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Difficulty: HardAtmosphere and Weather

An unsaturated air parcel at sea level is forced to rise over an approaching mountain ridge. As the parcel ascends, it expands and cools adiabatically until it reaches its dew point temperature at the lifting condensation level, after which thick clouds begin to form as the parcel continues upward. How does the rate of cooling of the air parcel change after condensation begins compared to before condensation occurred, and what is the physical reason for this change?

  1. A
    The cooling rate increases because the phase change of water vapor into liquid droplets absorbs thermal energy directly from the surrounding air parcel.
  2. B
    The cooling rate increases because newly formed water droplets conduct heat away from the air parcel into the colder surrounding atmosphere.
  3. The cooling rate decreases because the condensation of water vapor releases latent heat into the parcel, partially offsetting adiabatic expansion cooling.Answer
  4. D
    The cooling rate stops completely because the saturated air parcel transitions into a localized high-pressure system that maintains a constant temperature.

Answer

The cooling rate decreases because the condensation of water vapor releases latent heat into the parcel, partially offsetting adiabatic expansion cooling.
As an unsaturated parcel of air rises, it expands and cools at the dry adiabatic rate. Once it reaches its dew point, condensation occurs. Because the transition from vapor to liquid releases latent heat, this added thermal energy counteracts some of the cooling caused by expansion. Consequently, the parcel cools at a slower rate (the moist or saturated adiabatic lapse rate) once cloud formation begins.

Step-by-Step Solution

1
Analyze the thermal behavior of the unsaturated rising air parcel before reaching the lifting condensation level.
Before saturation, the parcel cools purely by adiabatic expansion at the dry adiabatic lapse rate of approximately 9.8∘C9.8^\circ\text{C} per kilometer (5.5∘F5.5^\circ\text{F} per 1000 ft1{}000\text{ ft}).
Lower atmospheric pressure at higher altitudes allows the rising air parcel to expand and do work on its surroundings, expending internal energy.
2
Determine the thermodynamic effect of the phase change occurring at and above the lifting condensation level.
When the air reaches its dew point, water vapor condenses into liquid water droplets, releasing latent heat of condensation into the parcel.
Condensation is an exothermic phase transition (g→lg \rightarrow l) where energy stored in molecular bonds is released into the immediate environment as sensible heat.
3
Compare the resulting saturated (moist) adiabatic lapse rate to the dry adiabatic lapse rate.
The release of latent heat partially counteracts the cooling from ongoing expansion, yielding a reduced lapse rate (the moist adiabatic lapse rate, typically 4∘C4^\circ\text{C} to 7∘C7^\circ\text{C} per kilometer).
Sensible heat addition slows the net temperature drop per unit of vertical ascent.

Key Concept

Dry vs. Moist Adiabatic Lapse Rates and Latent Heat Release during Condensation
Estimated Time:1m 30s
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Topics

Atmosphere and Weather

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