Take the previous lesson's water machinery, put it inside a parcel of air, and lift. This lesson is the payoff of the module: why rising air cools and at what exam-quotable rates, how condensation slows the cooling, and how comparing those rates with the real atmosphere's temperature curve answers the question behind all convective weather: if this air is nudged upward, does it come back or keep going?
The adiabatic process
A rising parcel climbs into lower pressure and expands; expansion is work, and the energy for it comes from the parcel's own heat, so the parcel cools as it rises without exchanging any heat with its surroundings. That heatless bookkeeping is the adiabatic process, and it runs equally in reverse: a descending parcel is compressed and warms adiabatically. For an unsaturated parcel the exchange rate is fixed and binding for the exam: the dry adiabatic lapse rate (DALR) of 1 °C per 100 m (3 °C per 1,000 ft), the same going up as coming down.
The humidity of the unsaturated riser changes character on the way. Its vapour content stays the same (nothing is added or removed), but the cooling closes the gap to the ceiling: the relative humidity of a rising unsaturated parcel increases, the temperature racing down toward the dew point. Climb far enough and the two meet: the parcel saturates, the previous lesson's condensation begins, and the cloud base marks the height where it happened. A descending parcel plays the film backwards: warming, relative humidity falling, cloud evaporating.
The saturated parcel
Saturation changes the arithmetic. A saturated riser still cools by expansion, but every metre of climb now condenses vapour, and condensation releases latent heat into the parcel: a stove burning inside the climb. The heating offsets part of the cooling, so the saturated parcel cools more slowly: the saturated adiabatic lapse rate (SALR), on average 0.6 °C per 100 m (1.8 °C per 1,000 ft) at lower levels. The value is an average because the stove burns in proportion to the vapour available: in warm, moisture-rich air the SALR is smaller still, while high and cold, with little vapour left to condense, the SALR creeps back up toward the DALR.
Stability: the parcel against the atmosphere
Stability is a comparison. Nudge a parcel upward and it cools at its adiabatic rate; whether it keeps rising depends on how fast the environment cools around it: the environmental lapse rate (ELR), the actual temperature curve a sounding measures. A parcel that arrives colder (denser) than its new surroundings sinks home: stable. One that arrives warmer (lighter) keeps accelerating upward: unstable. The definitions, qualitative and quantitative:
Stable — the ELR is less than the SALR: every displaced parcel, dry or saturated, ends up colder than its surroundings and returns.
Unstable — the ELR exceeds the DALR: every displaced parcel ends up warmer than its surroundings and keeps going.
Conditionally unstable — the ELR lies between the SALR and the DALR: stable for a dry parcel, unstable for a saturated one; the condition is saturation.
Indifferent (neutral) — the ELR equals the parcel's own adiabatic rate: a displaced parcel matches its surroundings and stays wherever it is put.