Everyday Life & Technology
Why does a window mist up?
A cool bottle helps explain little droplets on windows.
The short answer
Warm moist air can cool against cold glass. Invisible water vapour then becomes tiny liquid drops. Many drops make the glass look cloudy. This is condensation.
An imagined Wusel scene
Puschel and Flauschi investigate
A cool sealed water bottle stands on the garden table. Puschel“Has it had a tiny shower on the outside?”
Flauschi“The drops can come from moist air. Water vapour turns into liquid on the cool surface.”
Puschel“So there is not necessarily a hole in the bottle?” Flauschi“Exactly. A window can mist up in a similar way.”
They look through the bottle and at the drops. Puschel“Air carries water even when I cannot see it before.”
They put the bottle on a mat and leave the table dry. Their observation needs neither hot steam nor an open window.
Little Wusel quiz
Must drops outside a cold bottle come through a hole?
Choose one answer: a), b).
- a)No, they can form from water vapour in the air.
- b)Yes, otherwise there would be no drops outside.
Show answer
Correct answer: a): No, they can form from water vapour in the air. No. Water vapour in the air can turn into water outside.
Is water vapour itself visible?
Choose one answer: a), b).
- a)Yes, every white cloud is the gas itself.
- b)No, visible mist consists of droplets.
Show answer
Correct answer: b): No, visible mist consists of droplets. Water vapour is invisible. Visible mist consists of little droplets.
Word help
- condensation
- A gas becomes liquid, for example water vapour becoming drops.
Explore in more detail
Here you will find longer explanations and further details. You can explore them together.
Understand step by step
How it works – simply explained
Invisible water is in air
Air may contain water vapour, a gas rather than visible droplets.
Air cools near glass
A cold pane can cool nearby air. With enough moisture, some vapour becomes liquid droplets: condensation.
Droplets make the misty layer
Many tiny drops scatter light, making the pane look foggy. Water has not simply passed through the glass. Warmer glass or drier air may prevent condensation.
Where window droplets come from
Indoor air may contain invisible water vapour. Air cools beside a sufficiently cold pane and some vapour can become liquid droplets. The pane does not create new water. Depending on conditions, condensation may form inside or outside; this example explains the indoor side.
The key points again
Discovery 1
Air contains invisible water vapour. When moist air meets a sufficiently cool surface, liquid water can form there. This is called condensation.
Source: [1]
Discovery 2
Many tiny drops on a window can scatter light differently. The glass then looks misted up.
Source: [1]
Discovery 3
Similar drops form outside a cold bottle. This water need not come from inside the bottle; it can come from water vapour in the surrounding air.
Source: [1]