Learn Finnish: Ilma kuilussa muuttuu tukalaksi sitä mukaa kun lämpötila nousee. - The air in the shaft becomes uncomfortable as the temperature rises.

While kun just means "when" or "as" in a general time sense, sitä mukaa kun emphasizes a proportional, parallel progression. It means "at the same rate as" or "in proportion as", highlighting that the air gets increasingly uncomfortable exactly as the temperature goes up.

Why is the adjective tukala (uncomfortable) in the translative case (tukalaksi)?

The verb muuttua (to change or to become) requires the translative case (ending in -ksi) for the word describing the new state. This case is used specifically to highlight the transformation from one condition into another.

What exactly does tukala imply compared to a more common word like epämukava (uncomfortable)?

Tukala is a much stronger, more specific word than epämukava. It typically describes physical oppression, stifling heat, or a tight, suffocating situation. In this shaft, it means the air isn't just unpleasant, but becoming oppressively heavy or hard to breathe.

What is the literal breakdown of the phrase sitä mukaa kun?

Sitä is the partitive form of se (it), and mukaa is an old form related to the postposition mukaan (along with / according to). Literally it means something like "along with that when", but you should treat it as a fixed phrase linking two continuously changing actions.