Why is there a water vapor on the glass?

Windows in the winter perspire a lot regardless the ventilation. Frames are wet too. After opening the window leave, the glass is almost immediately foggy. There is an unattractive dark mark where the water drops stream down and they cannot be clean with detergents. You should the gravitational ventilation and make sure that You let clean air inside by intensive but short airing of the room or using the ventilators. The most often cause of appearance of the water vapor on glass is the high humidity in the room.