prize1-3, Exercise 2: Where the cloud begins

Model answer:

The air must cool from 15 to its dew point of 5 degrees, a fall of 10 degrees. At 10 degrees per kilometre, that takes 1 kilometre of rise, so cloud would begin at about 1,000 metres.
Assumption: the dew point stays at 5 degrees and the parcel rises as dry air. In practice the dew point falls slightly as air rises, so the true height is a little lower; the sum is an estimate.