User blog comment:Lightning the Skrill/Snow Wraith thermal vision gone wrong!/@comment-5690373-20150927215452/@comment-5690373-20151001133007

Like Angues said, it would be unlikely for animals of that size to be able to survive so far north if they are cold blooded. But Lightning is right too, most if not all reptiles are cold blooded. So let me propose an alternative explanation.

Just like Toothless was seen with a high temperature in the episode because of the fire within him, what if dragons use some form of internal fire to keep themselves warm? We know this can somehow happen, since the Gronckle and Scauldron both use elements from the environment that they heat up to shoot out later. It would also explain why dragons that live in the cold (like in the sea for tidal class dragons, in blizzards for the snow wraith and so on) don't usually shoot fire, but other things, they live in colder environments so they have adapted to conserve heat more efficiently.

And the Groncicle really is an odd case. Maybe its blood is not made of water or otherwise it would freeze to death. Also, since it is generally colder than the environment around it, it could probably use the liquid nitrogen fire to keep itself cold instead of warm. But then I don't know how liquid nitrogen is made, so this may not be physically possible.