Weather in Beijing is hot during the summer (I wore sleeveless shirts almost every day because I do not like hot weather very much), but don’t worry, the classrooms are always air-conditioned :3 I highly recommend bringing a fan, or buying one
Pollution: There has been weeks where I do not see the blue sky at all. I have asthma, so I wore a mask almost every day because without it on bad pollution days, my breathing would be uncomfortable. But pollution is not bad every single day. Most people don’t wear masks often.
Rain: BRING AN UMBRELLA! It sometimes rains EXTREMELY HARD (but not often). There have been times when the streets got flooded up to the shins and walking through that is not fun.
Here is a video when it rained once and walked from the dorm to the store on campus (WuMei).