Wow, it is very cold on the mountain top. I went to climb Mt. Huashan in May last year. It is hot in the downtown city and the foot of the mountain, but very cold on the top of the mountain, especially in the early morning. You may need warm overcoat. Well, it is not convenient for you to carry a thick over coat when climb the mountain. There are military down jacket rent on the mountain top. You can rent one.