A supplied parcel-cooling model
A fictional weather exercise starts an air parcel at 18 degrees Celsius at sea level. The parcel rises to a 2000 metre ridge, reaches saturation at 1000 metres and follows the supplied dry rate of 1 degree Celsius per 100 metres before saturation and saturated rate of 0.5 degree per 100 metres afterward. All condensed water is stated to fall out before descent. The parcel then descends to sea level following the supplied dry rate. The task asks for the temperature at the ridge and after descent. These deliberately stated rates are a teaching model, not universal measurements of a real mountain atmosphere. No flight route or clearance is supplied, and the arithmetic cannot establish freedom from mountain waves, cloud or turbulence. A complete answer follows the two ascent stages and retains the changed descent assumption.
1. Calculate ridge and final sea-level temperature.
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The first 1000 metres cool the parcel by 10 degrees to 8. The next 1000 metres cool it by 5 degrees to 3. Descending 2000 metres warms it by 20 degrees, producing 23 degrees Celsius.
2. Why is the final temperature higher than the start?
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Under the supplied model, part of ascent uses a smaller saturated cooling rate while descent is dry after moisture loss. The result follows those explicit assumptions rather than a universal forecast.