GEOL1340 Review Module 4 Spring22

Main Ideas

1. Forces That Control Wind

The review identifies four forces acting on the atmosphere:

• Gravity acts on air, though the lecture focuses mainly on the other three forces.
• Pressure-gradient force moves air from high pressure toward low pressure. Isobars are lines of equal pressure; when they are close together, the pressure gradient and winds are stronger.
• Coriolis force is the apparent deflection caused by Earth’s rotation:
  • To the right in the Northern Hemisphere.
  • To the left in the Southern Hemisphere.
  • There is no Coriolis deflection at the Equator.
• Friction slows winds near Earth’s surface. It matters near the ground but becomes negligible higher in the atmosphere.

In the middle and upper troposphere, pressure-gradient force and Coriolis force balance, producing geostrophic winds that flow roughly parallel to isobars. Near the surface, friction changes that balance, so winds cross isobars at an angle.

2. Wind Patterns Around Highs and Lows

Surface winds move outward from high-pressure areas and inward toward low-pressure areas:

• In the Northern Hemisphere, winds spiral clockwise and diverge around highs (anticyclones), and spiral counterclockwise and converge around lows (cyclones).
• In the Southern Hemisphere, the rotational directions are reversed.
• In three dimensions, air sinks in high-pressure systems and rises in low-pressure systems. Air that rises over a low eventually spreads outward aloft; air that sinks over a high spreads outward near the surface.

3. Global Pressure Belts and Prevailing Winds

The lecture describes several major pressure belts:

• Equator — Intertropical Convergence Zone (ITCZ), or doldrums: Warm air rises where the trade winds converge. Rising air cools, forming clouds and precipitation, which helps explain the wet conditions and tropical rainforests near the Equator.
• Around 30° north and south — subtropical highs: Air descends, warms, and becomes dry. Many of the world’s major deserts lie in these belts.
• Around 60° north and south — subpolar lows and the polar front: Warmer westerly air meets colder polar air. Their interaction can produce clouds and precipitation.
• Polar regions — polar highs: Cold air sinks and spreads outward as polar easterlies.

Surface air flowing back toward the Equator from subtropical highs forms the trade winds. Air flowing poleward from those highs forms the westerlies. Polar easterlies meet the westerlies near the polar front.

4. Jet Streams and Rossby Waves

Two major bands of fast upper-level winds are discussed:

• The subtropical jet stream, near 30° latitude.
• The polar jet stream, near the boundary between cold polar air and warmer air farther south.

Both generally flow from west to east, shift northward in summer and southward in winter, and help steer storm systems. Rossby waves are large north–south undulations in the flow near the polar front. When these waves become pronounced, they can carry very cold air far south. The lecture relates this pattern to a past Arctic cold spell in the United States and the associated Texas power outages.

5. Isobaric Pressure Surfaces

An isobaric surface connects locations with the same atmospheric pressure. These surfaces are not flat; they form ridges and troughs.

• High-pressure areas correspond to ridges in pressure surfaces.
• Low-pressure areas correspond to troughs.
• Upper-level geostrophic winds flow around these features, generally clockwise around highs and counterclockwise around lows in the Northern Hemisphere.

6. Local Winds Caused by Temperature Differences

The lecture reviews four types of local winds:

• Land and sea breezes
  • By day, land heats faster than water. Air rises over land, drawing surface air from sea to land.
  • At night, land cools faster than water. Air rises over the relatively warmer water, and surface winds blow from land out to sea.
• Mountain and valley breezes
  • By day, heated air rises upslope from the valley.
  • At night, cooled air flows downslope into the valley.
• Catabatic winds
  • These are downslope winds. The Santa Ana winds are an example: air flows from high pressure over the Great Basin toward Southern California, warms as it descends, and can accelerate through constricted valleys. The lecture also mentions winds flowing off Antarctic ice sheets.
• Monsoonal winds
  • In winter, land is cooler than the ocean; surface winds blow from land toward the ocean, and conditions are relatively dry.
  • In summer, land warms more than the ocean; moist air flows from sea to land, bringing heavy rainfall.

7. Ocean Circulation

The lecture distinguishes three broad ocean layers:

• Surface water, whose temperature varies with latitude and season.
• Intermediate water, where temperature decreases with depth.
• Deep water, which is cold and relatively uniform in temperature.

Two sources of deep water are described:

• North Atlantic Deep Water, formed as cold water sinks in the Labrador, Greenland, and Icelandic seas.
• Antarctic Bottom Water, formed as cold water sinks near Antarctica and flows beneath North Atlantic Deep Water.

The lecture presents these currents as part of a large-scale circulation loop: deep water eventually warms and rises, returning to the surface and joining warmer currents.

8. Ocean Gyres and the Equatorial Pacific

Large surface-current loops, or gyres, are driven by prevailing winds and the Coriolis effect. In general:

• Northern Hemisphere gyres circulate clockwise.
• Southern Hemisphere gyres circulate counterclockwise.

The review also compares normal equatorial Pacific conditions with La Niña and El Niño:

• Normal conditions: Trade winds push surface water westward. Water rises near the coast of South America, bringing nutrient-rich deep water to the surface and supporting productive fishing. Rainfall is concentrated farther west, including Australia and Southeast Asia.
• La Niña: The normal pattern intensifies, with stronger westward trade winds.
• El Niño: The pressure pattern shifts and trade winds weaken or reverse. Surface water moves eastward and becomes warmer near South America. This suppresses upwelling there and is associated with drought in Australia and Southeast Asia.

Review Questions and Key Takeaways

The lecture’s true-or-false and labeling questions reinforce that:

• Winds move from high toward low pressure, but pressure-gradient force alone does not make winds flow parallel to isobars.
• Surface winds diverge from highs and converge into lows; the rotational direction depends on the hemisphere.
• Sea breezes blow onshore by day and offshore at night.
• Summer monsoon winds blow from ocean to land.
• The El Niño pattern is distinct from normal Pacific conditions.
• The lecture recommends using the Module 4 animations to visualize circulation, jet streams, Rossby waves, and El Niño/La Niña.

Speakers and Sources Featured

• Instructor: Jennifer Lytwyn, identified by the channel metadata; referred to in the subtitles as the professor.
• Students: Unidentified students ask questions and respond to the review questions.
• Visual sources referenced: Lecture slides, diagrams, and animations from Module 4.