seegongsik
Grade 11-12 (age 16-18)

high school Atmospheric Thermodynamics

When a parcel of air rises, the pressure around it drops, it expands, and it cools on its own without exchanging heat with its surroundings; this is adiabatic cooling. As long as no water vapor condenses, it cools about 10 °C for every kilometer, but once it reaches its dew point and clouds start to form, condensation releases heat and it cools only about 5 °C per kilometer. Whether a parcel cools faster or slower than the air around it decides whether the atmosphere is stable or unstable, and the föhn, air that warms as it descends after crossing a mountain, works the same way. In the first diagram, changing the surface temperature changes the height of the cloud base, and in the second, changing the surrounding lapse rate changes the stability of the atmosphere.

Why is it colder up a mountain?

💡 Adiabatic change
①As a parcel of surface air rides up a mountain, the pressure around it drops with height
②With less pressure on it, the parcel expands, and because it uses its own energy to push the outside air away, its temperature falls
③A change in temperature that comes from a change in volume, without exchanging heat with the surroundings, is an adiabatic change
④When a parcel sinks, the opposite happens: it is compressed and warms (adiabatic compression)

Where do clouds form?

25
☁️ How clouds form
①A rising parcel meets lower pressure, expands, and cools
②Before any vapor condenses, it cools at the dry adiabatic lapse rate, about 10 °C per kilometer
③Its dew point also drops about 2 °C per kilometer, so the temperature and the dew point close in on each other by 8 °C per kilometer
④Condensation starts where they meet, and that height is the cloud base, about 125 × (surface temperature − surface dew point) m
⑤Above it, condensation releases heat and the parcel cools at the moist adiabatic lapse rate, only about 5 °C per kilometer

Atmospheric stability

7
25
Absolutely unstable
Γenv > Γdry (about 10 °C/km)
A rising parcel stays warmer than its surroundings and keeps rising, so convection is strong.
Absolutely stable
Γenv < Γmoist (about 5 °C/km)
A rising parcel becomes colder than its surroundings and tends to sink back, so convection is suppressed.
📐 Stability and clouds
①In an unstable atmosphere, air keeps surging upward, cumuliform clouds (cumulus and cumulonimbus) build, and showers or thunderstorms fall
②In a stable atmosphere, air does not rise easily, and stratiform clouds or fog form
③If the environmental lapse rate lies between the moist and dry adiabatic rates, the air is conditionally unstable: stable for unsaturated air but unstable for saturated air
④That is why forecasters check both the lapse rate and the humidity of the atmosphere to judge whether showers and thunderstorms will form

The föhn effect

🏔️ A hot, dry wind beyond the mountain
①On the windward side, air at 20 °C rises up the slope and cools 10 °C per kilometer, reaching its dew point and forming cloud at 1 km (10 °C)
②Above that it cools at the moist rate, reaching 5 °C at the 2 km summit, and loses water vapor as rain on the way
③Descending the far side, the now-dry air warms 10 °C per kilometer over 2 km and reaches 25 °C
④So the far side gets a wind 5 °C warmer and much drier than at the start; on the Korean Peninsula, the nopsae wind that blows into the western side of the mountains in late spring and early summer is a well-known example

Worked Examples and Exam Practice

Example 1
An unsaturated parcel rises 2 km. By about how many degrees does its temperature drop? (dry adiabatic lapse rate about 10 °C/km)
1
Temperature change = lapse rate × height risen.
2
10 °C/km × 2 km = 20 °C, so it cools by about 20 °C.
▸ about 20 °C
If condensation starts on the way, the parcel cools only about 5 °C per kilometer above that point. In the first diagram, air starting at 25 °C cools 15 °C up to 1.5 km and only 2.5 °C more over the next 0.5 km, reaching 7.5 °C at 2 km.
Example 2
The surrounding air has an environmental lapse rate of 12 °C/km. With a dry adiabatic lapse rate of 10 °C/km, is this atmosphere stable or unstable for unsaturated air?
1
A parcel rising 1 km cools 10 °C, but the surrounding air at that height is 12 °C colder than at the surface, so the parcel ends up 2 °C warmer than its surroundings.
2
A parcel warmer than its surroundings is lighter and keeps rising, so the atmosphere is unstable, convection is strong, and cumuliform clouds build.
▸ unstable (environmental 12 °C/km > dry adiabatic 10 °C/km)
With the environmental lapse rate set to 12 in the second diagram, the parcel 1 km up shows as warmer than its surroundings. The steeper the environmental lapse rate, the less stable the atmosphere.
CSAT-style problem
Which statement about adiabatic changes and atmospheric stability is correct?
①The dry adiabatic lapse rate is smaller than the moist adiabatic lapse rate
②Rising air expands adiabatically and cools
③If the environmental lapse rate is smaller than the adiabatic lapse rate, the atmosphere is unstable
④Water vapor absorbs latent heat when it condenses
⑤Cumuliform clouds develop readily in a stable atmosphere
▸ ② Rising air expands adiabatically and cools
1
Rising air meets lower pressure, expands, and cools on its own without exchanging heat with its surroundings (adiabatic cooling).
2
The dry adiabatic rate (about 10 °C/km) is larger than the moist rate (about 5 °C/km) (①), a smaller environmental lapse rate makes the air more stable (③), condensing vapor releases latent heat (④), and cumuliform clouds develop in unstable air (⑤).

Summary

Key numbers
dry adiabatic ≈ 10 °C/km | moist adiabatic ≈ 5 °C/km | dew point ≈ 2 °C/km
How fast a parcel cools depends on whether vapor condenses, and the cloud-base height can be estimated as 125 × (T − Td) m.
🎯 Exam Points
①The dry adiabatic lapse rate is about 10 °C/km and the moist adiabatic lapse rate about 5 °C/km
②If the environmental lapse rate exceeds the dry adiabatic rate, the air is absolutely unstable; if it is below the moist adiabatic rate, absolutely stable
③If the environmental lapse rate lies between the two adiabatic rates, the air is conditionally unstable
④The lifting condensation level (cloud base) is about 125 × (surface temperature − surface dew point) m
⑤Unstable air favors cumuliform clouds and showers, and stable air favors stratiform clouds and fog
← Previous
Geological Time & Environment
Next →
Ocean Dynamics

Last updated

Was this helpful? Support seegongsik