This summer is definitely more humid than it was last year.
蒸れる is "to be humid." So, if you have a humid hot, you have weather that is hot and humid (or you can just say humid, because that implies hotness too, usually). This is a useful word for summers in Japan, since you'll complain about this all the time.
The reading comes from 蒸れる and 暑い. Know those and you know this too.