Layers of the atmosphere

The correct answer is A, the troposphere. It is the lowest atmospheric layer, and people live at its lower boundary. Most weather, including clouds, rain, and storms, occurs there, so conditions in this layer affect everyday life directly.

The correct answer is B, changes in temperature with altitude. The main atmospheric layers are distinguished by how temperature changes as height increases—for example, it generally decreases with height in the troposphere and increases in the stratosphere. Ocean waves, soil colour, and river flow do not classify atmospheric layers.

The correct answer is B, the stratosphere. In order from Earth’s surface upward, the main layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Therefore, the stratosphere is immediately above the troposphere.

The correct answer is D, the exosphere. It is the outermost major atmospheric layer, where gas particles are extremely far apart, so the air is highly rarefied. Atmospheric density generally decreases with increasing altitude. (Source: NCERT Class XI Geography, *Fundamentals of Physical Geography*, chapter “Composition and Structure of Atmosphere.”)

Direct answer: B, a layered and relatively stable structure. The word stratosphere is linked with the idea of strata, or layers. Compared with the turbulent troposphere below, the stratosphere is more regularly layered and has less vigorous vertical mixing. Through much of it, temperature increases with height because ozone absorbs ultraviolet radiation, which contributes to its stable arrangement. A is wrong because very heavy rainfall is mainly associated with weather in the troposphere. C is wrong because soil is a surface feature and does not occur as a defining component of the stratosphere. D is wrong because river valleys are landforms, not atmospheric structures. Do not confuse “stable” with completely motionless: winds can still occur, but strong vertical convection is relatively limited. Memory cue: strato means layers, so think of stable atmospheric strata.

The correct answer is A, the troposphere. It contains most atmospheric water vapour, and clouds and most familiar weather phenomena—such as rain and storms—occur there. The higher layers listed are not the main region of everyday weather. (Source: NCERT Class XI Geography, *Fundamentals of Physical Geography*, chapter “Composition and Structure of Atmosphere.”)

The correct answer is B, the stratopause. The stratosphere lies above the troposphere and below the mesosphere, so its upper boundary is the transition between the stratosphere and mesosphere. The tropopause is the upper boundary of the troposphere, while the mesopause is the upper boundary of the mesosphere. (Source: NCERT Class XI Geography, *Fundamentals of Physical Geography*, chapter “Composition and Structure of Atmosphere.”)

The correct answer is D, the exosphere. From Earth’s surface outward, the main atmospheric layers are the troposphere, stratosphere, mesosphere, thermosphere and exosphere. The exosphere is the outermost layer, so it is farthest from the surface among the options.

The correct answer is B. Starting at Earth’s surface and moving outward, the layers are the troposphere, stratosphere, mesosphere, thermosphere and exosphere. This is the only option with the layers in that order.

The correct answer is C, protecting life from harmful ultraviolet (UV) radiation. Ozone in the stratosphere absorbs much of the Sun’s harmful UV radiation, reducing the amount that reaches Earth’s surface. Rainfall and soil formation result from other atmospheric and Earth-surface processes, not the ozone layer.

The correct answer is D, the troposphere. It is the lowest major atmospheric layer and begins at Earth’s surface. The stratosphere lies above it, followed by the mesosphere and thermosphere.

Correct answer: A, the troposphere. It is the lowest atmospheric layer, where air is densest because gravity keeps most atmospheric gases close to Earth’s surface. Air becomes thinner with increasing altitude, so the higher layers contain much less of the atmosphere’s mass.

Correct answer: B, air density decreases. Air density means the amount of air, or number of air molecules, in a given volume. As altitude increases, there is less air above a place, so the weight of the overlying air and the atmospheric pressure decrease. The molecules therefore become more widely spaced, making the air thinner and less dense in the upper atmosphere. A is wrong because density does not become higher upward. C is wrong because density changes with altitude. D is wrong because rainfall is not the controlling reason for the general decrease; the smaller overlying air column is the key idea. A useful cue is: “Higher means less air above, so lower density.”

The correct answer is C. Ozone in the stratosphere absorbs much of the Sun’s harmful ultraviolet radiation. If the ozone layer is damaged or depleted, more ultraviolet radiation can reach Earth’s surface, increasing risks such as skin and eye damage. Ozone depletion does not directly lower ocean salinity, greatly increase air pressure, or eliminate the mesosphere.

The correct answer is D. In names such as tropopause, stratopause, and mesopause, “-pause” refers to a boundary or transition level between atmospheric layers, where the vertical temperature pattern changes. For example, the tropopause lies between the troposphere and stratosphere. It is a transition level, not necessarily a sharp physical wall.

Correct answer: A. Most weather occurs in the troposphere, which contains most of the atmosphere’s water vapour. In the usual pattern, temperature decreases as altitude increases. The other statements are incorrect: the troposphere contains a mixture of gases, it is not the outermost layer, and the main concentration of ozone is in the stratosphere.

The correct answer is C, the mesopause. It is the boundary between the mesosphere and the thermosphere. The tropopause and stratopause are the boundaries above the troposphere and stratosphere, respectively.

The correct answer is B. Stratospheric ozone absorbs much of the Sun’s harmful ultraviolet radiation, reducing the amount that reaches Earth’s surface and can damage living cells. It does not block all ultraviolet radiation, but it provides important protection for life.

Correct answer: A, the troposphere. It is the lowest atmospheric layer and contains most of the atmosphere’s water vapour. Clouds and precipitation form there, and ordinary weather systems, including storms, occur mainly in this layer.

Correct answer: B, the mesosphere. From Earth upward, the main atmospheric layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The mesosphere therefore lies between the stratosphere and thermosphere. The ionosphere is a region that overlaps parts of several layers, rather than a separate layer in this sequence.

Answer: A, the troposphere. Gravity keeps most atmospheric gases close to Earth’s surface, so the lowest layer contains the greatest share of the atmosphere’s mass—about three-quarters. The layers above it have much thinner air.

Answer: B, the ozone layer and relatively stable air. The stratosphere contains the main concentration of atmospheric ozone, which absorbs ultraviolet radiation. Compared with the troposphere, the stratosphere is also relatively stable, with less vertical mixing. Meteors commonly burn in the mesosphere, while the exosphere is the outermost major layer.

Answer: D, the exosphere. It is the outermost major atmospheric layer, where gas particles are extremely far apart. The atmosphere becomes progressively thinner there and gradually merges into outer space rather than ending at a sharp boundary.

The correct answer is D. A meteoroid entering the atmosphere at high speed collides with air molecules. These interactions create drag and heat the object, so it may glow, lose material or break apart. Many meteors become visible as they pass through the mesosphere. This heating is often described as atmospheric friction, though collisions and drag are the more precise explanation.

The correct answer is A: the troposphere. It contains most atmospheric water vapour and nearly all ordinary weather. Temperature generally decreases with height in this layer. Rising moist air cools, and its water vapour can condense to form clouds.