[Hijikata doesn't expect that Eren will need to stay a titan for longer than a tag or two a few minutes, given how far he can see at that height. If there's no special snow around this area at all, they can travel farther into the Segment with both of them in human form! After all it might be a bit cumbersome even for titan Eren to carry all the sleigh containers at once. As for spending the night, Eren would be surprised at how comfortable Hijikata's temperature-controlled Luxury Tent™ is! But of course the sooner they can complete the mission the better.

"I don't want you to get cold"... Eren really is so endearing. Hijikata will never not feel joy at his blunt kindness, that's how used he is to being around jackasses who express even positive feelings with crass rudeness. Little does he know how much energy the titan transformation takes out of Eren each time (it always looks so effortless and immediate!), else he'd have volunteered to fly up right away!]

It's not the light overall, but a special weather phenomenon. It only shows up at night, then you can see those colours in the sky, like huge veils. It only happens in cold areas.

[The cold may be less important than the northern location, but he's a samurai, not a meteorologist!]

And Eren, being a titan is never the only useful thing you can do. If you couldn't transform, we'd just search by sleigh. Four eyes always see more than two.

[Don't reduce yourself to your usefulness as a titan shifter!! Hijikata will have none of that!]