Your iPhone's specs explain that it should survive through temperatures from -20 to 45 C just fine. Good news? That appears to be true. Bad news? Cold temperatures may cause your iPhone's moisture indicator to imply liquid damage.
Polish website Moje Jabluszko decided to run a few tests on iPhones to see just how winter temperatures affected the devices. They were particularly interested in whether the moisture indicator—or liquid sensor—located in the headphone jack of an iPhone would turn red—indicating moisture—due to temperature changes.
While their testing may not be entirely without flaws and doesn't account for air humidity in the first place, it does suggest that the liquid sensors are a bit inaccurate and may change color at -11 C instead of the -20 C indicated by the device's specs. Of course, one must keep in mind that condensation may play a large role in all of this.