Fogging between the panes in your window.
Dual-pane windows are common in houses and offices because of their improved insulation. They work by having two panes of glass separated by about a half-inch of space between the two panes, with the trapped air acting as an additional insulator. If the seal on any of the panes of glass fails, fog can build up between the windows, caused by the moist air. While normal fog moisture can be wiped off the surface, getting rid of fog within the windows requires some additional work.