Solution

ID: past-exam-of-the-short-options-paper-shared-by-prelims-and-parts-a-and-b-of-the-physics-course-of-the-university-of-oxford/2017/s25/2/1/solution

For dry ascent, and hydrostatic pressure change is , so . During condensation , and offsets expansion cooling, making the saturated ascent less steep.
For a saturated parcel with , use to obtain
With dilute vapor, and , this becomes . In terrestrial conditions the denominator's latent-heating correction dominates. Warming increases saturation humidity strongly, so latent heat increasingly compensates expansion cooling and the moist lapse rate falls. This is a statement about saturated convective adjustment, not every instantaneous environmental temperature profile.

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