lecture recording on 05 March 2025 at 10.20.18 AM

Air Parcel Behavior

• Air parcels behave differently based on their interaction with the atmosphere.

• Rising parcels perform work, leading to a decrease in temperature.

  • Adiabatic cooling: temperature decreases at a rate of approximately 10 degrees Celsius per kilometer (or 1 degree per 100 meters).

• Descending parcels experience compression, which leads to an increase in temperature. This is averaged, as it may not be a constant rate.

• These processes are reversible.

Lapse Rates

• Dry Adiabatic Lapse Rate (DALR): Rate at which an unsaturated air parcel cools while ascending.

  • Approximately 10 degrees Celsius per kilometer.

• Saturated Adiabatic Lapse Rate (SALR): Rate at which a saturated air parcel cools as it ascends; approximately 6 degrees Celsius per kilometer.

  • The rate varies due to latent heat released during condensation.

Lifting Condensation Level (LCL)

• The altitude where the air parcel's temperature equals its dew point, causing saturation and cloud formation.

• Temperature decreases at a rate of 1.8 degrees Celsius per kilometer as the parcel rises and approaches the dew point.

• Intersection of temperature and dew point lines on diagrams indicates saturation.

Atmospheric Diagrams

• Key diagrams in the study involve characteristics of air parcels and their elevation changes:

  • Emagrams and Tephigrams represent essential thermodynamic properties but won't be the focus of the course.

• Important to understand how to interpret these diagrams as they can predict air parcel behavior and weather phenomena.

  • Includes isobars (pressure), isotherms (temperature), and water vapor content (mixing ratio).

Effects of Precipitation on Air Temperature

• As a parcel rises, it cools at the DALR until it reaches the LCL; beyond that, cooling occurs at the SALR.

• If precipitation occurs, it can extract moisture from the air, altering the air's original temperature and humidity.

• Dry conditions can lead to a warmer parcel descending from a mountain range due to lost moisture through precipitation.

  • Comparison of initial water vapor in the parcel before and after precipitation highlights changes in temperature and relative humidity.

Example Scenario

• Starting with a parcel of air at 25 degrees Celsius and 11 grams of water vapor per kilogram:

  • As it rises, it cools by DALR until reaching the LCL.

  • If it precipitates on the way down, it may end up with significantly less water vapor and higher temperature on descent.

  • Finished air parcel at ground level may be significantly warmer and drier than the original parcel due to condensation.

Atmospheric Stability

• Understanding the cooling rates of rising and descending air parcels is crucial for determining atmospheric stability.

• Air parcels that cool less rapidly than the surrounding atmosphere can be destabilizing, leading to weather events like storms.

• The balance between DALR and SALR helps predict whether conditions are conducive to cloud formation and precipitation.