General circulation of the atmosphere and planetary winds

Because Earth is tilted and revolves around the Sun, the overhead Sun shifts between the Tropic of Cancer and the Tropic of Capricorn during the year. The belt of maximum heating and rising air follows this apparent movement, so the ITCZ migrates northward and southward seasonally. Hence option B is correct.

Pressure-gradient force is produced by a difference in atmospheric pressure over distance. Air is accelerated from areas of relatively high pressure toward areas of relatively low pressure; when the pressure changes sharply over a short distance, the pressure gradient and the resulting force are stronger. Therefore, option B is correct.

Wind is driven by the pressure-gradient force, which acts from areas of higher pressure toward areas of lower pressure. When the pressure difference over a given distance is large, the pressure gradient is steep and the force on the air is stronger. Consequently, wind generally moves faster, although friction and the Coriolis effect can modify its actual speed and direction. Thus, option C is correct.

The idealized three-cell model divides atmospheric circulation in each hemisphere into the Hadley cell from the Equator to about 30°, the Ferrel cell from about 30° to 60°, and the Polar cell from about 60° to the pole. Therefore, option A gives the correct three cells.

The subpolar low-pressure belt is generally located near 60° north and 60° south latitudes. Around this zone, relatively warmer westerlies meet colder polar easterlies at the polar front. The convergence and uplift of air reduce surface pressure and support cloud formation, cyclonic activity, and unsettled weather. Therefore, option B gives the correct latitude zone.

Polar regions receive low-angle solar radiation and lose heat, so the air becomes extremely cold. Cold air is denser than warm air and tends to sink toward the surface. This descending air increases the amount of air pressing on the ground, producing a zone of persistent surface high pressure called the polar high. Hence, option C is correct.

Trade winds are large-scale, persistent winds that blow from the subtropical high-pressure belts toward the equatorial low-pressure belt. Their direction is modified by the Coriolis effect, producing northeast trades in the Northern Hemisphere and southeast trades in the Southern Hemisphere. They are therefore planetary winds, so option D is correct.

Trade winds move from the subtropical high-pressure belts toward the equatorial low-pressure belt. Because Earth rotates, the Coriolis effect deflects moving air to the right in the Northern Hemisphere. Therefore, these winds approach the Equator from the north-east and are called north-east trade winds. They are a major part of the planetary wind system.

Trade winds flow from subtropical high pressure toward the equatorial low-pressure belt. In the Southern Hemisphere, the Coriolis effect deflects moving air to the left. As a result, the winds reach the Equator from the south-east and are called south-east trade winds. They are persistent planetary winds, not merely local winds.

Strong heating near the Equator makes warm, moist air rise in the Intertropical Convergence Zone. At upper levels, this air spreads away from the Equator toward both hemispheric poles. This poleward movement forms the upper branch of the Hadley circulation cells. Cooling and subsidence later occur near the subtropical latitudes.

Air that rises near the Equator moves poleward at high altitude and gradually cools. Around 30° north and south, it descends toward the surface. This subsiding air increases the weight and pressure of the atmosphere near the ground, producing the subtropical high-pressure belts. These belts help generate both trade winds and westerlies.

General circulation is the worldwide movement of air produced by unequal heating, pressure differences, Earth’s rotation, and the distribution of land and water. It transports heat from low latitudes toward higher latitudes and redistributes moisture. Consequently, it strongly influences temperature, rainfall, seasonal conditions, and the formation of regional climates.

Earth’s rotation causes the Coriolis effect, an apparent deflection of moving air relative to the rotating surface. Winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The effect is weak at the Equator and strongest toward the poles, and it helps determine the direction of planetary winds.

When air descends, it accumulates and exerts greater force on the surface below. This increases atmospheric pressure and generally produces a high-pressure area. Descending air also tends to warm by compression, reducing relative humidity and often suppressing cloud formation and rainfall. Subtropical high-pressure belts are a major example of this process.

When air rises, less air remains over the surface, so the weight of the air column and the surface pressure decrease. Rising air expands and cools, often reaching condensation levels where clouds and precipitation can form. Equatorial low pressure and subpolar low pressure are important examples associated with persistent or frequent upward motion.

The global wind pattern results from the general circulation of the atmosphere. Unequal solar heating creates pressure belts, while Earth’s rotation deflects moving air through the Coriolis effect. The Hadley, Ferrel, and Polar cells, together with land–sea contrasts and seasonal shifts, produce the major planetary winds: trade winds, westerlies, and polar easterlies.

Westerlies are the prevailing planetary winds of the middle latitudes. They flow generally from the subtropical high-pressure belts near 30° latitude toward the subpolar low-pressure belts near 60°. Earth’s rotation deflects them, so their direction is broadly from west to east. They influence the weather of the temperate regions.

Strong solar heating near the Equator raises the temperature of the surface air. The air expands, becomes less dense, and rises through convection. Its upward movement leaves relatively lower pressure at the surface, helping to produce the equatorial low-pressure belt. Therefore, warm air rising is the direct result.

Very low temperatures at the poles cool the air near the surface. Cold air contracts and becomes denser than warmer surrounding air, so gravity causes it to sink. This descending air produces relatively high surface pressure and forms an important part of the global pressure and wind circulation. Hence option D is correct.

Polar easterlies originate in the very cold, high-pressure regions around the poles. Because the source air is cold and generally contains little water vapour, these winds are characteristically cold and dry. They move generally from the east toward the west at high latitudes under the influence of pressure gradients and Earth’s rotation. Thus option A is correct.

Unequal solar heating creates temperature differences from the Equator toward the poles. These thermal differences produce broad pressure belts, and air moves between areas of different pressure. Earth’s rotation deflects the moving air, helping organize it into prevailing wind belts such as trade winds, westerlies, and polar easterlies. Therefore option A is correct.

The idealized three-cell model presents the broad global circulation of the atmosphere in each hemisphere. It represents the Hadley cell in the tropics, the Ferrel cell in the middle latitudes, and the Polar cell near the poles. Together, these cells help explain the major pressure belts and prevailing planetary winds, so option A is correct.

A low-pressure area has relatively less air mass per unit area at the surface than its surroundings. Air therefore moves horizontally from nearby higher-pressure regions toward the low-pressure centre, while rising motion commonly occurs over the low. Convergence and uplift can encourage cloud formation and unsettled weather, although the exact weather also depends on moisture and temperature. Hence option A is correct.

At the surface, air moves outward from a high-pressure centre toward surrounding areas of lower pressure. High pressure is commonly associated with descending air, which spreads after reaching the surface. Descending and diverging air generally suppresses cloud development, although local weather depends on moisture, temperature, and other conditions. Thus the correct effect described here is outward spreading, option B.

Land and water heat and cool at different rates. Land usually warms and cools more quickly, while the ocean changes temperature more slowly. This contrast creates alternating pressure differences that can drive local breezes, such as sea and land breezes, and can also support seasonal wind systems such as monsoonal circulation. Therefore option A is correct.