12.4: Review and Additional Resources

Summary

Review and assess your learning. Start with the "Important Terms and Concepts" to ensure you know the terminology related to the topic of the chapter and concepts discussed. Move on to the "Review Questions" to answer critical thinking questions about concepts and processes discussed in the chapter. Finally, test your overall understanding by taking the "Self-assessment quiz". 

Important Terms and Concepts

• Wind 

  The horizontal movement of air relative to the Earth's surface, flowing from zones of high pressure to zones of low pressure.

• Atmospheric Pressure

  The force per unit area exerted by the weight of the air column directly overlying a specific location.

• Global (Planetary) Winds

  Large-scale, permanent atmospheric circulation cells driven by uneven solar heating and the Coriolis effect (e.g., Polar Easterlies).

• Local Winds

  Mesoscale or microscale wind systems generated by local topography or differential heating between land and water (e.g., Chinook, Föhn).

• Hadley Cell

  A low-latitude convection cell where air rises at the equator, moves poleward, and sinks at roughly 30° North and South latitude.

• Ferrel Cell

  A mid-latitude atmospheric convection cell spanning from roughly 30° to 50°–60° North and South latitude.

• Polar Cell

  A high-latitude convection cell spanning from 50°–60° latitude up to the North and South Poles (90°).

• Trade Winds

  Planetary winds blowing from the subtropical highs toward the equatorial low, deflected westward by the Coriolis effect.

• Westerlies

  Mid-latitude planetary winds blowing poleward from the subtropical highs toward the subpolar lows, deflected eastward.

• Polar Easterlies

  Cold planetary winds blowing westward from the polar highs toward the subpolar lows.

• Pressure Gradient Force

  The primary driver of wind, created by differences in atmospheric pressure between two locations; acts at a right angle to isobars.

• Advection

  The horizontal transport of properties (like heat or moisture) within an air mass by the wind.

• Centrifugal Force

  An apparent outward force that acts on air masses traveling along a curved pathway, pushing them away from the center of rotation.

• Coriolis Force

  An apparent deflection of moving air caused by Earth's rotation, turning winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

• Friction

  The resistance encountered when wind interacts with Earth's surface terrain, which slows wind speed and diminishes the Coriolis effect.

• Geostrophic Wind

  A theoretical upper-atmosphere wind that blows at a constant speed parallel to straight, evenly spaced isobars, occurring when the pressure gradient force perfectly balances the Coriolis force.

• Gradient Wind

  A realistic upper-atmosphere wind model that blows parallel to curved isobars, accounting for the balance between the pressure gradient force, Coriolis force, and centrifugal force.

•  Yardangs 

  Ridge and furrow landscapes in arid regions where hard and soft rocks are present in alternating vertical bands.

• Furrow 

  A long, narrow trench, valley, or corridor carved out between rock ridges by erosion.

• Strata

  Layers of rock or horizontal bands of rock material.

• Denudation 

  The wearing away of the Earth's surface by natural processes like wind and water.

• Zeugen 

  A ridge and furrow landscape formed where rocks are arranged in alternating horizontal bands of hard and soft layers.

• Mesa 

  A flat, table-like land mass with a very resistant horizontal top layer and very steep sides, named after the Spanish word for table.

• Butte 

  An isolated, flat-topped hill formed when continued denudation through the ages reduces the surface area of a mesa.

• Abrasion 

  The mechanical scraping or wearing away of rock surfaces driven by blowing wind.

• Resistant 

  A characteristic of harder rock layers that allows them to withstand erosion and remain standing as ridges.

• Joints 

  Cracks or fractures in rock structures that, when exposed by weathering, become vulnerable to wind erosion.

• Gorge 

  A deep, narrow valley or canyon that often separates mesas in arid countries.

• Fault Blocks 

  A geological structure upon which mesas can form, with Table Mountain in South Africa serving as a primary example

• Corridor 

  A long, narrow valley created by the erosion of softer rock bands, separating steep-sided hard rock ridges.

• Stratum 

  A single distinct layer of rock material.

• Weathering 

  The initial process that opens up top rocks and exposes their joints, making them vulnerable to wind erosion.

• Sand Dune

  An accumulation of loose sand grains piled up and shaped by the action of the wind.

• Stoss Slope (Windward Side)

  The gentle, gradual incline facing the incoming wind. Grains of sand are actively pushed up this slope toward the peak.

• Slip Face (Leeward Side)

  The steep, sheltered downwind slope of a dune where sand collapses under its own weight once it passes the crest.

• Horns (Arms)

  The elongated, tapering tips extending from the sides of crescentic or curved dunes.

• Erg (Sand Sea)

  A vast, flat desert basin covered with sweeping, undulating waves of wind-blown sand, spanning up to tens of thousands of square kilometers.

Review Questions \(\PageIndex{1}\)

By international meteorological convention, how are winds named?

Answer

Winds are designated by the direction from which they originate (their source) rather than the direction toward which they blow.

What are the two structural categories into which global atmospheric circulation is broadly divided?

Answer

Global (or Planetary) Winds and Local Winds.

How is atmospheric pressure formally defined?

Answer

It is defined as the force per unit area exerted by the weight of the air column directly overlying a specific location.

According to the text, what five forces influence the speed or direction of horizontal winds?

Answer

Pressure gradient force, advection, centrifugal force, Coriolis force, and friction.

What causes a localized zone of low atmospheric pressure to form at the surface?

Answer

When thermal energy is introduced to an air parcel, the molecules move faster and the parcel expands, becoming less dense. The lighter air parcel ascends (lifts away), reducing the total mass of the air column pressing down on the ground..

How does the strength of the Coriolis force vary across the globe?

Answer

Its strength increases from zero at the equator to its maximum at the poles.

Why do surface winds cross lines of equal pressure at an angle rather than blowing parallel to them?

Answer

Friction from the Earth's surface slows down the wind speed, which diminishes the impact of the Coriolis force, preventing the wind from blowing completely parallel to the pressure lines.

What is the name of the low-pressure zone located at the equator (0° latitude)?

Answer

The Equatorial Low or the Intertropical Convergence Zone (ITCZ).

What creates the theoretical approximation known as the geostrophic wind?

Answer

It occurs when the pressure gradient force and the Coriolis force reach an exact equilibrium while acting in opposite directions, causing air to blow at a constant speed parallel to straight isobars.

In what type of rock arrangement do yardangs form?

Answer

Yardangs form where hard and soft rocks are present in alternating vertical bands.

How does the rock arrangement of a zeugen differ from that of a yardang?

Answer

A zeugen forms from alternating horizontal bands of hard and soft rocks, whereas a yardang forms from alternating vertical bands.

Explain the mechanical process of saltation as described in the text.

Answer

Saltation is a hopping or bouncing motion where wind lifts sand grains or pebbles briefly when its energy is high, but drops them back down as its energy drops. When a settling sand grain impacts the surface, it strikes other grains, launching them into the air to continue moving downwind.

What distinguishes the shape and movement of a barchan dune from a parabolic dune?

Answer

A barchan dune is a crescent-shaped dune whose elongated tips or horns point downwind in the direction of movement, occurring in dry areas lacking plants. In contrast, a parabolic dune is U-shaped with its horns pointing backward into the upwind direction because the outer arms are anchored in place by moisture and vegetation.

What environmental hazard do migrating barchans pose, and how do communities combat it?

Answer

Migrating barchans pose a major threat by encroaching on oases, where they can completely bury homes, palm trees, and fertile agricultural land. To halt this advance, communities plant long-rooted, sand-holding trees and specialized grasses to anchor the soil and block the shifting sand.

How do longitudinal dunes form, and how does their growth differ from transverse dunes?

Answer

Longitudinal dunes form parallel to the wind axis in regions where winds blow from more than one direction. Unlike transverse dunes, which migrate laterally across the desert floor perpendicular to the wind, longitudinal dunes do not migrate; instead, they lengthen in size over time.

What wind conditions are required to produce a star dune, and how does it grow?

Answer

Star dunes form in sandy ergs where complex wind systems blow from several different directions. Because the changing winds pile sand onto multiple sides of the structure, it is unable to collapse in a single direction, causing the dune to grow vertically into a pyramidal mound rather than moving laterally.

What unique mechanical property does loess possess, and what structural feature allows this to happen?

Answer

Loess possesses the unique mechanical property of vertical cleavage, which allows it to maintain exceptionally stable, near-vertical walls and cliffs when cut. This occurs because the windblown silt particles are remarkably uniform in size, densely packed, and have a high degree of vertical porosity.

What causes the strong, turbulent wind currents found in deserts during summer months?

Answer

During the summer, the bare desert surface gets intensely hot from solar radiation. The air above the ground heats up, expands, and rises rapidly, which creates a localized low-pressure zone at the surface. Strong, turbulent winds are generated when cooler, denser air from surrounding high-pressure systems rushes inward to fill this low-pressure void.

How does a soil moisture deficit make desert sand and minerals more vulnerable to wind displacement?

Answer

Because arid regions have very little rain and high evaporation rates, desert soils are severely dehydrated. Without water to form a liquid film between individual particles, there is no surface tension or cohesive bonding to hold them together. As a result, the loose sand grains and minerals sit unattached on the surface, making it easy for the wind to lift and move them.

Self-Assessment Quiz \(\PageIndex{1}\)

1. A wind that blows from the west toward the east is designated as a:
   1. Easterly wind
   2. Westerly wind
   3. Northerly wind
   4. Leeward wind
2. Which cell spans mid-latitudes from roughly 30° to 50°–60° North and South latitude?
   1. Hadley Cell
   2. Polar Cell
   3. Ferrel Cell
   4. Chinook Cell
3. When an air parcel experiences localized thermal cooling, what is the immediate structural result?
   1. The air parcel expands and its volumetric density decreases.
   2. The air parcel contracts in volume and its volumetric density increases.
   3. The molecules gain kinetic energy and ascend rapidly.
   4. The parcel becomes completely unaffected by gravity.
4. Which force acts as the primary driver of wind by moving air from higher pressure to lower pressure areas?
   1. Coriolis force
   2. Centrifugal force
   3. Advection
   4. Pressure gradient force
5. The apparent deflection that turns moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere is caused by the:
   1. Centrifugal force
   2. Rotation of the Earth (Coriolis force)
   3. Surface friction of rough terrain
   4. Thermal compaction of air parcels
6. The Subtropical High Pressure Belt, located around 30° North and South latitude, is characterized by:
   1. Warm, rising air forming continuous storms
   2. Freezing air colliding with the polar front
   3. Dry, sinking air from the Hadley and Ferrel cells
   4. Rising air driven by intense equatorial solar heating

7. Planetary winds blowing between the Subtropical Highs and the Subpolar Lows that are deflected eastward are called:
   1. Trade Winds
   2. Polar Easterlies
   3. Northeast Trades
   4. Westerlies
8. Winds in the upper atmosphere are primarily governed by the balance between which forces?
   1. Friction and Centrifugal force
   2. Pressure gradient force and Coriolis force
   3. Advection and Friction
   4. Surface terrain and Gravity
9. What type of wind is characterized by air curving around a low-pressure center (cyclone) and blowing slower than it would under purely geostrophic conditions?
   1. Supergeostrophic wind
   2. Subgeostrophic wind
   3. Planetary wind
   4. Trade wind
10. Around 60° North and South latitude, the collision of cold polar air and warmer subtropical air forces the warmer air upward, creating the:
    1. Subtropical High Pressure Belt
    2. Equatorial Low
    3. Subpolar Low Pressure Belt
    4. Polar High Pressure Belt
11. How does a lack of water film impact individual desert soil particles and sand grains?
    1. It increases surface tension, locking the grains into a rigid matrix.
    2. It triggers rapid potential evaporation within the rock minerals.
    3. It removes the cohesive bonds needed to keep surface sediments attached to one another.
    4. It creates structural obstacles that absorb kinetic energy from eolian currents.
12. During desert summer months, what primary atmospheric change occurs immediately after the ground absorbs intense solar radiation and heats the air directly above it?
    1. The air contracts, becomes denser, and sinks to the ground.
    2. The air expands, becomes less dense, and ascends rapidly.
    3. The air creates a massive localized high-pressure system.
    4. The air loses its kinetic energy and slows down wind currents.
13. Why does the absence of tall trees specifically contribute to higher wind velocities across arid landscapes?
    1. It eliminates structural barriers that would otherwise absorb kinetic energy.
    2. It increases the potential evaporation rate across the surface.
    3. It prevents localized thermal low-pressure zones from ascending.
    4. It forces surrounding high-pressure systems to sink into the topsoil.
14. Why does the mechanical wearing down of solid rock formations via wind abrasion occur most intensely in the zone located 10 to 40 centimeters above the desert floor?
    1. Turbulent wind currents lose all their velocity and kinetic energy above 40 centimeters.
    2. Heavier, highly abrasive quartz sand grains travel close to the ground through saltation.
    3. Deflation exclusively removes fine-grained particles like clay and silt at this specific height.
    4. Human activity and overgrazing have artificially lowered the bedrock profile in arid basins.
15. Which physical mechanism of wind erosion is uniquely characterized by the continuous mid-air collision and friction between moving sediments, resulting in smoother and smaller particles?
    1. Deflation
    2. Abrasion
    3. Attrition
    4. Desertification

Answer

1. B
2. C
3. B
4. D
5. B
6. C
7. D
8. B
9. B
10. C
11. C
12. B
13. A
14. B
15. C

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