むしあつく is the adverbial form. It's an i-adjective, so the dictionary form is むしあつい (蒸し暑い in kanji).
むしあつい means "hot and humid", not just "hot", so it's not used for hot and dry weather. "Hot" is just あつい.
むしあつく is the adverbial form. It's an i-adjective, so the dictionary form is むしあつい (蒸し暑い in kanji).
むしあつい means "hot and humid", not just "hot", so it's not used for hot and dry weather. "Hot" is just あつい.