Layers of the atmosphere

The correct answer is B. The exosphere is the outermost, very thin region of the atmosphere, where the gas becomes increasingly sparse and gradually merges into outer space rather than ending at a sharp boundary. Clouds and most rainfall form much lower down, mainly in the troposphere.

The correct answer is B, the stratosphere. The main protective ozone layer is in the stratosphere, where ozone absorbs much of the Sun’s harmful ultraviolet radiation. Some ozone also occurs in the troposphere, but it is not the protective ozone layer meant here.

The correct answer is A. Ozone in the stratosphere absorbs much of the Sun’s harmful ultraviolet (UV) radiation before it reaches Earth’s surface. This helps reduce UV-related harm to people and other living things. Ozone does not affect earthquakes or remove sea salts.

The correct answer is B, the troposphere. It is the lowest major atmospheric layer, where temperature generally decreases as altitude increases. This temperature change with height is described by the lapse rate.

The correct answer is B, the troposphere. It lies below the stratosphere, and its temperature generally decreases with altitude. In the stratosphere above it, temperature generally increases with altitude, largely because ozone absorbs ultraviolet radiation.

The correct answer is A, the mesosphere. In this layer, temperature generally decreases with altitude, and the air is very thin compared with the lower atmosphere. The stratosphere differs because its temperature generally increases with altitude, while the exosphere is the outermost atmospheric region.

The correct answer is A. Stratospheric ozone absorbs much of the Sun’s biologically harmful ultraviolet radiation, including a large part of UV-B. Without this protection, more harmful UV would reach Earth’s surface, increasing risks such as skin and eye damage. Tides, seismic-wave speeds, and magnetic-pole movement are not controlled by the ozone layer.

The correct answer is B, the stratosphere. Most atmospheric ozone is concentrated there, where it absorbs much of the Sun’s harmful ultraviolet radiation.

The correct answer is B, the troposphere. It is the lowest atmospheric layer and lies directly above Earth’s land and oceans. Surface heating, evaporation, and the exchange of heat and moisture between land, sea, and air strongly affect this layer; most everyday weather occurs there.

The correct answer is C, the mesosphere. Many meteoroids become intensely heated as they encounter air and lose material while passing through this layer, so they burn up before reaching the ground. Not every meteoroid burns up there, but the mesosphere is the standard answer for most meteors.

A, the troposphere, is correct. It contains most of the atmosphere’s water vapour, and its air is continually mixed, allowing clouds, rain, and storms to develop. Therefore, most weather occurs in this lowest atmospheric layer.

B, the stratosphere, is correct. Ozone in this layer absorbs much of the Sun’s ultraviolet radiation, warming the surrounding air. As a result, temperature generally increases with altitude through much of the stratosphere. In the troposphere and mesosphere, temperature generally decreases with altitude.

A is correct. The atmospheric layers, from lowest upward, are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The tropopause is the boundary between the troposphere and stratosphere. The other named boundaries are the stratopause between the stratosphere and mesosphere, and the mesopause between the mesosphere and thermosphere.

D is correct. The exosphere is the outermost atmospheric region. Its gas particles are extremely far apart, and the atmosphere thins gradually until it merges with the surrounding space rather than ending at a sharp boundary. Dense rain clouds form much lower in the atmosphere, mainly in the troposphere.

B, the stratosphere, is correct. Most atmospheric ozone is concentrated there, broadly around 15–35 km above Earth, though it is spread across a range of altitudes rather than forming a thin sheet. Ozone absorbs much of the Sun’s harmful ultraviolet radiation.

B, the stratosphere, is correct. It contains most atmospheric ozone, which absorbs ultraviolet radiation and warms the surrounding air. Compared with the troposphere, it has much less water vapour and far less ordinary cloud and weather activity. The troposphere is the main layer where weather occurs.

The correct answer is A. The tropopause is the transition boundary between the troposphere and the stratosphere, and it marks the top of the troposphere, where most weather occurs. Its altitude varies with location and season, so it is not found only at sea level.

The correct answer is A, the troposphere. Most atmospheric water vapour and nearly all ordinary weather processes occur in this lowest layer, so clouds are concentrated there. The higher layers are much drier.

The correct answer is C. In the ionosphere, energetic solar radiation ionises some atmospheric gases, producing charged particles such as positive ions and free electrons. It is a region of the upper atmosphere, not a layer made entirely of ozone or a region where rainfall forms. Its charged particles can also affect radio-wave propagation.

The correct answer is A, the troposphere. The Earth's surface warms the air above it, so temperature generally decreases with height in this layer. The troposphere contains most of the atmosphere's water vapour, and most familiar weather—such as clouds and rain—occurs there.

The correct answer is A. In the troposphere, the surface warms the air above it, so temperature generally decreases with height. In the stratosphere, ozone absorbs ultraviolet radiation and warms the air, so temperature generally increases with height. These are the usual vertical trends in the two layers, not changes limited to nighttime.

The correct answer is B, the troposphere. Air warmed near Earth's surface can rise and mix vertically. As moist rising air expands and cools, water vapour may condense into tiny droplets or ice crystals, forming clouds. The stratosphere is comparatively stable, while the extremely thin upper layers are not where ordinary weather clouds form.

The correct answer is A. The mesosphere begins above the stratopause, the upper boundary of the stratosphere, and extends up to the mesopause, above which the thermosphere begins. Thus, it lies above the stratopause and below the mesopause.

The correct answer is A. Atmospheric pressure and the number of gas molecules in a given volume generally decrease with altitude, so gas density also generally decreases. The exosphere is extremely rarefied, while the stratosphere still contains gas; its density is not zero.

The correct answer is A, the troposphere. It extends upward from Earth’s surface, and winds and convection mix many pollutants emitted at ground level within this layer. The higher layers are not the main zone for the initial mixing of ordinary surface emissions, although some pollutants can be transported upward.