Lecture Notes: Atmospheric Stability

Introduction to Atmospheric Stability

• Atmospheric stability helps determine the likelihood of vertical motion in the atmosphere.
• Stable Atmosphere: Prevents vertical motion.
• Unstable Atmosphere: Allows vertical motion.

Vertical Motion and Adiabatic Processes

• For a parcel to rise, it may cool to the dew point temperature, potentially forming a cloud.
• Adiabatic Process: Temperature changes in a parcel without heat exchange with the environment.
  • Rising parcels expand, increasing volume, decreasing kinetic energy, and lowering temperature.
• Example: Helium balloon rising and expanding until it pops.

Lapse Rates

• Lapse Rate: Change in temperature with height.
• Dry Adiabatic Lapse Rate: Used when air temperature > dew point temperature.
  • Cools at 10°C per kilometer.
• Wet Adiabatic Lapse Rate: Used when air temperature equals dew point temperature (saturated).
  • Cools at around 6°C per kilometer.

Calculating Parcel Temperature Changes

• Example Calculation:
  • Air temperature of 25°C with a dew point of 5°C:
    • At 1 km: Cools to 15°C using dry adiabatic lapse rate.
    • At 2 km: Cools to 5°C using dry adiabatic lapse rate.
  • At dew point, switch to wet adiabatic lapse rate.

Meteorological Graphs and Lifting Condensation Level (LCL)

• Diagrams plot parcel temperature changes:
  • Red Line: Dry adiabatic lapse rate.
  • Blue Line: Wet adiabatic lapse rate.
  • Green Line: Dew point lapse rate.
• Lifting Condensation Level (LCL):
  • Height where parcel cools to dew point, potential base height of a cloud.
  • Marks switch to wet adiabatic lapse rate.

Determining Atmospheric Stability

• Compare parcel's adiabatic temperature to environmental temperature.
• Environmental Lapse Rate (ELR): Measured by weather balloons, averages around 6°C.
  • Stable Atmosphere: Parcel temperature is colder than environment at all heights.
    • Results in sinking parcel, warming, and inability to form clouds.
    • Forecast: Usually sunny and dry.
  • Unstable Atmosphere: Parcel temperature is warmer than environment.
    • Allows rising parcels, often leading to severe weather.
• Conditional Atmosphere:
  • Starts stable, becomes unstable if lifted.
  • Forecasts often predict potential rain with lifting mechanisms.

Weather Implications

• Stable atmospheres lead to clear weather.
• Unstable atmospheres cause extreme weather due to significant cloud growth.
• Conditional atmospheres require lifting mechanisms to potentially cause storms.