General circulation of the atmosphere and planetary winds

Correct answer: C. When a meandering river downcuts after rejuvenation.

Explanation: The correct answer is C. After rejuvenation, a meandering river may gain renewed vertical erosive power and cut down into its former floodplain or valley floor. If it retains much of its winding course as it deepens the valley, steep-sided incised meanders form. Rejuvenation can follow uplift or a fall in base level.

Correct answer: B. Under the influence of two dominant wind directions

Explanation: The correct answer is B. A seif, or longitudinal, dune is a long ridge of sand that commonly forms under two dominant wind directions, often when wind direction changes seasonally. The other choices describe processes unrelated to the formation of this wind-built dune.

Correct answer: A. A glacier abrades the valley floor and sides broadly, whereas a river mainly incises its channel

Explanation: The correct answer is A. A river usually concentrates its erosion in a relatively narrow channel, cutting downward and commonly forming a V-shaped valley. A glacier fills a wider part of a valley; rock fragments in the moving ice abrade the floor and sides, and the ice can also pluck rock. This broader erosion commonly produces a U-shaped cross-profile. Glacier movement is driven mainly by gravity, not only by wind.

Correct answer: A. Upper troposphere and lower stratosphere

Explanation: Direct answer: A, the upper troposphere and lower stratosphere near the tropopause. Jet streams are relatively narrow belts of very fast, mainly west-to-east winds high above Earth. They form near strong horizontal contrasts in temperature and pressure, especially around the tropopause. A is correct because this transition region contains the main jet-stream belts. B is wrong because the mesosphere is much higher and has a very thin atmosphere with different circulation. C is wrong because the exosphere is the outermost, extremely diffuse region, not the normal location of jet streams. D is wrong because the upper thermosphere is far above the weather-producing atmosphere and does not contain ordinary jet streams. The exact height varies with latitude and season, so “near the tropopause” is safer than a single fixed height. Memory cue: “Jet streams ride the tropopause.”

Correct answer: A. Strong temperature and pressure gradients in the upper troposphere

Explanation: A is correct. Jet streams are narrow bands of fast winds found near the tropopause. Strong temperature contrasts between neighbouring air masses are associated with pressure gradients aloft, which help drive these winds. The other options describe processes or features in different atmospheric regions and do not explain jet streams near the tropopause.

Correct answer: A. Temperature and pressure gradients in the upper troposphere

Explanation: Correct answer: A. Strong horizontal temperature contrasts between air masses create pressure differences in the upper troposphere. The resulting pressure-gradient force helps accelerate the air, while Earth’s rotation deflects its motion. Together, these effects help produce fast, narrow wind belts near the tropopause called jet streams.

Correct answer: D. Strong horizontal temperature and pressure gradients

Explanation: D is correct. Unequal heating between regions creates horizontal temperature contrasts and associated pressure gradients high in the atmosphere. Earth’s rotation helps organize this flow into narrow bands of very fast winds near the tropopause, called jet streams.

Correct answer: A. A ridge is an extension of high pressure, whereas a trough is an extension of low pressure

Explanation: A pressure ridge is an elongated area or extension of relatively high pressure from a high-pressure centre. A trough is an elongated area or extension of relatively low pressure from a low-pressure centre. Their identification depends on the arrangement and values of isobars, not on temperature lines alone. Therefore, option A correctly states the fundamental difference between the two map features.

Correct answer: A. Convergence of cold polar air and warm westerlies causes uplift

Explanation: The subpolar low-pressure belt forms near about 60° latitude in both hemispheres. Here, relatively cold polar easterlies meet warmer westerlies. Their convergence forces air to rise along the polar front. Rising air reduces surface pressure and creates a dynamic, or frontal, low-pressure belt. This zone is also associated with frequent cyclones and unsettled weather because of the meeting of contrasting air masses.

Correct answer: A. Unequal solar heating of the Earth

Explanation: The general circulation of the atmosphere is fundamentally driven by unequal solar heating. Low latitudes receive more solar energy than high latitudes, creating temperature and pressure differences. Air then moves in response to these pressure gradients, while Earth’s rotation and the Coriolis effect modify its direction. Thus, unequal heating is the basic cause.

Correct answer: B. Indirect mid-latitude cell

Explanation: The Ferrel cell occupies the middle latitudes, approximately between 30° and 60° in both hemispheres. Unlike the Hadley and polar cells, it is not driven directly by a simple heating contrast; it is an indirect circulation produced largely by the interaction of the neighbouring cells and the mid-latitude westerlies. Therefore, it is called an indirect mid-latitude cell.

Correct answer: A. Towards the equator

Explanation: At the surface, very cold air subsides over the poles and creates polar high pressure. It then flows equatorward toward the subpolar low-pressure belt near 60° latitude. Earth’s rotation deflects this equatorward flow, producing the polar easterlies. The broad surface movement is therefore from the poles toward the equator, not toward the poles.

Correct answer: B. Deflecting the direction of moving air

Explanation: The Coriolis force is an apparent force associated with Earth’s rotation. It does not create heat, eliminate pressure, or directly stop rainfall; instead, it changes the apparent direction of moving air. Winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, helping produce the observed directions of planetary wind belts.

Correct answer: B. Northeast trade winds

Explanation: In the Northern Hemisphere, the surface air of the Hadley cell moves from the subtropical high-pressure belt toward the equatorial low-pressure belt. The Coriolis effect deflects this flow to the right, so the winds approach the equator from the northeast. They are therefore called the northeast trade winds. In the Southern Hemisphere they are called southeast trades.

Correct answer: B. From southeast to northwest

Explanation: In the Southern Hemisphere, air flows at the surface from the subtropical high-pressure belt toward the equatorial low-pressure belt. Earth’s rotation deflects this equatorward movement to the left, so the winds generally come from the southeast and move toward the northwest or the equator. These are known as the southeast trade winds.

Correct answer: A. They blow from west to east

Explanation: A wind is named according to the direction from which it originates or appears to come. Westerlies generally blow from the west toward the east in the middle latitudes, although their paths can be modified by continents, pressure systems, and seasonal conditions. The name does not mean that they occur only on western coasts or that they cannot bring rain.

Correct answer: A. From polar high pressure to subpolar low pressure

Explanation: Polar easterlies are planetary winds that blow from the permanent polar high-pressure belts toward the subpolar low-pressure belts. Very cold, dense air accumulates near the poles and produces high pressure; air then moves outward toward the relatively lower pressure around about 60° latitude. Earth’s rotation deflects this flow, giving it an easterly direction.

Correct answer: A. Descending air from the upper atmosphere

Explanation: In the idealized three-cell model, air heated near the equator rises and travels poleward aloft in the Hadley cells. Around 30° north and south, this upper-level air cools sufficiently and descends. The sinking air increases surface pressure, suppresses cloud formation, and helps create the subtropical high-pressure belts and many subtropical deserts.

Correct answer: A. Convergence of winds

Explanation: The subpolar low-pressure belt forms near about 60° latitude, where the relatively warmer westerlies meet the colder polar easterlies. This surface convergence forces air upward, lowering pressure at the surface. The rising air also supports cloud formation and unsettled weather. Thus, convergence of winds is an important reason for the subpolar low.

Correct answer: A. From the equatorial region toward the poles

Explanation: Because the equatorial region receives more solar energy than it loses while the poles receive less, a global energy imbalance develops. Atmospheric circulation transports surplus heat from the tropics toward higher latitudes, while ocean currents also contribute. This poleward transfer helps moderate tropical overheating and polar cooling, maintaining Earth’s overall heat balance.

Correct answer: A. Deflection would be very small or absent

Explanation: The rotation of Earth produces the Coriolis effect, which deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. If Earth stopped rotating, this rotational deflection would be absent or extremely small. Pressure differences would still drive winds, so winds would not stop; their paths would be more direct.

Correct answer: A. Bring moisture and support convection

Explanation: Trade winds generally blow from subtropical high-pressure belts toward the equatorial low-pressure belt. When they travel over warm oceans, they pick up water vapour and carry it toward tropical land or convergence zones. Rising moist air cools, condenses, forms clouds, and may produce convectional rainfall. The exact amount of rainfall depends on topography, season, distance from the sea, and the position of the pressure belts, but bringing moisture and supporting convection is the correct general effect.

Correct answer: A. Due to descending dry air

Explanation: Around 25°–35° north and south, air that rose near the equator gradually descends as part of the Hadley circulation. Descending air becomes compressed and warms adiabatically, so its relative humidity decreases and cloud formation is suppressed. These subtropical high-pressure belts therefore have clear skies, little convection, and very low rainfall. Many major deserts, such as the Sahara and Australian deserts, occur in or near these belts. Thus, descending dry air is the best answer.

Correct answer: A. Continental barriers are fewer

Explanation: The Southern Hemisphere has a much greater proportion of ocean surface, especially in the middle and higher latitudes where the westerlies occur. With fewer large continents and mountain barriers, these winds encounter less friction and are interrupted less often. They can therefore travel around the globe in a more continuous belt, particularly over the Southern Ocean. The Coriolis force still operates there, so the greater strength and consistency are mainly related to the limited continental obstruction, making A correct.

Correct answer: A. Pressure gradient and Coriolis force

Explanation: The pressure-gradient force starts the movement of air by accelerating it from areas of higher pressure toward areas of lower pressure. As Earth rotates, the Coriolis force deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The balance between these forces, together with friction near the surface, determines the observed direction of planetary winds. Among the choices, pressure gradient and Coriolis force are the fundamental controlling factors, so A is correct.