沉闷 (chén mèn): oppressive (của thời tiết)

On a summer afternoon, the sky is covered with dark clouds, and the air is so oppressive that it's hard to breathe.

Before the rainstorm, the weather is unusually oppressive, and even the leaves are completely still.

In this oppressive room without windows or a fan, I feel like I'm suffocating.

The climate in the tropical rainforest is always so oppressive and humid, making people feel uncomfortable all over.