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Atmospheric Stability and Weather Systems

Atmospheric Stability

  • Stable Parcel: If a parcel of air is displaced vertically, it will return to its original position.
  • Neutral Stability: If a displaced parcel remains in its new position, it is in neutral stability.
  • Unstable Parcel: If a displaced parcel continues to accelerate away from its original position, it is unstable.

Comparison of Moist vs. Dry Air

  • Surface Temperature Increase: An increase in temperature leads to a greater surface pressure exertion from dry air compared to moist air due to added water vapor (H₂O).
  • Molecular Weights: Dry air consists of Nitrogen (N₂) and Oxygen (O₂); moist air has H₂O, which reduces pressure at the same volume.

Stuve Diagram Analysis

  • Basics: Stuve diagrams are used to analyze the vertical structure of the atmosphere, plotting temperature and dew point.
  • Wind Speed: 50 knots = 1.15 mph, represented at various pressure levels from 1000 mb to 100 mb.
  • Free Convection Level: The pressure level at which a lifted parcel is warmer than its environment, often around 425 mb in scenarios.

Atmospheric Soundings and Environmental Lapse Rate (ELR)

  • Temperature and Dew Points: Identified on soundings as thick black lines (temperature) and thin gray lines (lifted parcel temperature).
  • Lifting Condensation Level (LCL): The pressure level where clouds form, identified by the temperature and dew point intersection on a sounding.
  • Parcel at 275 mb: can ascend and cross dew point at interaction.

Adiabatic Processes

  • Dry Adiabatic Lapse Rate (DALR): Cool rising air cools at 10°C/km; sinking air warms at 10°C/km.
  • Moist Adiabatic Lapse Rate (MALR): Varies with altitude, approximately 6°C/km in lower troposphere and 4°C/km in upper troposphere when saturated.

High vs. Low Pressure Systems

  • High Pressure Systems: Comprised of dry air, characterized by stable weather patterns.
  • Low Pressure Systems: Comprised of moist air, leading to cloud formation and potential precipitation.

Forces and Force Balances

  1. Pressure Gradient Force (PGF): The force that initiates wind, strongest with a steep gradient.
    • Vertical PGF: Gravity balances the vertical differences.
    • Horizontal PGF: Initiates horizontal wind movement.
  2. Gravity: Acts downward, strongest near the surface.
  3. Friction: Affects wind speed and direction near the surface based on roughness and density.
  4. Coriolis Effect: Influences the direction of winds, with right deflections in the Northern Hemisphere and left in the Southern Hemisphere.

Geostrophic Balance and Jet Streams

  • Geostrophic Wind: Occurs in balance with PGF and Coriolis force at higher altitudes, leading to jet streams.
  • Types of Jet Streams: Subtropical, polar, and arctic jets; jet streaks indicate areas of strong winds within these streams.

Fronts

  1. Cold Front: Characterized by colder air mass moving to displace warmer air, often associated with thunderstorms.
  2. Warm Front: Warmer air rises over colder air, resulting in rain varieties depending on temperature stability and moisture.
  3. Stationary Front: Cold and warm air masses meet, leading to prolonged rain events.
  4. Occluded Front: Weather transitions between cold and warm fronts can lead to complex precipitation patterns.
  5. Dry Lines: Often represent a strong boundary between moist and dry air masses, potentially leading to severe thunderstorms.
  6. Upper Level Fronts: Influence weather patterns through descending air, with minimal surface changes.