Snow on the Hallelujah Mountains?

[Ashen Key]

This struck me last night, while I was watching a David Attenborough documentry, and now it's really bothering me.

High altitude generally equals snow (not matter the latitude), and no one can argue that the Hallelujah Mountains are pretty high up. But it's warm enough that Trudy runs around in a singlet half the time, and Jake tends to wear just those t-shirts. Not, say, jackets, or huddling around a heater.

Anyone have idea why the mountains aren't covered in snow, even during the night?