Atmospheric Circulation: Coriolis Effect, Global Winds, and Deserts
Course Information and Logistics
Office Hours: Rescheduled to Friday at noon; original Tuesday session cancelled due to travel. Office located on the Fifth Floor of the building.
Review Session: Via Zoom, link on Canvas, on September 23 (Tuesday evening) at 5:30 ext{ PM}, the evening before the exam.
Exam: Will cover topics one through eight, inclusively.
Learning Objective: Understand how the atmosphere works and its role in weather and climate.
Atmospheric Circulation: Recap and Drivers
Earth View: Atmospheric circulation cells with distinct belts of high pressure (purple) and low pressure (gray).
Non-rotating Earth: Would have only one circulation cell in each hemisphere.
Rotating Earth: Results in three circulation cells in each hemisphere.
Pressure Definitions:
Low Pressure: Air is rising above a point on the Earth's surface.
High Pressure: Air pressure is coming down on a point on the Earth's surface.
The Coriolis Effect: An Experimental Demonstration
Experiment Setup: Volunteers Richard (receiver), Phoebe (thrower), and Kenzie staged a football throwing experiment in the classroom.
"Omaha" Play (Standing Still):
Phoebe threw while standing still.
Observation: One perfect throw, one short, both online (on target).
"Blue" Play (Phoebe Running):
Phoebe ran (approximately 16 ext{ mph}) while throwing the ball.
Observation: Throws were consistently off target, specifically to the right of Richard, the intended target (e.g., three seats over, ten seats over).
Interpretation/Inference (Conservation of Momentum):
Phoebe's running imparted momentum to the ball, moving it horizontally in her direction of motion.
When she threw the ball, it received a second vector of force towards the target.
The ball, upon leaving her hand, still retained the momentum from her running motion.
The combined effect of these two vectors (one towards the target, one from her horizontal movement) resulted in the ball being deflected to the right of the intended target.
Analogy to Earth's Rotation:
Imagine Phoebe running at the Equator (rotational speed of 1000 ext{ mph}) and throwing a ball towards Richard at the North Pole (rotational speed of 0 ext{ mph} relative to Phoebe).
The same principle of deflection to the right would apply, but on a much larger scale.
The Coriolis Force and its Global Impact
Definition: A deflective force acting on moving objects (like air masses and ocean currents) on a rotating body.
Direction of Deflection:
Northern Hemisphere: Objects moving through the atmosphere are deflected to the right of their direction of motion.
Southern Hemisphere: Objects moving are deflected to the left of their direction of motion (a mirror image).
Scale of Effect:
Significant: On large spatial scales (planetary or hemispheric) and over long time scales (hours to days).
Insignificant: On small scales (like a classroom experiment or the flushing of a toilet) and short time scales (a few seconds).
Impact on Circulation Cells: This force is responsible for breaking up the single circulation cell per hemisphere (expected on a non-rotating Earth) into the observed three cells.
Misconceptions:
Toilet Flushing Direction: The Coriolis effect does not influence the direction toilets flush; this is a common misconception due to the small spatial and temporal scales involved.
Earth's Distance and Seasons: Another common misconception (even held by educated individuals) is that the Earth's distance from the sun controls seasons, rather than axial tilt.
Jupiter as an Example:
Jupiter, a rapidly rotating planet (about 2.5 times faster than Earth), exhibits more atmospheric circulation cells (six cells per hemisphere).
This demonstrates the universality of the Coriolis effect, where faster rotation leads to a greater number of circulation cells.
Jupiter's Great Red Spot is a persistent high-pressure storm, observed since the 1830s.
Global Pressure Belts and Surface Wind Patterns
Idealized View: Earth shows continuous belts of high pressure at 30^ ext{o} N/S latitude and 90^ ext{o} (poles), and low pressure at the Equator (0^ ext{o}) and 60^ ext{o} N/S latitude.
Complication by Continents: Landmasses disrupt these continuous belts, resulting in more localized high and low pressure systems, especially over continents (e.g., at 30^ ext{o} N).
Major Surface Wind Patterns (Named by Origin):
Westerlies: Prevail between 30^ ext{o} and 60^ ext{o} latitude, blowing from west to east.
Northeast Trade Winds: Prevail between the Equator ( 0^ ext{o}) and 30^ ext{o} North, blowing from the northeast to the southwest.
Regions of Calm or Difficult Sailing:
The Doldrums: Located near the Equator (0^ ext{o}). Air rises vertically, leading to very little horizontal wind, making sailing difficult.
The Horse Latitudes: Located around 30^ ext{o} N/S latitude. Air sinks vertically, also leading to very little horizontal wind and difficult sailing conditions.
Etymology: Named because early European explorers, stuck in these calm regions and running low on fresh water and provisions, would jettison their horses overboard to lighten the load and conserve resources. Jim Morrison's father, a marine, recounted tales related to this.
Moisture Transport, Rainfall, and Deserts
Tropical/Equatorial Regions:
Warmest ocean water leads to high evaporation, forming warm, moist air masses.
Warm air rises, and as it ascends, it cools.
Cooling reduces the air's capacity to hold moisture, leading to condensation and heavy rainfall in the tropics.
Air Movement Aloft:
At the top of the troposphere, the air diverges, flowing poleward (towards 30^ ext{o} N/S).
As this air moves towards the poles, it continues to cool.
30^ ext{o} N/S Latitudes:
The cool, dry air descends at these latitudes. As it descends, it warms up.
Warming air increases its capacity to hold moisture.
This results in very dry conditions, as the descending air absorbs moisture rather than releasing it.
Prediction of Desert Locations: Based on this atmospheric circulation and moisture transport, deserts are predicted to occur at approximately 30^ ext{o} N/S latitude and at the poles.
Real-World Confirmation: All major deserts in the world are indeed found near 30^ ext{o} N/S latitude (e.g., Sahara, Arabian, Australian, Atacama, Namib, etc.), validating the atmospheric principles.
Historical Navigation and Modern Applications
Columbus's Voyages (1492):
Columbus, sailing from Spain (Europe) to the Caribbean (Bahamas), utilized the Northeast Trade Winds, which carried his ships southwestward directly to the New World.
For his return journey to Western Europe, he likely used the Westerlies, sailing upwind relative to the trades but downwind relative to the prevailing westerlies.
This demonstrates that ancient mariners, even without explicit knowledge of the Coriolis effect, understood and exploited global wind patterns for navigation.
Modern Sailing Example (Stephen Stills' Song):
The song "Southern Cross" describes a sailing journey from Santa Catalina Island (off Los Angeles) to Tahiti, utilizing the trade winds for a "downhill run" to Papati Bay, possibly via the Marquesas.
This highlights that contemporary sailors also strategically use global wind directions for efficient travel.
Low vs. High Pressure Systems: Surface Characteristics
Low Pressure Systems:
Airflow: Air converges at the ground and rises.
Appearance/Weather: Rising air cools, leading to condensation, cloud formation, and precipitation (e.g., rain, storms).
High Pressure Systems:
Airflow: Air descends from above, hits the ground, and diverges.
Appearance/Weather: Descending air warms, increasing its capacity to hold moisture; thus, no condensation occurs, leading to clear skies and dry weather.
Reconstructing Atmospheric Circulation from First Principles
One can deduce the three-cell atmospheric circulation model without memorization by starting with fundamental principles:
Identify Key Latitudes: Equator (0^ ext{o}), Poles (90^ ext{o}), and the cell boundaries at 30^ ext{o} and 60^ ext{o} latitude.
Equator: Air is warm and rises, then diverges aloft.
Poles: Air is cold and sinks, then diverges at the surface.
Complete Hadley Cell: The rising air at the Equator flows poleward aloft and descends at 30^ ext{o}. Surface flow is back towards the Equator.
Complete Polar Cell: The sinking air at the Pole flows equatorward at the surface and rises at 60^ ext{o}. Aloft flow is back towards the Pole.
Complete Ferrel Cell: The middle cell (30^ ext{o} to 60^ ext{o}) is driven by the adjacent Hadley and Polar cells. Air at 30^ ext{o} sinks (from Hadley), and air at 60^ ext{o} rises (to Polar). This determines the direction of flow within the Ferrell cell, ensuring continuous circulation without air flowing against itself in an unstable manner.
Upcoming Topic
The next class will discuss cyclones, hurricanes, and climate change, including voting on whether to implement a 10 versus 3 proposed solution in class activities.