system
Given the following post, respond as if you're commenting on Reddit:
human
[Back to the Future] What exactly is the time machine travelling through that causes it to frost over completely?

The very first time that Doc Brown tests it (and the others too, I think), the time machine arrives completely frozen over, to the point that ice cracks when he opens the doors and it's painful to touch.

The parking lot is wet, meaning it didn't freeze naturally from the air temperature, either.

Additionally, where does the water come from? The DeLorean was driven to the lot inside a closed truck and the rain has obviously stopped, meaning that it couldn't have been wet beforehand.
gpt
Theoretical: the time displacement field extending from the aluminum body of the car on outwards saps all energy right down to an atomic level, resulting in the body of the car becoming quite close to absolute zero. One the field is gone it begins absorbing the ambient warmth around it again (there was some delay before Doc reached the car to touch it) but it's still extremely cold for a short while. 

The DeLorean isn't actually traveling through anything between the two points of spacetime, as seen when we're along for the ride to a different time. A quick flash of light from the formed time displacement field and then you're in the targeted spacetime once it fades down again. 

The moisture is just condensed humidity from the air flash frozen to the body of the car. Like your windshield frosting over in winter.