Layers of the atmosphere

A is correct. In much of the stratosphere, temperature increases with altitude, which discourages rising air from continuing upward. This limits convection and strong vertical mixing, so conditions are generally more stable than in the weather-active troposphere. Turbulence can still occur, so the stratosphere is not always completely smooth.

A is correct. The mesosphere contains very little ozone to absorb incoming ultraviolet radiation, so it receives relatively little solar heating. Its temperature generally falls as altitude increases, reaching very low values near the mesopause. Water vapour is concentrated mainly in the lower atmosphere, and air pressure decreases—not increases—with height.

C, the mesosphere, is correct. Temperature generally decreases with height through the mesosphere and reaches very low values near the mesopause, the boundary between the mesosphere and thermosphere. The troposphere also usually cools with height, but it ends at the tropopause, not the mesopause.

Air pressure is the force exerted by the weight of the air above a given level. As altitude increases, the remaining air column becomes shorter and less massive, so its weight and the pressure it produces decrease. This relationship applies across the atmosphere, although the rate of decrease may vary with temperature and density. Therefore, option A is correct; gravity does not suddenly increase, water vapour does not always rise, and permanent clouds do not cause the pressure change.

The correct answer is C, the thermosphere. Its thin gases absorb energetic solar radiation, including ultraviolet and X-ray radiation, causing the measured temperature to rise sharply with altitude. The air is extremely sparse, so this high temperature does not mean it would transfer heat like dense, warm air near the surface.

The correct answer is A, the stratosphere. Temperature generally decreases with height in the troposphere, but in the stratosphere ozone absorbs ultraviolet radiation and warms the air. As a result, temperature generally increases with altitude through the stratosphere. In the mesosphere, the decreasing trend returns.

The correct answer is A. The atmosphere is divided into troposphere, stratosphere, mesosphere, and thermosphere according to how temperature changes with altitude. Each layer has a characteristic trend caused by different absorption and transfer of solar or terrestrial energy. Temperature is not uniform, and pressure alone does not define every boundary. Thus thermal structure provides a clear scientific basis for identifying the layers.

The correct answer is A. The exosphere is extremely rarefied, so particles collide infrequently. A sufficiently fast particle can travel a long distance without losing much energy in collisions and may escape if its speed and energy are high enough to overcome Earth's gravitational binding. Low density alone does not guarantee escape; the particle must also have sufficient energy.

The correct answer is A. The thermosphere is defined by its temperature pattern, while the ionosphere is a region containing ions and free electrons. These regions overlap: a large part of the ionosphere lies within the thermosphere, although the ionosphere also extends into neighboring upper atmospheric regions.

The correct answer is A, the mesosphere. Ozone absorbs ultraviolet radiation and warms the stratosphere. Above the stratopause, this ozone heating becomes much less significant, and temperature generally decreases with altitude through the mesosphere, reaching its lowest values near the mesopause.

The correct answer is A. Temperature generally decreases with altitude through the mesosphere and reaches a minimum near the mesopause. By contrast, it generally increases with altitude in the stratosphere and thermosphere, while it usually decreases in the troposphere. The words “always” in B and D also make those claims too absolute.

The correct answer is C, the mesosphere. Temperature generally decreases with height through the mesosphere and reaches its minimum near the mesopause, the mesosphere’s upper boundary. Above it, in the thermosphere, temperature generally increases with height as gases absorb high-energy solar radiation.

The correct answer is A. Stronger heating near the equator drives rising air and deeper convection, making the troposphere thicker and its upper boundary—the tropopause—higher. Colder polar air generally produces a shallower troposphere and a lower tropopause. The stratosphere is present over the poles, gravity does not disappear at the equator, and ozone is not completely absent at the poles.

Correct answer: A. Temperature measures the average kinetic energy of particles, not the total heat transferred to a person or object. The thermosphere’s few particles can have very high kinetic energy, but the gas is extremely thin, so collisions—and thus heat transfer—are limited. Its temperature would not feel like that of dense, hot air near Earth’s surface.

Correct answer: A. The top of the troposphere is the tropopause. Temperature generally decreases with height through the troposphere; near the tropopause, this decrease becomes very small and the temperature is nearly constant. Above it, in the stratosphere, temperature generally increases with height. The other conditions do not define this boundary.

Correct answer: A. The D layer is the lower region of the ionosphere. Daylight solar radiation increases ionisation there, and collisions in this relatively dense region absorb some radio-wave energy, weakening certain signals. The stratopause and tropopause are atmospheric boundaries, not ionospheric layers.

Correct answer: A. Meteors enter the atmosphere at extremely high speeds. In the mesosphere, collisions with gas particles and compression of the air ahead convert kinetic energy into heat, causing the meteor and surrounding gas to glow; some meteor material may also ablate. The gas is thinner than in the troposphere, but the meteor’s speed is high enough for substantial heating.

Correct answer: A. Charged particles from the Sun are guided by Earth’s magnetic field toward the upper atmosphere, especially near the poles. When they collide with atmospheric gases such as oxygen and nitrogen, those gases emit light, producing an aurora. Water-vapour condensation, by contrast, is associated with clouds and precipitation.

Correct answer: A. In the troposphere, temperature generally decreases with altitude. At the tropopause, the boundary with the stratosphere, this decrease ends; in the stratosphere, temperature is initially fairly stable and then generally rises with altitude. Thus the tropopause marks the transition from the tropospheric cooling trend to the stratospheric warming trend.

Correct answer: A. The mesopause is the boundary between the mesosphere and thermosphere. In the mesosphere, temperature generally falls as altitude increases and reaches a very low minimum near this boundary. That is why the mesopause is counted among the coldest regions of the atmosphere. The exosphere, clouds, and sea level do not explain this temperature minimum.

The correct answer is A, the ionosphere. This region of the upper atmosphere contains ions and free electrons produced by solar radiation. These charged particles can affect the propagation of radio waves, which is why the ionosphere is associated with some forms of long-distance radio communication. The troposphere is the lowest atmospheric layer, while the stratopause is a boundary between layers.

The correct answer is A, the thermosphere. Its sparse gases absorb high-energy solar radiation, including extreme ultraviolet and X-rays. This increases the kinetic energy of the particles, so the measured temperature rises with altitude. The stratosphere’s temperature increase is mainly associated with ozone absorbing ultraviolet radiation; in the troposphere and mesosphere, temperature generally decreases with height.

The correct answer is A. The stratosphere is relatively stable, so vertical mixing is limited; it is also much drier than the troposphere, with little rainfall to wash pollutants out. As a result, some pollutants can persist there for a long time. This does not mean that every pollutant persists equally or is unaffected by chemical reactions.

The correct answer is A. In the stratosphere, temperature generally rises with altitude because ozone absorbs ultraviolet radiation. In the mesosphere, temperature generally falls with altitude, reaching a minimum near the mesopause. The troposphere usually cools upward, so option B reverses both relevant trends.

The correct answer is A, the mesopause. Temperature first decreases through the troposphere and reaches a minimum near the tropopause. It then rises through the stratosphere, before decreasing again through the mesosphere to a second major minimum near the mesopause. The stratopause, by contrast, is associated with a relative temperature maximum.