Almost every night at this time of the year, my single-glazed windows invite the warm-air molecules to stay awhile as the temperature outside begins to plummet. In short, this leads to condensation.
With wooden windows (particularly those of which are neglected from regular maintenance), condensation can lead to a case of wet rot as this water runs down the glass and is absorbed by the horizontal surfaces. Regular maintenance (painting, putty, etc.) can prevent this but it won’t stop the condensation from forming.