https://rpg.stackexchange.com/questions/124338/how-do-people-travel-in-the-north-of-faer%c3%bbn-during-winter
Hmm. I know there's a supplement for the Great Glacier in older editions (https://www.drivethrurpg.com/product/16832/FR14-The-Great-Glacier-2e?it=1) that probably has some useful info for this, but I don't have a copy.