You need to find fire essence. Ayane the kitsune from the pine forest gives you a vague its location - the desert. Bring it to her. After taming the fire crystal, Ayane will guide you to the snowy mountains to the East of her forest. Taming the fire essence will allow Ada to ignore the cold and proceed further to get the air crystal.
If you are stuck or don’t know what to do next - try the ”Help” button in the game menu. Just hit ”Escape” on the map to call it.