Windows fog up when water vapor in the air cools on the glass and condenses into tiny liquid droplets. This often happens when warm, humid indoor air touches a cold window, but fog can also form outside when the glass is cooler than the outdoor air.
The glass cools air below its dew point
Air contains water vapor. If moist air touches a surface colder than its dew point—the temperature at which the air becomes saturated—some vapor changes into liquid droplets. The droplets scatter light, making the glass look cloudy. [1]
Where the condensation forms matters
Fog on the room-facing side can result from indoor humidity meeting cold glass. Fog on the outdoor side can occur when the exterior pane cools overnight. If moisture appears between sealed panes, the window’s insulating seal may have failed; that is different from ordinary surface condensation.
Why it often clears later
When the glass or surrounding air warms, the droplets evaporate back into water vapor and the window becomes clear. Ventilation can reduce excess indoor moisture from cooking, showers, or drying clothes. [2]
Is fog on a window water leaking through it?
Usually no. Surface fog is condensed water vapor from the air. Moisture trapped between panes can indicate a damaged seal.
Sources and evidence
- National Weather Service — Dew Point ↗
Defines dew point and describes condensation as water vapor changing to liquid when air cools to saturation.
- University of Minnesota Extension — Controlling Moisture Problems in Your Home ↗
Identifies window condensation as a symptom of excess indoor moisture and discusses ventilation.
Sources consulted: 15 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; the explanation was checked against linked institutional, medical, or scientific references. This article was prepared with AI assistance and has not received independent expert review.