It'd been one of the spells he and Hermione had experimented on, a lifetime ago, so he was able to control it precisely, though it had taken a lot of power to affect that much mass. Hermione's body should now be at almost exactly five degrees Celsius.

I feel like this comes off as a bit of an ass pull. It's the suspicious specificity that does it, I think.

It would be easy to prevent that feeling, if you care to and if it's not just me, with a throwaway line in an earlier chapter.