Meteorological question about stability

[NecPluribusImpar]

Hello,

I'm doing my ATPL training, and already finished half of the exams for the authorities. As I was studying meteo, I encountered this question:
Question: The stability in a layer increases by advection of

1. moist air in the lower part
2. warm air in the lower part
3. cold air in the lower part
4. dry air in the upper part

As the answer in my book is very doubtful to me, I was wondering if somebody else could answer this question. Can you also give a small explication how you come to your answer.

Thanks in advance

[FrenchScotPilot]

I'll give it a try.


The stability of the atmosphere depends on its ability to resist vertical motion.

There are 2 things to have a look at: moisture advection and temperature advection.

a. Moisture advection:
Since water vapour is lighter than air, moisture decreases air density, causing it to rise. Since moist air cools at a slower rate, it is generally less stable than dry air since the moist air must rise higher before its temperature cools to that of the surrounding air. The dry adiabatic lapse rate (unsaturated air) is 3°C per 1,000 feet. The moist adiabatic lapse rate varies from 1.1°C to 2.8°C per 1,000 feet.

It’s not answer 1.

Dry air is heavier and will want to sink, therefore dry air aloft decreases stability as less dry air below will tend to rise and replace it.

It’s not answer 4.

b. Temperature advection:
Advecting warm air in the lower parts of the troposphere will make the parcel warmer than its surrounding therefore, it’ll keep rising.

It’s not answer 2.

Conclusion:
The combination of moisture and temperature determine the stability of the air. Cool, dry air is very stable and resists vertical movement. The greatest instability occurs when the air is moist and warm, as it is in the tropical regions in the summer.

Correct answer is 3.

Regards,
FSP

[NecPluribusImpar]

The answer you gave is also the correct one as my book gave me. Thank you very much for this information!