What causes ice to form on the inside of windows?

Ice forms on the inside of windows when warm, moist indoor air comes into contact with a window surface that has dropped below the freezing point of water. The moisture in the air condenses onto the cold glass and then freezes. This typically happens during very cold weather when the window is poorly insulated or single-paned, allowing outdoor temperatures to chill the glass surface significantly. Higher indoor humidity makes the problem worse, as there is more moisture available to condense and freeze. Common sources of indoor humidity include cooking, breathing, and bathing. Improving insulation with double or triple-pane windows, reducing indoor humidity, or improving air circulation near windows can help prevent interior ice formation.