General circulation of the atmosphere and planetary winds

Westerlies are the prevailing winds of the mid-latitudes and generally move toward the east. In the Northern Hemisphere, the Coriolis effect deflects moving air to the right. Thus, the wind approaches from the southwest and flows generally toward the northeast or east. The exact direction varies with pressure systems, but southwest is the standard general-circulation answer.

Westerlies move generally from west toward east in the middle latitudes. In the Southern Hemisphere, the Coriolis effect deflects moving air to the left. This causes the prevailing westerly flow to approach from the northwest and move toward the southeast or east. Therefore, northwest is the appropriate general direction, even though individual weather systems can produce temporary variations.

Jet streams are narrow bands of very fast air movement located near the tropopause, generally in the upper troposphere. They form where strong horizontal temperature contrasts create large pressure differences with height, especially near major atmospheric circulation boundaries. Although their exact altitude varies, they are not surface winds, ocean currents, or movements inside the solid Earth.

At the Earth’s surface, air moves because a pressure gradient force acts on it. Air generally flows from areas of higher atmospheric pressure toward areas of lower pressure. The Coriolis force and friction may alter the direction and speed, but they do not change this basic pressure-gradient principle.

The doldrums are the calm or weak-wind zone near the equator, within the equatorial low-pressure belt. Strong heating causes air to rise there, while the northeast and southeast trade winds converge near the Intertropical Convergence Zone. Rising air and weak horizontal winds produce the characteristic calm conditions.

Trade winds are the prevailing easterly winds of the tropics, generally found between the subtropical high-pressure belts and the equatorial low-pressure belt. The northeast trades occur in the Northern Hemisphere and the southeast trades in the Southern Hemisphere. Their regularity results from persistent pressure gradients and Earth’s rotation.

The westerlies are the prevailing winds of the middle latitudes. They generally move from west to east because air flows from the subtropical high-pressure belts toward the subpolar low-pressure belts and is deflected by the Coriolis effect. Their flow is important for the movement of temperate weather systems and cyclones.

The Coriolis effect deflects moving air to the right of its path in the Northern Hemisphere and to the left in the Southern Hemisphere. This apparent deflection results from Earth’s rotation and affects the direction of planetary winds, cyclones, and ocean currents. The effect is weakest at the Equator.

The Sun is the principal energy source for the atmosphere. Because Earth is spherical and its axis is tilted, solar heating is unequal: the equatorial region generally receives more direct energy than the polar regions. This unequal heating creates temperature and pressure contrasts, which drive convection and large-scale atmospheric circulation. Ocean salinity and lunar gravity may influence other processes, but they do not provide the main energy driving planetary winds.

Subtropical high-pressure belts are located approximately around 25°–35° north and south, conventionally described as near 30° in each hemisphere. Air that has risen near the Equator moves poleward at high altitude and descends around these latitudes, producing the subtropical highs. Their descending, relatively dry air is associated with many of the world’s major desert regions. Therefore, option B is correct.

Trade winds are the persistent winds of the tropical circulation cells. They begin in the subtropical high-pressure belts, near 30° north and south, and move toward the equatorial low-pressure belt or ITCZ. Earth’s rotation deflects them to the right in the Northern Hemisphere, producing northeast trade winds, and to the left in the Southern Hemisphere, producing southeast trade winds. The pressure-belt direction is therefore stated correctly in option B.

In the Northern Hemisphere, trade winds move generally from the subtropical high-pressure belt toward the equatorial low-pressure belt. As moving air travels, the Coriolis effect deflects it to the right of its path. Consequently, the winds approach the Equator from the northeast and are called the northeast trade winds. Their overall direction is therefore from northeast toward southwest, making option B correct.

The Coriolis force is an apparent deflective force observed because Earth rotates on its axis. Air and water moving over the rotating surface appear to bend from a straight path: toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. The effect is zero at the Equator and becomes stronger toward the poles. Earth’s revolution around the Sun is not the primary cause, so option B is correct.

The magnitude of the Coriolis effect depends on latitude and is proportional to the sine of latitude. At 0°, the latitude of the Equator, this value is zero, so moving air is not deflected horizontally by the Coriolis force there. The effect gradually increases toward the poles and reaches its maximum at the poles. Therefore, option B is correct; the force is not restricted to daytime or nighttime.

The Intertropical Convergence Zone, or ITCZ, is a belt near the thermal Equator where the northeast and southeast trade winds meet. Their convergence forces warm, moisture-laden air to rise. Rising air cools, condenses, and produces clouds, heavy convection, and frequent rainfall. Thus, the defining event asked in the question is the convergence of trade winds, making option B correct.

The idealised three-cell model divides circulation in each hemisphere into the Hadley, Ferrel, and Polar cells. The Hadley cell occupies roughly 0°–30°, the Ferrel cell approximately 30°–60°, and the Polar cell about 60°–90°. Together, they describe the broad meridional transfer of heat and the associated planetary wind belts. Therefore, option A is correct.

The three major permanent or planetary wind belts are the trade winds, the westerlies and the polar easterlies. They blow with broadly consistent global patterns because of permanent pressure belts and Earth’s rotation. Monsoon, sea, valley, Loo, Chinook and Foehn winds are seasonal, local or regional winds, not permanent planetary winds.

The prevailing westerlies are planetary winds that blow broadly from west to east in the middle latitudes of both hemispheres. Because many travelling cyclones, fronts and pressure systems are embedded in this flow, they are commonly carried from west toward east. Local circulation and unusual weather can create temporary exceptions.

Jet streams are narrow bands of very strong winds found mainly near the tropopause, which is the upper part of the troposphere. Strong horizontal temperature contrasts and the pressure-gradient and Coriolis forces help maintain them. They are not surface winds at sea level and are not geological features of the crust.

Earth’s rotation causes the Coriolis effect, an apparent deflection of moving air relative to the rotating surface. The deflection is to the right of the motion in the Northern Hemisphere and to the left in the Southern Hemisphere. It changes wind direction but does not create the original pressure-driven motion, and its strength increases toward the poles and with wind speed.

Trade winds are the persistent tropical easterlies that blow from the subtropical high-pressure belts toward the equatorial low-pressure belt. They occupy roughly the 0°–30° latitude belts in both hemispheres and form the surface branch of the Hadley cells. Earth’s rotation deflects them to the right in the north and to the left in the south.

The polar regions receive weak, oblique solar radiation and lose heat efficiently, so the air near the surface becomes extremely cold. Cold air is denser and heavier; it subsides and exerts greater pressure on the surface, producing a persistent polar high-pressure belt. This is different from equatorial low pressure, which is mainly linked with intense heating and rising air.

Friction is strongest close to Earth’s surface because air interacts directly with terrain, vegetation, buildings, and other obstacles. Its influence decreases rapidly with height, so winds in the free atmosphere encounter much less resistance and can attain greater speeds. This weak-friction environment helps explain the fast flow of upper-air winds and jet streams.

The first step is to identify the locations of the global pressure belts because the pressure-gradient force drives air from high-pressure belts toward low-pressure belts. After establishing this basic flow, Earth’s rotation and the Coriolis force explain the deflection into trade winds, westerlies, or polar easterlies. Thus, pressure-belt location provides the essential starting framework.

The equatorial region receives intense solar heating throughout the year. Warm, moist air rises through strong convection, creating a belt of low pressure. At the surface, the northeast and southeast trade winds converge toward this zone, producing the calm conditions traditionally called the doldrums. Rising air cools, condenses and forms deep clouds, so frequent convectional rainfall occurs. Thus, surface convergence and vigorous upward convection together explain both the calm air and the rainfall.