A saturated air parcel near the warm tropical surface rises 1 km. Compared to a dry unsaturated parcel rising the same distance, the saturated parcel will:
ACool at the same rate — both follow the first law of thermodynamics without heat exchange
BCool more slowly — condensation releases latent heat into the parcel, partially offsetting adiabatic cooling
CCool more quickly — carrying moisture increases the total heat the parcel must lose
DStop cooling once condensation begins, then resume cooling at the dry rate
As a saturated parcel rises and cools, water vapor condenses and releases latent heat back into the parcel. This heat partially compensates for the adiabatic cooling due to expansion, so the net cooling rate is lower than the dry adiabatic lapse rate (~9.8°C/km). Near warm tropical surfaces where moisture content is high, the SALR can be as low as 3–4°C/km. The cooling does not stop — it just proceeds more slowly than it would without condensation.
Question 2 Multiple Choice
The environmental lapse rate in a particular region is 11°C/km. A dry (unsaturated) air parcel is displaced upward. How will it behave?
AIt accelerates upward — it cools at 9.8°C/km and stays warmer than the surrounding environment, which is cooling faster at 11°C/km
BIt sinks back — it cools faster than the environment and becomes colder than its surroundings
CIt remains neutrally buoyant — the parcel always equilibrates with the environment
DIt rises only until it becomes saturated, then follows the saturated lapse rate
Atmospheric stability is determined by comparing the parcel's cooling rate to the environmental lapse rate. The dry parcel cools at 9.8°C/km; the environment cools at 11°C/km (faster). After rising 1 km, the parcel has cooled 9.8°C but the environment has cooled 11°C — so the parcel is now warmer than its surroundings and positively buoyant. It will accelerate upward. This is absolute instability: the environment cools faster than the DALR.
Question 3 True / False
The saturated adiabatic lapse rate is approximately constant at 5–6°C/km regardless of the temperature or moisture content of the air.
TTrue
FFalse
Answer: False
The SALR is not constant — it varies strongly with temperature and moisture content. Warmer air holds exponentially more moisture (following the Clausius-Clapeyron relation), so a warm saturated parcel releases far more latent heat per kilometer of ascent than a cold one. Near the warm tropical surface, the SALR can be as low as 3–4°C/km. In the cold upper troposphere, where almost no moisture remains, the SALR approaches the DALR (~9.8°C/km). This contrasts with the DALR, which depends only on gravity and the specific heat of dry air and is essentially constant.
Question 4 True / False
Adiabatic lapse rates describe the temperature profile of the environment (how temperature actually varies with altitude in the atmosphere), not the behavior of rising or sinking air parcels.
TTrue
FFalse
Answer: False
This is a key misconception. Adiabatic lapse rates describe how the temperature of an air parcel changes as it moves vertically without exchanging heat with its surroundings. The *environmental* lapse rate is the actual observed temperature profile of the surrounding atmosphere, measured by weather balloons. Atmospheric stability is determined by comparing these two: if the environment cools faster than the parcel, the parcel stays warmer than its surroundings and rises freely. The two rates are distinct and must not be confused.
Question 5 Short Answer
Why is the saturated adiabatic lapse rate lower than the dry adiabatic lapse rate, and why does the SALR vary with temperature while the DALR does not?
Think about your answer, then reveal below.
Model answer: The SALR is lower because condensation releases latent heat into the rising parcel, partially offsetting the adiabatic cooling from expansion. The net cooling rate is therefore less than the DALR. The SALR varies with temperature because warmer air holds exponentially more moisture — a warm saturated parcel releases far more latent heat per kilometer than a cold one, making its effective cooling rate much lower. The DALR is constant because it depends only on gravity and the specific heat of dry air, both of which are essentially fixed across atmospheric conditions.
The difference between DALR and SALR is the engine of convective instability. When the environment cools faster than the SALR, any parcel (wet or dry) is buoyant and convection is vigorous. When the environment cools at a rate between SALR and DALR (conditional instability), only saturated parcels are buoyant — explaining why thunderstorms require moisture and a trigger to force parcels to their condensation level.