On a calm morning, an air parcel is lifted to its lifting condensation level. Before saturation, it cools at 9.8°C/km (dry rate) and is denser than its surroundings — stable. After saturation, it cools at 5°C/km while the environment cools at 7°C/km. What happens?
AThe parcel remains stable because it is still cooler than the environment when averaged over the whole ascent
BThe parcel becomes positively buoyant — it now cools more slowly than the environment, so it is warmer than its surroundings and accelerates upward
CThe environment's lapse rate must change before the parcel can become unstable
DThe parcel stabilizes again once latent heat is exhausted and the dry rate resumes
This is conditional instability in action. A parcel is buoyant when it is warmer than its surroundings at the same altitude. Unsaturated, the parcel cools at 9.8°C/km while the environment cools at 7°C/km — the parcel cools faster, becoming denser and sinking back. Saturated, the parcel cools at 5°C/km — slower than the environment's 7°C/km — so the parcel is progressively warmer than its surroundings and accelerates upward. The atmosphere hasn't changed; the parcel's lapse rate changed when condensation began releasing latent heat. This is the trigger for deep convection and thunderstorm development.
Question 2 Multiple Choice
Why is the moist adiabatic lapse rate significantly lower (gentler) in warm tropical air than in cold polar air?
ATropical air has higher pressure aloft, which reduces the rate of adiabatic expansion and therefore cooling
BWarm air holds much more water vapor; more vapor condenses per kilometer of ascent, releasing more latent heat that partially offsets adiabatic cooling
CThe Coriolis effect in tropical regions deflects rising parcels horizontally, reducing vertical cooling
DTropical air contains more CO₂, which absorbs the heat released during condensation
The moist lapse rate is variable precisely because it depends on how much latent heat is released per kilometer of ascent — and that depends on how much water vapor is available to condense. Warm air near the tropics can hold far more water vapor than cold polar air (the Clausius-Clapeyron relationship makes water vapor content strongly temperature-dependent). A rising tropical parcel condenses more moisture per kilometer, releases more latent heat, and thus cools more slowly — perhaps 4–5°C/km. A rising polar parcel condenses little moisture, releases little latent heat, and its moist lapse rate approaches the dry rate of 9.8°C/km.
Question 3 True / False
The moist adiabatic lapse rate averages about 6°C/km but varies depending on temperature and moisture content — unlike the dry adiabatic lapse rate, which is essentially constant.
TTrue
FFalse
Answer: True
The dry adiabatic lapse rate (9.8°C/km) depends only on the specific heat of dry air and gravitational acceleration — both nearly constant. The moist rate adds a latent heat term whose magnitude varies with how much water vapor condenses per unit of ascent. Since water vapor content depends strongly on temperature (warm air holds more), the moist lapse rate is steepest in cold, dry air (approaching the dry rate) and gentlest in warm, humid air (as low as 4°C/km in the tropics). This variability is why the moist rate is described as approximately 6°C/km rather than a precise constant.
Question 4 True / False
The moist adiabatic lapse rate is a fixed constant of approximately 6°C/km, similar to how the dry adiabatic lapse rate is fixed at approximately 9.8°C/km.
TTrue
FFalse
Answer: False
This is one of the most common misconceptions about lapse rates. The dry rate is fixed because it depends only on constants (specific heat of dry air, gravitational acceleration). The moist rate is variable because the latent heat released per kilometer of ascent depends on how much water vapor condenses — which depends on temperature. A tropical surface parcel may cool at only 4–5°C/km; a cold high-latitude parcel may approach 9°C/km. Treating the moist rate as a fixed constant leads to errors in stability analysis, especially when comparing tropical and polar atmospheric profiles.
Question 5 Short Answer
Why does the difference between the dry and moist adiabatic lapse rates create 'conditional instability' in the atmosphere?
Think about your answer, then reveal below.
Model answer: An atmosphere is conditionally unstable when its environmental lapse rate falls between the dry (~9.8°C/km) and moist (~6°C/km) rates. In this condition, an unsaturated parcel cools faster than the environment (dry rate > environmental rate) and is stable. But once the parcel reaches saturation — its condensation level — it switches to the lower moist rate and now cools more slowly than the environment. The parcel becomes warmer than its surroundings, positively buoyant, and accelerates upward without further forcing. The condition is the saturation threshold; stability is conditional on whether the parcel is dry or saturated.
This mechanism explains how a clear-sky morning can produce towering afternoon thunderstorms. Surface heating lifts parcels; once they reach their lifting condensation level and saturation, the latent heat engine takes over and drives explosive vertical development. The atmosphere hasn't changed — what changed is the parcel's phase from unsaturated to saturated, crossing the threshold from stable to unstable behavior.