Flying North to Sundsvall

I caught the train to Malmö in southern Sweden but returning I needed to get back in time to teach so it was the plane for the homeward trip.
As we flew north from Stockholm the sea was open. As we neared Sundsvall it was still open but dotted with ice floes. As we flew over Sundsvall the sea between the island of Alnön and the mainland was frozen, apart from a narrow break where boats had broken a way through to the harbour.