Above the cloud base, a rising parcel is saturated, and every bit more cooling condenses some vapor. Condensing releases latent heat inside the parcel. That changes how fast it cools.
Key idea
A saturated parcel cools more slowly than a dry one: about 6 °C per km on average (the moist adiabatic lapse rate). It is about 4 °C per km in warm, humid air near the ground and approaches the dry rate (9.8 °C per km) in very cold air high up. Problems here use 6 °C per km unless they say otherwise.
Why it works
Expansion still cools the parcel by 9.8 °C for each km. But as it cools, vapor condenses, and each gram of vapor that condenses gives about 2500 J back to the parcel's air. That pays back part of the cooling. Warm saturated air condenses a lot of vapor for each degree of cooling (saturation climbs so steeply with temperature), so it gets a big payback and cools only about 4 °C per km. Cold air has little vapor left to condense, so it cools almost as fast as dry air.
Worked example: Climbing inside a cloud
A parcel is saturated at its cloud base, 1 km up, at 16 °C. It keeps rising to 5 km. What is its temperature? (Use 6 °C per km.)
- It climbs $5 - 1 = 4$ km while saturated.
- Cooling: $4 \times 6 = 24$ °C.
- $16 - 24 = -8$ °C. A dry parcel would have cooled 40 °C, to −24 °C.
Answer: −8 °C
Worked example: From the ground to the top of a cloud
Ground air at 26 °C with a dew point of 18 °C is lifted to 4 km. Use 10 °C per km (dry), 2 °C per km for the dew point and 6 °C per km (moist). Find its temperature at 4 km.
- Cloud base: the spread of 8 °C closes at 8 °C per km, so the base is at 1 km.
- At the base: $26 - 10 \times 1 = 16$ °C (and the dew point is $18 - 2 = 16$ °C).
- From 1 km to 4 km, moist: $16 - 6 \times 3 = -2$ °C.
Answer: −2 °C
Common mistake
Thinking the released latent heat warms a rising cloud parcel. The parcel still cools as it rises; the latent heat only slows the cooling, from 9.8 to about 6 °C per km. Check: in any moist-ascent answer, the parcel's temperature at the top must be lower than at the cloud base.
| Air at the cloud | Moist lapse rate (about) |
|---|---|
| 30 °C, near sea level | 3.6 °C per km |
| 20 °C, near sea level | 4.3 °C per km |
| 0 °C, near sea level | 6.5 °C per km |
| −20 °C, about 5.5 km up | 7.7 °C per km |
| −40 °C, about 9 km up | 9.0 °C per km |