In 1966, the Tivoli gardens, where the restaurant scene in Copenhagen takes place, were open from May 1st to September 30th. During that period, the temperature would never be at freezing point in Osterfjord, Norway, where the opening scene takes place.
Or their shadow, anyway. On the road-level shot of Armstrong's taxi leaving the farm (Gromek's motorbike is visible on the left of the screen), just at the very bottom of the image can be seen the shadow of the camera (4:3 television version only).
When Professor Armstrong is on the boat, the heater aboard ship is broken, and the close-up of the thermometer shows it is freezing - yet it spite of the fact that he and all the extras are wearing heavy coats, their breath yields no steam, which would have been profuse at that temperature.