Corrected entry: Throughout the entire movie, none of the characters' breath forms steam. There are many things in the film that point out it's very cold (Stoik mentioning winter is almost here, they must be in the arctic circle because the Northern Lights are visible, etc.) If it was even fairly cold, breath would form a steam cloud in the air but, somehow, this never happens.
Fawkes the Phoenix
11th May 2010
How to Train Your Dragon (2010)
1st Apr 2010
How to Train Your Dragon (2010)
Corrected entry: When Astrid meets Toothless for the first time Hiccup says "She's a friend", referring to Toothless. It's established many times that Toothless is a "He".
Correction: 1) The Northern Lights are not just visible in the arctic circle. 2) Given what's seen of the current climatic conditions and the abundant vegetation, it seems clear that it's not that cold at that point.