During my last encounter my 3lvl wizard fell into a trap with freezing water below. He managed to save himself using Levitate, but during my party’s post-game discussion there was a question of getting out of freezing water into an even more freezing air (now wet through) and how it should impact a PC.
A spellcaster using a Ready action can keep any spell with a casting time of 1 action on concentration until start of his next turn.
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs.
If it’s a fire spell and you hold its energy, then it seems reasonable it would release some heat, doesn’t it?
Also, even though you can keep cast and readied spell only until start of your next turn, Fire Bolt is a cantrip so you could just repeat the whole process as long as needed to get oneself sufficiently warmed.
And no, my wizard sadly doesn’t know Prestidigitation.