General circulation of the atmosphere and planetary winds

Strong solar heating near the equator warms the surface and causes moisture-laden air to rise by convection. As this air rises, it expands and cools; water vapour then condenses around condensation nuclei, forming clouds and frequent heavy convectional rainfall. Therefore, rising warm, moist air is the correct cause.

Convergence means that air flows from different directions toward and into the same region. When air converges near the surface, it generally cannot continue moving horizontally, so it is forced to rise. This rising motion is commonly associated with low pressure, cloud formation, and unsettled weather.

Divergence occurs when air flows outward from a common region toward surrounding areas. At the surface, it is usually associated with descending air and high pressure because air spreads away from the centre of the pressure system. Divergence therefore tends to reduce cloud formation and promote more stable weather.

Planetary winds are driven mainly by global pressure differences, but Earth rotates while the air moves across its surface. This rotation produces the Coriolis effect, which deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Consequently, planetary winds follow curved rather than perfectly straight paths.

Permanent, or planetary, winds are parts of the global circulation system and generally blow in fairly regular belts over large areas, such as trade winds and westerlies. Local winds develop over smaller regions because of local differences in heating, pressure, or relief, such as land and sea breezes.

The westerlies are prevailing planetary winds that blow in the middle latitudes from the subtropical high-pressure belts toward the subpolar low-pressure belts. Valley winds, sea breezes, and land breezes are local winds because they develop over limited areas due to local differences in heating and pressure. Therefore, option C is correct.

Pressure belts shift northward and southward during the year because the zone receiving the Sun’s most direct or vertical rays also moves seasonally between the Tropic of Cancer and the Tropic of Capricorn. This changes the distribution of heating, temperature, and air pressure. Hence, option A is correct.

The Inter-Tropical Convergence Zone, or ITCZ, is the region where the trade winds converge and air rises. It follows the seasonal migration of the belt of maximum solar heating, shifting northward in northern-hemisphere summer and southward in southern-hemisphere summer. Thus, option B is correct.

During summer, the zone of maximum solar heating moves into the summer hemisphere. The major pressure belts, including the equatorial low-pressure belt and subtropical belts, shift generally toward that hemisphere. This seasonal north-south movement is important in explaining monsoon circulation. Therefore, option B is correct.

The seasonal movement of the ITCZ changes the location of low pressure and convergence. Winds are redirected toward this shifting low-pressure zone, so their direction, strength, and rainfall distribution change. In South Asia, the northward movement of the ITCZ during summer is closely associated with the southwest monsoon. Hence, option A is correct.

The idealized global pressure pattern contains seven belts: one equatorial low-pressure belt, two subtropical high-pressure belts, two subpolar low-pressure belts, and two polar high-pressure belts. The belts occur as paired features in the two hemispheres, except for the single equatorial belt. Therefore, the total is seven and option C is correct.

The northeast trade winds blow toward the equatorial low-pressure belt from the Northern Hemisphere, while the southeast trade winds approach it from the Southern Hemisphere. These two flows converge near the ITCZ, producing rising air and frequent clouds and rainfall. Thus, trade winds occur on both sides, making option B correct.

The subpolar low-pressure belts are located approximately around 60° north and 60° south. They form near the zone where the relatively warmer westerlies meet the colder polar easterlies, causing air to rise and producing a belt of low pressure. Therefore, option C, sixty degrees, is correct.

The subtropical high-pressure belts are found approximately around 30° north and 30° south. Air that rises near the equator moves poleward at high altitude and gradually descends around these latitudes, creating high pressure. These regions are also called the horse latitudes. Hence, option B is correct.

Polar high-pressure belts occur near the North and South Poles, close to 90° north and 90° south. Very cold air over the polar regions becomes dense and sinks, producing high surface pressure. From these belts, polar easterly winds flow toward lower latitudes. Therefore, option D is correct.

Trade winds are the prevailing tropical surface winds that blow from the subtropical high-pressure belts toward the equatorial low-pressure belt. As they travel over warm oceans, they acquire water vapour and transport moisture toward the Intertropical Convergence Zone, where rising air can produce clouds and rainfall. Therefore, option A is correct.

Westerlies are the prevailing winds of the middle-latitude belt, generally between about 30° and 60° in both hemispheres. They move from subtropical high-pressure areas toward subpolar low-pressure areas and strongly influence travelling cyclones, fronts, and day-to-day weather in these latitudes. Hence, option A is correct.

The Roaring Forties are a belt of powerful westerly winds found approximately between 40° and 50° south latitude in the Southern Hemisphere. Their strength is associated with the small amount of land that obstructs the winds and the strong pressure gradient in these latitudes. They are called “roaring” because they can produce rough seas and severe weather. Therefore, option B is correct.

Oceans store and redistribute large amounts of solar heat and provide abundant water for evaporation. This heat and moisture enter the atmosphere, influence air pressure, support convection and cloud formation, and affect winds and rainfall across regions. Oceans therefore act as major energy and moisture sources in atmospheric circulation, making option A correct.

At the surface, air moves from areas of relatively higher pressure toward a low-pressure centre. This produces horizontal convergence. The converging air is usually forced upward, and if it contains sufficient moisture, rising and cooling can lead to cloud formation and precipitation. Therefore, option B is correct.

Surface air generally moves outward from a high-pressure centre toward surrounding areas of lower pressure. This horizontal spreading is called divergence. Because air is leaving the centre near the ground, air from above tends to descend to replace it, so high pressure is commonly linked with sinking air and clearer weather. Hence, option B is correct.

Trade winds form the lower, surface branch of the Hadley circulation. They blow from the subtropical high-pressure belts toward the equatorial low-pressure belt, carrying tropical air and, frequently, oceanic moisture toward the Intertropical Convergence Zone. This makes option A the correct description of their global-circulation role.

In geography, a wind is generally named for the direction from which it originates or comes, not the direction toward which it travels. For example, a westerly wind comes from the west and moves toward the east, while a northerly wind comes from the north. Temperature and rainfall may describe a wind’s characteristics, but they do not provide the basic directional name. Hence, option B is correct.

Within the Hadley cell, intense heating near the equator causes air to rise. At the upper troposphere, this air moves away from the equatorial region toward the subtropics and, in a broad sense, toward the poles before descending near about 30° latitude. Therefore, option A is the best answer.

Near 30° north and south, air descending from the upper branch of the Hadley cells creates subtropical high-pressure belts. Descending air warms adiabatically, lowering relative humidity and suppressing cloud development and rainfall. Persistent dryness in these belts helps explain the location of many major hot deserts, so option A is correct.