Topic 3_Ch 8_"Adiatbatic Processes and Atmospheric Stability"

Chapter 8: Adiabatic Lapse Rates and Atmospheric Stability

Adiabatic Processes

  • Rising Air Parcel:

    • Expands as it rises, leading to cooling.

    • Temperature decreases because it does work.

  • Sinking Air Parcel:

    • Compressed as it sinks, causing temperature to increase.

    • Temperature increases because work is being done on it.

  • Reversibility:

    • Adiabatic processes are reversible.

Dry Adiabatic Lapse Rate (DALR)

  • A rising, unsaturated parcel of air cools at a constant rate of 10°C/km until temperature (T) equals the dew point temperature (Td).

  • A sinking, unsaturated parcel heats at the same rate.

Saturated Adiabatic Lapse Rate (SALR)

  • Saturation: When saturated air rises, water vapor condenses, releasing latent heat.

  • Cooling Rate: The rate of cooling is not constant; it decreases due to the release of latent heat.

  • Average Cooling Rate: An average value of 6°C/km is often used.

Lifting Condensation Level (LCL)

  • As air rises, Td decreases at the dew-point lapse rate of 1.8°C/km.

  • LCL occurs when temperature equals Td; at this height, clouds begin to form.

Thermodynamic Diagrams

  • Emagrams: These diagrams display five sets of lines that provide information about mixing ratios and temperature relationships.

Environmental Lapse Rate (ELR)

  • Defined as the change in temperature with height within the atmosphere.

  • The average ELR in the troposphere is approximately 6.5°C/km.

  • The actual ELR can vary significantly in time and space, derived from atmospheric soundings and plotted on a tephigram for weather forecasting.

Atmospheric Stability

  • Definition: Measures the tendency of air to move vertically after being disturbed.

  • Classification of Air Types:

    1. Unstable

    2. Stable

    3. Neutral

Stability Types

  • Stable Air:

    • Rising parcel becomes cooler than surrounding air, pushed downwards.

    • Stable if: ELR < DALR (dry air) or ELR < SALR (saturated air).

  • Unstable Air:

    • Rising parcel stays warmer than surrounding air, pushed further upward.

    • Unstable if: ELR > DALR (dry air) or ELR > SALR (saturated air).

  • Neutral Air:

    • Displaced air parcel remains at the same temperature as its surroundings.

    • Neutral if: ELR = DALR (dry air) or ELR = SALR (saturated air).

Conditionally Unstable Environment

  • Saturated air is unstable and rises; dry air is stable but may become unstable if forced to rise (Level of Free Convection - LFC).

Factors Influencing Stability

  • Stability can increase if ELR increases due to:

    • Surface warming (e.g., clear-day heating).

    • Cooling at the top of the air layer (e.g., from radiative cooling).

  • Stability can also increase if ELR decreases due to:

    • Cooling of surface air (e.g., nocturnal radiative cooling).

    • Warming of the air layer at the top (e.g., warm front advecting over cold air).

Changes in Stability with Height

  • Daytime Conditions: Typically lead to increased instability due to heating.

  • Nighttime Conditions: Generally stable due to cooling effects.