很闷热。
Hěn mēn rè
Very stuffy-hot.
闷热 = muggy, single compound adjective
今天好闷热，空气都是湿的。
Jīn tiān hǎo mēn rè, kōng qì dōu shì shī de
Today very muggy, the air is all wet.
Natural description
又闷又热，像在蒸笼里一样，喘不过气来！
Yòu mēn yòu rè, xiàng zài zhēng lóng lǐ yī yàng, chuǎn bù guò qì lái!
Both stuffy and hot, like being inside a steamer, can't catch breath!
蒸笼 = bamboo steamer; 喘不过气 = can't catch breath, very vivid
More Chinese (Mandarin) Weather Phrases