Dry Adiabatic Process

Knowing the rate at which rising air cools is vital in determining the stability of the atmosphere. We have briefly introduced dry, moist, and saturated adiabatic processes in previous readings, but because these concepts are so important to the discussion on atmospheric stability, we will take a few minutes to review them, as well as introduce a couple of others that are of equal importance.

The Dry Adiabatic Process

When a parcel of air rises, it expands, and the temperature decreases. Likewise, when air sinks, it compresses, and the temperature increases. This phenomenon occurs without adding or withdrawing energy from the parcel and is illustrated by the equation of state and Poisson's equation. When a parcel of air, either dry or containing water vapor, rises or sinks without the addition or extraction of heat, that process is said to be a dry adiabatic process. Even though a parcel of air may contain moisture, if the parcel is rising, then it cools according to the dry adiabatic lapse rate until it reaches the dew point temperature (Td).A comparsion of the dry and moist adabatic laspe rates (Note: The dew point temperature for a rising air parcel is not equal to the dew point temperature of the same parcel at the surface. As the parcel rises, the dew point temperature decreases slightly in response to the decrease in pressure.) We refer to the pressure where the actual temperature equals the dew point temperature as the Lifting Condensation Level (LCL). At the LCL, the cooling process becomes a moist or saturated adiabatic process.
Quick Quiz: At what point does the dry adiabatic lapse rate become the moist (saturated) adiabatic lapse rate?
at the tropopause
at the lifting condensation level
at a height of 500-mb pressure
all of the above

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