Arrow-right Camera
The Spokesman-Review Newspaper

The Spokesman-Review Newspaper The Spokesman-Review

Spokane, Washington  Est. May 19, 1883
Clear Night 52° Clear
News >  Family

Ask Dr. Universe: Why do mirrors fog up when you breathe on them?

A couple, reflected in a mirror, look at the moving images projected on the walls and reflecting mirrors as other patrons enter during a virtual exhibit titled "Immersive Van Gogh" on Thursday, Feb. 18, 2021, at the Lighthouse ArtSpace in Chicago.  (Shafkat Anowar/Associated Press)
A couple, reflected in a mirror, look at the moving images projected on the walls and reflecting mirrors as other patrons enter during a virtual exhibit titled "Immersive Van Gogh" on Thursday, Feb. 18, 2021, at the Lighthouse ArtSpace in Chicago. (Shafkat Anowar/Associated Press)
Washington State University

Washington State University

Dr. Universe: Why do mirrors fog up when you breathe on them? – Zinnia, 7, Richmond, Virginia

Dear Zinnia,

That’s a great observation. When you breathe out, you let a couple of different things into the air.

Not only do you breathe out carbon dioxide, but you also breathe out teeny tiny droplets of water. These water droplets are so small, we can’t see them with our eyes.

Scientists actually have a name for these little droplets of water in the air: water vapor. You may remember from our question about the states of matter that there are all kinds of different gases, liquids and solids in our world. Water vapor is a kind of gas.

My friend Cigdem Capan, a physics instructor at Washington State University, said one big factor that can help water move between these different states of matter is temperature.

When you breathe on a mirror, you are helping water move from a gas state to a liquid state. The surface of the mirror is a lot colder than the water vapor that comes from your warm human body. If you breathe on a mirror, you can easily feel that heat releasing into the air.

As water vapor in your breath reaches the mirror’s cool surface, the vapor droplets mix to form a liquid. When this happens, you can see thousands of super-tiny liquid droplets form on the mirror: fog.

Scientists also call this transition from a gas to a liquid condensation. It’s the same process that helps form big, fluffy clouds in the sky, tiny drops of morning dew and the water droplets on the outside of your cool water glass.

“If you are wearing eyeglasses and you are wearing a face mask, you can also see the glass fog up,” Capan said.

That’s condensation, too. While you might not always be able to see the water vapor from your breath, when the temperatures drop, it is a bit easier to observe this condensation in action.

It’s been pretty cold here in the Northwest, so I’ve noticed this happen when I go outdoors. As we breathe into the chilly air, the warm water vapor condenses into tiny droplets of liquid water – and even some solid water or ice – that form a kind of miniature cloud. It’s pretty fun to watch.

Whether you fog up the cool air, a window or your glasses, you may have also noticed that the moisture doesn’t stick around forever.

Try breathing on the surface of a glass mirror or windowpane, and watch what happens. Eventually, the liquid droplets disappear from the mirror. Why do you think that might be?

Share your ideas with your friends or family, and see if you can work together to figure out where those water droplets go. If you need a hint, do a little bit of research on how puddles dry up or investigate the water cycle on our planet.

Tell us what you discover at dr.universe@wsu.edu.

Sincerely,

Dr. Universe

Ask Dr. Universe is a project from Washington State University. Submit a question at askdruniverse@wsu.edu.

The Spokesman-Review Newspaper

Local journalism is essential.

Give directly to The Spokesman-Review's Northwest Passages community forums series -- which helps to offset the costs of several reporter and editor positions at the newspaper -- by using the easy options below. Gifts processed in this system are not tax deductible, but are predominately used to help meet the local financial requirements needed to receive national matching-grant funds.

Active Person

Subscribe to the Coronavirus newsletter

Get the day’s latest Coronavirus news delivered to your inbox by subscribing to our newsletter.