The specific DNA sequence has been doubled. This cycle of Denature-Anneal-Extend is repeated, usually about 35 times, by using a thermocycler over 2-3 hours (the "chain reaction" of PCR). This produces 34,359,738,368 copies of DNA (That's 2^35)!
Explanation : Water Vapor is a source of warmth, since the heat that is radiated from Earth is absorbed by the water vapor. This is why greenhouses use water vapor as well.