-related warming with the stratosphere, and the gradual transition to space with the exosphere.

Correct answer: A. Most atmospheric ozone is concentrated in the stratosphere. Ozone absorbs much of the Sun’s harmful ultraviolet radiation, reducing how much reaches Earth’s surface and helping protect living things. This absorption also warms the stratosphere. Most clouds and weather occur in the troposphere, and air is densest near Earth’s surface.

Correct answer: A. The exosphere is the outermost part of Earth’s atmosphere, where gas becomes extremely sparse with increasing altitude. There is no sharp boundary separating it from space, so the transition is gradual. The other choices describe conditions that do not apply to the exosphere.

C, the mesosphere, is correct. As a meteoroid enters the atmosphere, collisions with air particles heat it and remove material from its surface through ablation. Many small meteoroids become visible as meteors in the mesosphere, where there are enough air particles for frequent collisions. Very large objects may travel lower, so they do not all burn up at exactly the same height.

C, the stratosphere, is correct. Ozone in the stratosphere absorbs much of the incoming ultraviolet radiation, warming this layer. As a result, temperature generally increases with altitude through much of the stratosphere. The thermosphere also warms with altitude, but the ozone clue points specifically to the stratosphere.

B, the stratosphere, is correct. It contains very little water vapour compared with the troposphere, and its air is generally stable, which limits vertical convection. As a result, ordinary clouds, rain, and thunderstorms occur much less often there than in the troposphere, where most weather takes place.

The correct answer is C, the mesosphere. Temperature generally decreases with altitude in this layer, and many meteoroids heat up and ablate as they encounter atmospheric particles there. The troposphere is the layer most closely associated with weather, while the stratosphere generally warms with height because ozone absorbs ultraviolet radiation.

The correct answer is A. The exosphere is the outermost, extremely tenuous region of Earth’s atmosphere. Hydrogen and helium are very light gases that can be present there; because of their low mass and the exosphere’s conditions, they can escape into space more readily than heavier gases. Rain clouds form much lower in the atmosphere, and ozone consists of oxygen atoms.

The correct answer is A. The troposphere contains most atmospheric water vapour and is where rising, cooling air commonly forms clouds and precipitation. The stratosphere is much drier and more stable, so it is not the main zone of rainfall. The other choices are incorrect: the stratosphere is part of the atmosphere, contains gases, and lies below the exosphere.

The correct answer is A. Many small meteoroids encounter air in the mesosphere and become intensely heated as they lose energy and material; some break apart or burn up before reaching the ground. This reduces the number of small objects that strike Earth’s surface. It does not form rivers, repair the ozone layer, or make the monsoon permanent.

The correct answer is A. As moist air cools, water vapour can condense into tiny droplets on small particles. Dust, sea salt, and other aerosols can provide these surfaces, which are called condensation nuclei. The droplets may then grow and contribute to cloud formation.

A is correct. Sound is a mechanical wave: particles in a material medium vibrate and pass the disturbance to neighbouring particles. In a very thin upper atmospheric layer, particles are far apart, so there are fewer interactions to carry the sound energy onward. Sound propagation can therefore be very weak; it does not require ozone specifically and cannot travel through a vacuum.

A, the troposphere, is correct. It is the lowest atmospheric layer and contains most atmospheric water vapour and much of the dust near Earth’s surface. Heating from the ground causes air to rise and sink, producing strong vertical mixing and much of the weather we experience. The stratosphere is comparatively stable, while the exosphere and upper thermosphere are extremely thin.

A is correct. The stratopause is the upper boundary of the stratosphere and the transition to the mesosphere. It also marks a change in the temperature pattern: temperature generally rises with height through the stratosphere, then decreases with height in the mesosphere.

The correct answer is C: the troposphere contains roughly three-quarters of the atmosphere’s mass. Gravity concentrates air near Earth’s surface, and the troposphere is the lowest atmospheric layer. The thermosphere may have very high temperatures, but its gas is extremely sparse; temperature does not mean it contains more mass.