The guy at the service station told my SO that you shouldn’t add air to tires in very cold weather because the valve could freeze open. However, I’ve tried searching the internet for corroboration, and the only thing I could find about it one way or the other was this old Q&A which suggests it’s not that big a worry.
I have a tire that needs air, and I’m thinking it’s probably safer to fill it than not, even when it’s this cold out, but what’s the straight dope?
Fill it. I’ve never had one freeze open, but if it does you can just poke it with a small screwdriver until it unsticks, then refill the tire.
The danger of driving on a low tire far outweighs the slight possibility of the inconvenience of a stuck schrader valve.
Fill it. Air coming quickly out of a compressor at say 100 psi into a tire holding 30 psi could cause some freezing from the rapid pressure drop. However that would mean that the air compressor wasn’t maintained properly to drain off moisture or there’s already some moisture on the valve from rain or high humidity. One reason for the valve cap. However it’s very, very unlikely to freeze up the valve.
FYI we use to have to deflate some aircraft tires before removing them from the aircraft. We would remove the Schrader valve core and let the air escape from these rather larger and high pressure tires. If it was humid out, some ice could build up on the valve.
There is the issue that if you fill it to a particular pressure in cold weather, then when the weather warms up the pressure will be a bit higher. Even in an extreme case, though, that won’t be much more than about 10%.