Nanjing, Hangzhou are incredibly hot and humid in late June, and I understand July and August also. Take plenty of towlettes (wash and dry) with you to help keep you cool. I carried a hand fan also and used it. Finding cool, bottled ice tea is not hard to find, but also carry water with you and drink it. The hotels we were in were air-conditioned, but sometimes not very much. Occasionaly we got a cool taxi, and that was a real relief after a hot, humid walk...and most of the sites have extensive walking.