General circulation of the atmosphere and planetary winds

Strong surface winds require a substantial horizontal pressure gradient. In the equatorial low-pressure belt, intense and fairly uniform heating causes air to rise through convection, while the horizontal pressure differences near the surface are relatively small. With weak horizontal forcing, surface winds remain light and variable; this calm region is traditionally called the doldrums. Option A correctly links weak pressure gradient with strong vertical convection.

A simple one-cell model assumes that warm equatorial air rises, travels directly toward the poles, sinks, and returns at the surface. Earth’s rotation introduces Coriolis deflection, while the unequal heating of different latitudes creates several pressure belts. These processes divide each hemisphere into Hadley, Ferrel, and Polar cells, so a three-cell model represents global circulation more realistically.

The Hadley cell is driven by strong heating near the equator. Air rises in the equatorial low-pressure belt, moves poleward aloft, descends near about 30° latitude, and then returns at the surface toward the equatorial low. The near-surface return flow is the trade-wind system, so it moves from subtropical highs toward the equator.

The Ferrel cell occupies the middle latitudes between the Hadley and Polar cells. Unlike those thermally direct cells, its circulation is not produced simply by heating at one boundary and cooling at another. It is an indirect, dynamically maintained circulation shaped by the surrounding cells, pressure gradients, and mid-latitude westerlies.

In the Hadley circulation, air rises near the equator and travels poleward in the upper troposphere. Around 25°–35° latitude, this air descends. Subsidence compresses and warms the air, producing a stable, dry condition and increasing surface pressure. This descending branch is the main process forming the subtropical high-pressure belts.

The Intertropical Convergence Zone forms where intense solar heating produces rising air and a broad equatorial low-pressure region. During the year, the zone of maximum overhead solar radiation migrates north and south between the tropics. The ITCZ follows this shifting thermal equator, usually with a seasonal lag and a stronger displacement over land.

Wind names indicate the direction from which the wind originates, not the direction toward which it travels. In the Northern Hemisphere, air moves from the subtropical high-pressure belt toward the equatorial low-pressure belt. Earth’s rotation deflects this southward flow to the right, giving it a north-easterly direction: from the north-east toward the south-west.

Westerlies occur mainly between the subtropical high-pressure belts and the subpolar low-pressure belts. Their basic pressure-gradient flow is toward higher latitudes, but Earth’s rotation deflects the moving air. The resulting prevailing wind has a west-to-east component in both hemispheres, although local and seasonal variations can alter its exact direction.

The subpolar low-pressure belt lies near about 60° latitude, where the relatively warm westerlies from lower latitudes meet the cold polar easterlies. Their convergence forces air to rise, lowering surface pressure and supporting frontal activity and frequent cyclonic weather. Thus, wind convergence is the principal circulation-based cause of this belt.

Walker circulation is an east–west, or zonal, circulation pattern along the equatorial Pacific and other tropical regions. Differences in sea-surface temperature and pressure produce rising air over warmer waters, upper-level return flow, sinking over cooler waters, and near-surface easterly flow. Its longitudinal arrangement distinguishes it from the mainly north–south Hadley circulation.

A geostrophic wind occurs when the pressure-gradient force pushing air toward lower pressure is balanced by the Coriolis force deflecting the moving air. This ideal balance is most applicable above the frictional boundary layer. As a result, the wind flows approximately parallel to isobars rather than directly across them, with speed related to the pressure gradient.

Near the surface, friction with terrain, vegetation, buildings, and other obstacles slows the wind. Because the Coriolis force depends on wind speed, it also becomes weaker. The pressure-gradient force then has a stronger influence, so the wind acquires a component directed toward lower pressure and crosses isobars at an angle. Coriolis force does not disappear entirely.

Unequal solar heating creates pressure differences and initiates atmospheric motion, but it does not explain the complete wind pattern. Earth’s rotation produces the Coriolis effect, which deflects moving air, while surface friction slows and changes near-surface winds. Land, sea, and relief also modify circulation locally.

The tropical easterly jet is a fast east-to-west current in the upper troposphere that develops over the tropical region during the Northern Hemisphere summer. Its appearance is associated with the monsoon circulation and the heated Tibetan Plateau. It is an upper-air feature, not a local surface breeze.

During the winter season, the subtropical westerly jet lies south of the Himalayas and helps steer temperate low-pressure systems known as western disturbances. These systems travel eastward and may bring winter rain to northwestern India and snowfall in the Himalayan region, which is important for crops and water supply.

The three-cell model of atmospheric circulation is an idealized representation. In reality, the apparent movement of the Sun changes the seasonal distribution of heating. Land–sea contrasts, mountains, ocean temperatures, and changing pressure patterns also disturb the ideal arrangement. Consequently, pressure belts and the boundaries of the Hadley, Ferrel, and Polar cells shift northward or southward during the year rather than remaining fixed at particular latitudes.

Water vapour is an important carrier of energy in the atmosphere. Evaporation absorbs latent heat from the surface, stores that energy in water vapour, and transports it with moving air. When the vapour rises and condenses into clouds, latent heat is released. This release warms the surrounding air, strengthens convection, and influences tropical circulation, rainfall, and storm development.

Near the surface, friction slows the wind and makes it cross isobars at an angle toward lower pressure. If friction were absent, the pressure-gradient force and Coriolis force could approach a balance, producing geostrophic flow. In that ideal condition, the wind would move approximately parallel to isobars rather than directly from high pressure to low pressure.

The subpolar belt is associated with a broad zone of low pressure and the polar front. Here, relatively warm air moving poleward with the westerlies meets cold, dense air moving equatorward with the polar easterlies. Their strong temperature contrast creates instability, frontal lifting, and frequent development of temperate cyclones. As a result, weather is often cloudy, windy, and changeable, making the zone a stormy belt.

Earth’s rotation generates the Coriolis effect, which deflects moving air and helps produce several broad circulation cells in each hemisphere. In a non-rotating Earth model, this deflection would disappear. Heated air would rise near the equator, move more directly toward the poles aloft, descend there, and return equatorward near the surface.

The subpolar low-pressure belt develops near about 60° latitude where the westerlies from the subtropical high-pressure belt meet the polar easterlies from the polar high-pressure belt. Their convergence forces air to rise, producing low pressure and frequent cyclonic activity. Therefore, option A is correct; equatorial heating and polar radiation explain different pressure belts.

Air flows equatorward from the subtropical high-pressure belt toward the equatorial low-pressure belt. In the Northern Hemisphere, the Coriolis effect deflects this moving air to the right. Consequently, the wind approaches the equator from the northeast and is named the northeast trade wind. Option A is correct.

In the Southern Hemisphere, air moves generally northward from the subtropical high-pressure belt toward the equatorial low-pressure belt. The Coriolis effect deflects moving air to the left, so the flow approaches the equator from the southeast. This produces the southeast trade winds, making option A correct.

Horse latitudes lie near the subtropical high-pressure belts, around 30° north and south. Air that has risen near the equator moves poleward aloft and then descends in these regions. Descending air warms adiabatically, lowers relative humidity, suppresses cloud formation, and often produces light surface winds. Therefore, option A is correct.

Strong solar heating near the equator causes warm air to rise and move poleward in the upper troposphere. In the Hadley cell, this upper branch transports sensible and latent energy from the equatorial region toward the subtropics, where the air eventually descends. The surface return flow is different, so option A is correct.