General circulation of the atmosphere and planetary winds

Westerlies are the prevailing planetary winds of the middle latitudes, generally found between about 30° and 60° 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. Their exact path can vary because of pressure systems and landforms, but west-to-east is the standard global pattern. Option B is correct.

Pressure belts and wind direction are the foundation for understanding general atmospheric circulation. Unequal heating creates pressure differences, and air moves from high-pressure areas toward low-pressure areas. The arrangement of these belts, together with the effect of Earth’s rotation, determines the direction and broad pattern of planetary winds.

The broad, recurring arrangement of global pressure belts produces persistent pressure gradients across the latitudes. These gradients help maintain the regular planetary wind systems, such as trade winds, westerlies, and polar easterlies. Earth’s rotation modifies their direction through the Coriolis effect, but the pressure-belt arrangement is the basic controlling factor.

General circulation redistributes energy received unevenly on Earth. The tropics receive more solar heating than the polar regions, so atmospheric and oceanic circulation transports sensible and latent heat away from warmer low latitudes toward cooler higher latitudes. This reduces, though it does not eliminate, the global temperature contrast.

Trade winds are planetary winds because they blow persistently and over extensive tropical regions as part of the global circulation system. Sea breezes and valley winds are local or periodic winds, while the loo is a regional, hot, seasonal wind. Therefore, trade winds are the best example among the given choices.

Unequal solar heating produces temperature differences on Earth. Temperature differences lead to differences in air density and atmospheric pressure, and the resulting pressure gradient makes air move. Thus, the linked sequence of unequal heating, temperature contrast, pressure contrast, and wind movement is the basic driving mechanism of atmospheric circulation.

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. The effect is weakest at the equator and strongest toward the poles, helping shape global wind belts and circulation.

Trade winds originate in the subtropical high-pressure belts, located approximately around 30° north and south, and blow toward the equatorial low-pressure belt. The pressure gradient drives this movement, while the Coriolis effect deflects the winds, producing northeast trades in the Northern Hemisphere and southeast trades in the Southern Hemisphere.

The northeast and southeast trade winds converge in the equatorial region, forming the Inter-Tropical Convergence Zone, or ITCZ. Converging air is forced to rise, leading to cloud formation, convection, and frequent heavy rainfall. The ITCZ shifts seasonally with the apparent movement of the Sun and is an important part of global circulation.

In the Northern Hemisphere, air moves from the subtropical high-pressure belt toward the equatorial low-pressure belt. The Coriolis effect deflects this equatorward flow to the right, giving it a north-easterly direction. Therefore, these winds are called the northeast trade winds. In the Southern Hemisphere, the corresponding winds are southeasterly.

In the Southern Hemisphere, air flows from the subtropical high-pressure belt toward the equatorial low-pressure belt. The Coriolis effect deflects this movement to the left, so the winds approach from the southeast. They are therefore called the southeast trade winds, in contrast to the northeast trades of the Northern Hemisphere.

Westerlies are the prevailing planetary winds of the middle latitudes. They move from the subtropical high-pressure belts, located near 30° north and south, toward the subpolar low-pressure belts near 60°. Earth’s rotation deflects them, but their pressure-gradient direction is from subtropical high pressure to subpolar low pressure. Therefore, option A is correct.

Westerlies are named according to the direction from which they originate: they come from the west and generally move toward the east. They occur mainly between the subtropical high-pressure belts and the subpolar low-pressure belts in both hemispheres. Although the Coriolis effect modifies their exact path, the usual direction remains west to east, making option B correct.

Polar easterlies originate in the polar high-pressure belts. At the poles, intense cooling makes the air very cold and dense, so it descends and creates high pressure at the surface. Air then flows outward toward the subpolar low-pressure belt. Earth’s rotation gives these winds an easterly component, but their source is the polar high-pressure area. Thus option C is correct.

Polar easterlies flow outward from the polar high-pressure belts toward the subpolar low-pressure belts, which are situated near 60° north and south. The pressure gradient drives the surface flow, while the Coriolis effect changes its exact direction and produces the easterly character. They do not normally travel directly to the equator or to the subtropical high-pressure belt. Therefore, option A is correct.

In the equatorial region, strong and nearly year-round solar heating warms the surface and the air above it. Warm air becomes less dense and rises by convection, producing the equatorial low-pressure belt and the Intertropical Convergence Zone. At higher levels it moves away from the equator as part of the Hadley circulation. Therefore, option C is correct.

After rising near the equator, air moves poleward in the upper troposphere and descends around 30° north and south. Descending air compresses and warms, which suppresses cloud formation and increases surface pressure. This creates and maintains the subtropical high-pressure belts, associated with many of the world’s dry deserts. Hence, option D is correct.

The Ferrel cell occupies the middle latitudes, roughly between 30° and 60° in both hemispheres. Its surface winds generally move from the subtropical high-pressure belt toward the subpolar low-pressure belt and are deflected by the Coriolis effect. These prevailing west-to-east winds are called westerlies. Trade winds belong mainly to the Hadley cell, so option A is correct.

In the polar cell, intense cooling over the polar surface makes the air extremely cold and dense. This dense air sinks near the poles, producing the polar high-pressure belts. At the surface, it then flows toward the subpolar low-pressure zone as polar easterlies. The sinking branch is therefore located near the poles, not at the equator or only over oceans. Option B 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. Together, these cells explain the major planetary pressure belts and prevailing winds. Thus option A is the only correct group.

The Coriolis effect is nearly zero at the equator because its strength depends on the sine of latitude, and the sine of 0° is zero. It becomes progressively stronger toward the poles. Therefore, moving air and ocean currents are deflected least at the equator and most at high latitudes.

The Coriolis effect results from Earth’s rotation and deflects moving objects such as winds and ocean currents. It is weakest at the equator and becomes progressively stronger as latitude increases, reaching its greatest value at the poles. Hence, high latitudes show it most clearly.

The governing concept is the global pressure-belt system and the rising branch of the Hadley circulation. The doldrums occur in a broad zone near the Equator, where strong solar heating warms the surface and causes air to rise. Persistent rising air lowers surface pressure and produces weak, variable winds; this is why the belt is also called the equatorial low-pressure belt. The Inter-Tropical Convergence Zone generally lies within or near this zone, where trade winds converge. Polar highs and subtropical highs are high-pressure belts, while the subpolar low is located much farther from the Equator. Hence option D is correct.

Horse latitudes occur near 30° north and south, within the subtropical high-pressure belts. Air descends in these belts, producing stable conditions and generally weak or calm surface winds. These calm conditions historically affected sailing ships, which is why the region became known as the horse latitudes. Thus, option A is correct.

In subtropical high-pressure belts, air descends after moving poleward at the upper levels of the Hadley circulation. Descending air becomes warmer and drier through compression, suppressing cloud formation and rainfall. This explains the dry climates and many major deserts near 30° latitude.