system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do most cars default to not circulating the air in the car instead of circulating it? For climate purposes it seems like circulating the air in the car would be better for constant temperature and humidity.
gpt
Humidity goes up with a bunch of warm bodies exhaling next to cold glass.  Your cars defroster might not be able to defrost the windows even on max high settings unless you get the moisture out.  This is why the ac will turn on automatically when using the defrost.

Also, fresh air to breath is nice when in a full car.