CA

Atmospheric Circulation, Cyclones, and Hurricanes

Exam Preparation & Course Logistics

  • Office Hours: Rescheduled to noon today at 5.210.
  • Sample Exam Questions:
    • Two versions: one with answer key, one without.
    • Recommended approach: Take the version without the answer key first under some time pressure (doesn't have to be exact exam time).
    • Reasoning: Struggling while solving fosters learning; passively reading answers with solutions already highlighted results in less effort and less effective learning.
    • Exam format: 40 questions in 50 minutes; time pressure will be present but is manageable if accustomed to a finite time limit.

Atmospheric Circulation: Foundational Principles

  • Drivers: Atmospheric circulation explains phenomena like where it's rainy, where deserts form, and temperature distribution (hot/cold areas).
  • Pressure Systems Review:
    • Low Pressure System: Air converges (flows inward), rises. Often associated with condensation and precipitation because rising warm, moist air cools, and cool air holds less moisture, leading to condensation.
    • High Pressure System: Air descends from above and diverges (flows outward) at the surface.
  • Coriolis Effect: The planet's rotation deflects wind pathways. This effect will be applied to understand the rotation of cyclones/hurricanes.

Cyclones, Hurricanes, and Typhoons: Circulation Dynamics

  • Definition: These terms refer to large-scale, low-pressure storm systems characterized by high wind speeds and heavy rains.
  • Visualizing a Low-Pressure System (Northern Hemisphere):
    • Imagine an 'L' for low pressure at the center.
    • Pressure Gradient: In a non-rotating scenario, wind blows from high pressure (margins) to low pressure (center), following the pressure gradient inward (black arrows in the conceptual diagram).
    • Coriolis Effect in Northern Hemisphere: The Earth's rotation deflects these inward-flowing winds to their right.
    • Regional Wind Flow: The combination of inward pressure gradient and rightward deflection results in a counter-clockwise swirl (regional scale blue arrows).
    • Time Scale: These systems take days to weeks to build and are on a regional/continental scale (over 1000 km across); the Coriolis effect is not applicable to small-scale, short-term phenomena like a toilet flushing.
  • Generalized Wind Flow (Northern & Southern Hemispheres):
    • Northern Hemisphere Low Pressure: Surface winds flow inward and counter-clockwise.
    • Northern Hemisphere High Pressure: Surface winds flow outward and clockwise (opposite of low pressure, with rightward deflection).
    • Southern Hemisphere Low Pressure: Surface winds flow inward and clockwise (mirror image of Northern Hemisphere).
    • Southern Hemisphere High Pressure: Surface winds flow outward and counter-clockwise (mirror image of Northern Hemisphere).
  • Global Distribution: Hurricanes/cyclones commonly originate near the Equator and are largely confined to tropical regions, where ocean water temperatures are high.

Driving Forces of Hurricanes

  • Latent Heat: Heat stored or released during a substance's phase change. For hurricanes, the key phase changes are:
    • Evaporation (liquid water to water vapor): Requires energy input to break hydrogen bonds between water molecule (H_2O) atoms (oxygen and hydrogen). This energy is stored as latent heat in the water vapor.
      • Molecular structure: Water molecules are bent at an angle, making them polar (net negative charge on oxygen, positive on hydrogen). Hydrogen bonds form between the hydrogen of one water molecule and the oxygen of another.
      • Breaking these hydrogen bonds during evaporation stores energy.
      • Latent heat of evaporation can be thought of as potential energy.
    • Condensation (water vapor to liquid water): Releases the stored latent heat energy in the form of heat.
  • Sea Surface Temperature (SST): Often called the