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.