General circulation of the atmosphere and planetary winds

D is correct. A terminal moraine is a ridge of glacial debris deposited at the glacier’s snout. It marks the glacier’s farthest advance during a particular period, often where the glacier paused before retreating.

Direct answer: A, the troposphere. The troposphere is the lowest major layer of Earth’s atmosphere and begins at the surface. It contains most of the atmosphere’s water vapour and dust, so clouds, rain, winds, storms, and most everyday weather develop there. Moving upward, the stratosphere lies above the troposphere, followed by the mesosphere; the exosphere is the outermost and extremely thin region. Option A is correct because it names the layer directly touching Earth’s surface. Option B is wrong because the stratosphere is above the troposphere. Option C is wrong because the mesosphere is higher still. Option D is wrong because the exosphere is the outermost layer, not the lowest. Memory cue: “tropo” is the weather layer at the top of the ground.

Direct answer: B, the troposphere. The troposphere is the lowest atmospheric layer and contains most of the atmosphere’s water vapour, dust, and relatively dense air. Earth’s surface warms the air above it, producing convection and vertical mixing. As moist air rises, it may cool, condense, and form clouds, rain, snow, winds, and storms. Therefore most familiar weather events occur in the troposphere. Option A is wrong because the stratosphere is above the troposphere and is comparatively stable, with much less ordinary weather activity. Option C is wrong because the mesosphere is much higher and is not the main weather layer. Option D is wrong because the thermosphere is still higher and has extremely thin air. Memory cue: troposphere is the “weather sphere.”

Direct answer: B, the troposphere. It is the lowest atmospheric layer and contains most of the atmosphere’s water vapour, clouds and much of its dust. Unequal heating causes convection: warm air rises, cooler air sinks, and this movement helps produce winds, clouds, rain and storms. Therefore it is the main weather-making layer. A, the stratosphere, is comparatively stable and contains much less weather activity. C, the mesosphere, is much higher and is known for very low temperatures and meteor-related phenomena rather than ordinary surface weather. D, the thermosphere is also far higher and has very thin, highly energetic gases. Memory cue: “tropo” is the layer of change and weather close to Earth.

Direct answer: A, the troposphere. Climate and daily weather depend strongly on air movement, water vapour, clouds, and exchanges of heat near Earth’s surface. The troposphere is the lowest layer and contains most of the atmosphere’s mass and nearly all its water vapour. Unequal heating creates convection, while condensation of water vapour produces clouds and precipitation. Winds, storms, and many other everyday weather events therefore occur mainly in this layer. B, the exosphere, is too thin and far from the surface for ordinary weather. C, the upper thermosphere is also extremely rarefied and is not the main region of cloud and rain formation. D, the mesopause is only a boundary, not a weather-making layer. Memory cue: “Tropo is the layer of change—weather changes happen there.”

Answer: A. The troposphere is the lowest major atmospheric layer and contains most of the atmosphere’s water vapour. Unequal heating of Earth’s surface causes convection and produces rising and sinking air, winds, clouds, precipitation, storms, and many other daily weather processes. Weather forecasting therefore depends heavily on observations of temperature, pressure, humidity, wind, and cloud movement in this layer. B is wrong because satellites occupy many different orbits and are not all fixed in the troposphere. C is wrong because ozone depletion is not exclusive to the troposphere. D is wrong because the exosphere lies far above the troposphere. Memory cue: almost all familiar day-to-day weather is tropospheric weather.

The Sun heats Earth’s surface unevenly because of differences in latitude, surface conditions, seasons, and the angle at which sunlight arrives. Unequal heating creates temperature contrasts and differences in air pressure. Air then moves from areas of relatively higher pressure toward areas of lower pressure, while Earth’s rotation modifies its direction. Thus, unequal solar heating is the fundamental energy source behind general atmospheric circulation.

Planetary winds are large-scale, nearly permanent wind systems that operate across broad belts of the Earth. Their pattern is produced mainly by global pressure belts, unequal solar heating, and the deflecting effect of Earth’s rotation. The major examples are the trade winds, the westerlies, and the polar easterlies. They differ from local or periodic winds, which change over smaller areas or shorter time intervals.

Trade winds are part of the tropical planetary-wind system. At the surface, air moves outward from the subtropical high-pressure belts near 30° latitude toward the equatorial low-pressure belt. Earth’s rotation deflects these winds: they become north-easterly in the Northern Hemisphere and south-easterly in the Southern Hemisphere. Therefore, their pressure-belt movement is from subtropical highs toward the equatorial low.

The westerlies are the prevailing planetary winds of the middle-latitude belt, extending approximately from 30° to 60° in both hemispheres. They generally blow from the subtropical high-pressure belts toward the subpolar low-pressure belts. Because of the Coriolis effect, their direction has a strong west-to-east component. They are important in the movement of weather systems across the temperate regions.

Polar easterlies originate in the cold, dense air of the polar high-pressure belts near the poles. This air flows toward the subpolar low-pressure belts around 60° latitude. Earth’s rotation deflects the flow, giving it an easterly character, so the winds are called polar easterlies. The pressure-gradient direction is therefore from the polar high-pressure area toward the subpolar low-pressure area.

The Coriolis force is an apparent force associated with Earth’s rotation. It does not create the original movement of air, but it changes the direction of moving air and ocean currents. Deflection is toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere; it is negligible at the equator and stronger toward the poles. This effect helps shape global wind belts.

Because Earth rotates, the Coriolis effect changes the apparent path of moving air. In the Northern Hemisphere, the deflection is to the right of the direction in which the air is travelling; in the Southern Hemisphere it is to the left. The force acts at right angles to the motion and influences wind direction without simply stopping or vertically lifting the air. Therefore, option B is correct.

The Coriolis effect results from Earth’s rotation and acts on moving air rather than on stationary air. In the Southern Hemisphere, moving air is deflected to the left of its direction of travel. The corresponding rule in the Northern Hemisphere is deflection to the right. The exact path also depends on the original wind direction, so the effect should not be described as always pointing north or east. Hence option B is correct.

The Doldrums are the belt of light, variable, and often calm winds near the equatorial low-pressure belt. Strong heating causes air to rise there, producing low pressure and frequent cloud formation and rainfall. The northeast and southeast trade winds converge in this region, so horizontal surface winds are weak. Therefore, option C is correct. It should not be confused with the trade-wind belts, westerlies, or polar regions.

The Intertropical Convergence Zone, or ITCZ, is a near-equatorial belt where the northeast trade winds of the Northern Hemisphere meet the southeast trade winds of the Southern Hemisphere. Their convergence forces warm, moist air to rise, producing clouds, convection, and substantial rainfall. The ITCZ shifts seasonally with the apparent movement of the Sun. Thus, option A correctly describes its main atmospheric feature.

The Hadley cell extends roughly from the equator to 30° latitude in each hemisphere. Intense heating near the equator makes air rise and creates an equatorial low-pressure belt. Aloft, this air moves toward the subtropics, descends there, and forms subtropical high pressure. At the surface, air returns from the subtropical highs toward the equatorial low, becoming the northeast and southeast trade winds. Hence, option A is correct.

Air mainly rises in the equatorial branch of the Hadley cell because the surface receives strong and relatively direct solar heating throughout the year. The heated air expands, becomes less dense, and rises, producing the equatorial low-pressure belt. It later moves poleward aloft before descending near 30° latitude. The subtropical high-pressure belt is therefore a sinking branch, not the main rising branch. Option B is correct.

Within the Hadley cell, air rises near the equator and moves poleward in the upper troposphere. Around 25° to 35° latitude, it cools sufficiently and descends, creating the subtropical high-pressure belts. This sinking air is associated with dry and generally clear conditions in many subtropical regions, including major desert belts. The equatorial zone is a rising, low-pressure region, so option A is the correct answer.

The Ferrel cell occupies the middle-latitude belt, approximately from 30° to 60° in both hemispheres. It lies between the Hadley cell nearer the tropics and the Polar cell nearer the poles. Unlike the thermally direct Hadley and Polar cells, the Ferrel cell is an indirect circulation cell strongly influenced by travelling cyclones and anticyclones. Surface westerlies are characteristic of this belt. Therefore, option B is correct.

At the poles, intense cooling makes the air very cold, dense, and heavy. It sinks and produces a surface area of high pressure. Air then flows outward from this polar high toward the subpolar low-pressure belt near 60° latitude. The Coriolis effect deflects this surface flow, producing polar easterlies. Consequently, the pressure source of the near-surface flow is the polar high, so option A is correct.

A wind is named according to the direction from which it originates, not the direction toward which it travels. Easterlies therefore come from the east and usually move toward the west. This naming convention applies to planetary winds such as polar easterlies and tropical easterlies. It does not mean that the winds occur only in eastern countries or blow only at night. Thus, option A correctly states the basis of the name.

Westerlies are named because their source direction is the west: they blow from west toward east. The name describes where the wind comes from, just as easterlies come from the east. In the global circulation system, prevailing westerlies are especially associated with the mid-latitudes and are influenced by the pressure gradient between subtropical highs and subpolar lows. Their name does not refer to countries, coastlines, or a straight path from the poles. Option A is correct.

The Northern Hemisphere trade winds are called the northeast trade winds because they originate from the northeast and generally blow toward the equatorial low-pressure belt. They begin near the subtropical high-pressure belt and are deflected by the Coriolis effect to the right of their path. In the Southern Hemisphere, the corresponding winds are the southeast trades. Therefore, option B is the correct answer.

The trade winds in the Southern Hemisphere blow from the subtropical high-pressure belt toward the equatorial low-pressure belt. Because of the Coriolis effect, they are deflected to the left and generally move from the south-east toward the north-west. Therefore, they are called south-east trade winds.