schwül
humid
Describes weather that feels unpleasant and oppressive because of a combination of high humidity and warmth.
Heute ist es so schwül, dass man kaum atmen kann.
It's so humid today that you can hardly breathe.

humid / sultry
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humid
Describes weather that feels unpleasant and oppressive because of a combination of high humidity and warmth.
Heute ist es so schwül, dass man kaum atmen kann.
It's so humid today that you can hardly breathe.

The adjective 'schwül' (humid/sultry) is closely related to the noun 'die Schwüle' (humidity, sultriness). 'Die Schwüle' describes the state of humid, oppressive weather itself.
Wegen der großen Schwüle konnte ich kaum schlafen.
Because of the intense humidity, I could hardly sleep.
You can recognize the meaning of 'schwül' (humid/sultry) from the context. If people are talking about 'Wetter' (weather), 'Luft' (air), 'Hitze' (heat), or 'Sommer' (summer), it means oppressive, humid warmth. In connection with 'Atmosphäre' (atmosphere), 'Stimmung' (mood), or 'Blick' (look/gaze), it has the erotic, sensual meaning.
An diesem schwülen Sommertag sind alle ins Freibad gegangen.
On this humid summer day, everyone went to the outdoor pool.
When talking about the weather, 'schwül' (humid) is very often used with 'es ist' (it is) to describe the current state of the atmosphere. Common combinations are also 'schwüle Hitze' (humid heat) or 'ein schwüler Tag' (a humid day).
Es ist heute sehr schwül und drückend.
It is very humid and oppressive today.
fresh
'frisch' and 'schwül' both often describe air or weather. In this context, however, they are opposites. 'frisch' means cool, pleasant air that feels good. 'schwül' means warm, moist air that feels heavy and unpleasant. After rain or in the morning, the air can be fresh; on hot summer days before a thunderstorm, it is often humid.

Im Bus ist es heute so schwül, dass alle die Fenster öffnen wollen.
It is so humid on the bus today that everyone wants to open the windows.
Nach dem Unterricht gehen wir kurz nach draußen, weil die Luft so frisch ist.
After class, we go outside for a short time because the air is so fresh.