What causes condensation?
Condensation forms when warm or humid air contacts a cold surface. Moisture is in the air all around us and warmer air can hold more moisture. When the air around us cools it contracts and condenses, and when the temperature drops, the first place you will see any condensation is on the windows.
Windows are the coldest surfaces indoors as they are our portal to the outside world. During colder months, indoor air is much warmer and holds more moisture than the outdoor air which is colder and dry. Warm and humid indoor air cools and contracts as it comes into contact with the cooler windows, the moisture then condenses on the glass.