Air Cooling in the Atmosphere

Introduction - Lapse Rates

• Discussion of heating and cooling mechanisms in the troposphere.
• Focus on stable air and its cooling pattern.
• Introduction to Normal Lapse Rate (NLR) or Environmental Lapse Rate (ELR).

Reasons for Troposphere Cooling with Altitude

• Bottom-up heating: Energy comes in at the surface, more reradiation near the surface.
• Density changes: More greenhouse gases near the surface, trapping heat.
• Air sinks and warms: Cold air at higher levels descends and warms up.

Normal Lapse Rate (NLR) / Environmental Lapse Rate (ELR)

• Stable Air Day: Temperature decreases ~3.5 degrees Fahrenheit per 1000 ft ascent.
• Example: Hiking in the Grand Canyon - colder at higher elevations and warmer as you descend.

Grand Canyon Example

• North Rim: Highest elevation, coldest temperatures.
• South Rim: Lower elevation, warmer than the North Rim.
• Inner Canyon: Warmest due to lowest elevation.
• The temperature gradient follows the NLR/ELR.

Heat vs. Temperature

• Heat: Total energy, influenced by the number of molecules and the energy they contain.
• Temperature: Average kinetic energy.
• Example with Red Solo Cup vs. a swimming pool.

Adiabatic Processes and Unstable Air

• Adiabatic Rates: Temperature changes due to air expansion/compression.
• Expansion: Air cools as it rises and expands.
• Compression: Air warms as it sinks and compresses.
• Practical example: Fire extinguisher cooling, blowing air on hand (open vs. pursed lips).

Dry Adiabatic Lapse Rate (DALR)

• Conditions: Unstable air, non-saturated humidity.
• Rate: Approx. 5.5 degrees Fahrenheit per 1000 ft.
• Cooling on ascent: -5.5°F per 1000 ft.
• Warming on descent: +5.5°F per 1000 ft.

Calculation Methods

• Using lapse rates for stable and unstable conditions to determine temperature changes with altitude.
• Stable Air Example: Descent of temperature using NLR/ELR.
• Unstable Air Example: Additional cooling using DALR due to expansional cooling.
• Example Comparison: Stable air vs. unstable air temperature calculations at different elevations.

Sinking Air Example

• Sinking air in subtropical regions (like Sahara Desert) causes significant heating.
• Example calculation at tropopause: yields major temperature rise as air compresses and descends.

Adiabatic vs. Environmental Lapse Rates

• Stable Air: Use NLR/ELR (~3.5°F per 1000 ft).
• Unstable Air: Use DALR (~5.5°F per 1000 ft) if unsaturated.
• Impact of compression and expansion on temperature.

Wet Adiabatic Lapse Rate (WALR)

• Conditions: Air is saturated at 100% humidity.
• Impact of Condensation: Releases latent heat, slowing cooling rate.
• Rate: Typically 2.8°F to 3.2°F per 1000 ft; often rounded to 3°F for simplicity in calculations.

Example Calculation: Dry vs. Wet Lapse Rates

• Scenario: Air parcel rising to 12,000 ft with initial temp of 90°F, reaching dew point at 8,000 ft.
• Steps:
  1. Calculate cooling using DALR from 0 to 8,000 ft.
  2. Calculate cooling using WALR from 8,000 to 12,000 ft.
  3. Combine to find final temperature.

Summary

• Importance of selecting the correct lapse rates for temperature calculations (NLR/ELR, DALR, WALR).
• Differences in temperature change mechanisms for stable vs. unstable air.
• Adiabatic processes critical in meteorology for understanding atmospheric temperature profiles.