General circulation of the atmosphere and planetary winds

Earth's rotation produces the Coriolis effect, which deflects moving air from a straight path. Winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection strongly influences the directions of planetary wind belts, so rotation is the correct answer.

The pressure-gradient force causes air to move from areas of relatively high atmospheric pressure toward areas of relatively low pressure. Differences in pressure therefore initiate wind movement. Earth's rotation and surface friction may alter the final direction, but the basic movement is from high pressure to low pressure.

Global heating is uneven, so broad belts of low and high pressure develop across Earth. Air moves because of the pressure gradient between these belts, forming the major planetary wind systems. The Coriolis effect then deflects the moving air, but pressure differences provide the basic driving force.

Horse latitudes lie near the subtropical high-pressure belts, around 30 degrees north and south. Air descends in these belts, producing stable conditions and often light, calm, or variable winds. Historical sailing ships could become becalmed there, which explains the traditional name horse latitudes.

Strong solar heating near the equator warms moist air, making it less dense and causing it to rise through convection. As the air rises, it expands and cools, water vapour condenses into clouds, and frequent heavy convectional rainfall occurs. This rising-air belt is associated with the equatorial low-pressure zone.

The equatorial region receives more solar energy than it loses, while the polar regions receive less solar energy and lose heat. Atmospheric circulation, together with ocean currents, transports surplus heat from low latitudes toward higher latitudes. This transfer helps reduce the global imbalance in heating.

Permanent, or planetary, winds are broad-scale wind systems that blow in fairly regular belts throughout the year. The three principal groups are trade winds in the tropics, westerlies in the middle latitudes, and polar easterlies near the poles. The other choices are local winds or weather disturbances.

Trade winds are persistent planetary winds that blow generally from the subtropical high-pressure belts toward the equatorial low-pressure belt. Before engines and modern navigation, their regular direction and dependable strength helped sailing ships cross tropical oceans, supporting exploration, transport, and international maritime trade.

Westerlies are the prevailing planetary winds of the middle latitudes, generally found between about 30 and 60 degrees in both hemispheres. They are named for the direction from which they originate, so they come from the west and usually move toward the east, although local pressure patterns can modify their path.

Polar easterlies originate in the cold, high-pressure regions near the poles. The air there has a very low temperature and generally contains little water vapour, so these winds are characteristically cold and dry. They move generally from the polar high-pressure areas toward the subpolar low-pressure belts. Therefore, option B correctly describes them; the other options do not match their polar origin.

The idealised three-cell model explains atmospheric circulation in each hemisphere through the Hadley cell, Ferrel cell and Polar cell. The Hadley cell dominates the tropics, the Ferrel cell occupies the middle latitudes, and the Polar cell operates near the poles. These cells together describe the broad meridional transfer of heat and momentum, so option A is correct.

Solar radiation is not received equally at all latitudes. Differences in the angle of insolation produce unequal heating, creating temperature contrasts. These contrasts cause variations in air density and pressure, which help establish global pressure belts and the planetary wind system. Thus, changing solar heating with latitude produces temperature differences and pressure belts, making option A correct.

The subpolar low-pressure belt is located near 60° latitude in both hemispheres. At the surface, the prevailing westerlies moving from the subtropical high-pressure belt and the polar easterlies moving from the polar high-pressure belt converge there. This convergence encourages rising air and unsettled weather. Therefore, option B is the correct answer.

Global wind circulation redistributes energy and water vapour around the planet. Winds transport warm air away from low latitudes and carry colder air toward lower latitudes; they also move moisture from oceans toward land and other regions. These transfers influence temperature, humidity, cloud formation and precipitation. Hence, option A correctly explains the weather effect of global circulation.

Pressure differences provide the basic force that moves air from high pressure toward low pressure. Earth’s rotation produces the Coriolis effect, which deflects moving air from a straight path. If the planet did not rotate, this deflection would disappear, although pressure differences caused by unequal heating would remain. Planetary winds would therefore move more directly, making option A correct.

Around 30° north and south, air descending from the upper branch of the Hadley circulation creates subtropical high pressure. As this air sinks, it warms adiabatically and its relative humidity decreases. The resulting stable, dry conditions suppress cloud development and rainfall. Persistent dryness helps deserts develop, so option A correctly explains their distribution.

The equatorial region receives intense solar heating, so surface air becomes warm, expands and becomes less dense. It rises through convection, producing the equatorial low-pressure belt. The trade winds also converge near the Inter-Tropical Convergence Zone, strengthening upward motion. Air generally descends in the subtropical high-pressure belts, not at the equator. Therefore, option B is correct.

Convergence means that air currents move toward and come together over a particular area. When surface winds converge, air cannot continue accumulating indefinitely at the ground, so it is commonly forced to rise. This rising motion is often associated with low pressure, cloud development and unsettled weather. Thus, option A gives the correct meaning of convergence.

Divergence occurs when air flows outward from a common region toward surrounding areas. At the surface, this outward spreading is commonly associated with high-pressure systems and sinking air from above, which replaces the air moving away. Subsiding air usually produces stable conditions and fewer clouds. Therefore, option B correctly defines divergence; option A describes convergence.

Trade winds are the major permanent or planetary winds of the tropics. They blow from the subtropical high-pressure belts towards the equatorial low-pressure belt. Because of the Coriolis effect, they are deflected to become north-easterly trades in the Northern Hemisphere and south-easterly trades in the Southern Hemisphere. Therefore, option A is correct.

The westerlies are the dominant planetary winds of the middle latitudes. They generally blow from the subtropical high-pressure belts towards the subpolar low-pressure belts, with a west-to-east component. This broad flow helps steer many mid-latitude depressions, fronts, and other weather systems from west to east. Thus, option B is correct.

Solar heating is unequal: low latitudes receive more energy than high latitudes. Atmospheric circulation, through rising and sinking air, planetary winds, cloud movement, and the transport of moisture, moves sensible and latent heat away from surplus-heating regions towards regions with less heating. This redistribution reduces thermal contrasts and supports Earth's heat balance, so option A is correct.

The north-e easterly and south-easterly trade winds converge in the equatorial region. Their meeting produces a belt of rising warm, moist air and relatively low surface pressure called the Inter-Tropical Convergence Zone, or ITCZ. It shifts seasonally with the apparent movement of the Sun. Therefore, option B is correct.

Planetary winds are called permanent winds because they occur over large parts of the Earth and have broadly regular directions determined by global pressure belts and the rotation of the Earth. Permanent does not mean that their speed never changes or that they are completely unaffected by seasons; it means their general pattern is persistent and predictable. Hence, option A is correct.

Land heats and cools more rapidly than water, while the sea changes temperature more slowly because of its higher heat capacity, mixing, and evaporation. This thermal contrast creates differences in air temperature and pressure between continents and oceans. Seasonal and daily pressure differences then produce monsoon, coastal, and other circulation patterns. Thus, option A is correct.