In the Curse of Strahd adventure, the sidebar about Amber Temple's temperature (p. 181) says:
The Amber Temple is a cold, dark place carved out of the snowy slope of Mount Ghakis. The temperature throughout the complex is -10 degrees Fahrenheit (-23 degrees Celsius). Characters who don't have heat sources, cold weather gear, or magic to protect them are subject to the effects of extreme cold, as described in the "Weather" section in chapter 5, "Adventure Environments," of the Dungeon Master's Guide. If Kasimir Velikov (see chapter 5, area N9a) is with the party, his ring of warmth protects him from the effects of the extreme cold.
It specifically mentions "heat sources", and I was wondering what counts as a heat source for the purposes of not succumbing to the cold of Amber Temple.
Is a torch good enough? Or are we talking more like a campfire (say, the sort that someone might make during a long rest)? I imagine Curse of Strahd itself doesn't expand much more on this, but there might be some generic rules in the DMG or similar that I haven't managed to find with regards to temperature?
I ask because there will be at least one party member who won't have any (relevant) magic items, cold resistance or cold weather gear by the time they enter Amber Temple, and I'd like to know what counts as a heat source so I (as the DM) know when to ask for or waive the need for saving throws vs. exhaustion.
For reference, the "Extreme Cold" (Dungeon Master's Guide, p. 110) section:
Whenever the temperature is at or below 0 degrees Fahrenheit, a creature exposed to the cold must succeed on a DC 10 Constitution saving throw at the end of each hour or gain one level of exhaustion. Creatures with resistance or immunity to cold damage automatically succeed on the saving throw, as do creatures wearing cold weather gear (thick coats, gloves, and the like) and creatures naturally adapted to cold climates.