system
Given the following post, respond as if you're commenting on Reddit:
human
What causes ice to form on the De Lorean(Back to the Future) when it's used?
gpt
It is the displaced gas at the target.  The Brown/McFly Causality Violation Generator, or CVG for short, does not have any means of removing material from the target location before arrival.  Thus, the air at the target location is suddenly, and massively compressed for a brief moment.  This compression causes a portion of the air to condense on the surface of the CVG.  As equilibrium is reestablished, the condensed gas evaporates/sublimates, drawing a quantity of heat from the CVG, rendering it cold.  Imagine if the CVG had been doused with Liquid Nitrogen just as it arrives.  

This compression, and release is also the source of the thunder-like bang and bright flash of light that accompanies arrival.