heat and humidity; humid heat
Refers to the uncomfortable combination of high temperature and high humidity, typical of Japanese summers.
The heat and humidity are especially severe this summer.
The humid heat makes it hard to sleep even at night.
Noun form of 蒸し暑い (mushiatsui, 'hot and humid'), from 蒸し (mushi, 'steaming') + 暑さ (atsusa, 'heat').