How does frost get onto the window at night?

On a cold winter morning, look at your window. White, feathery patterns cover the glass. Touch them with one finger, and they feel icy cold. They look like little ferns made of ice.

Last night, that same glass was clear. Nobody painted those patterns. So how does frost get onto the window at night? Let's find out together.

The air in your home holds water. You can't see it, because the bits of water are tiny. Your warm breath puts water into the air. A warm bath does too.

When night comes, the sun goes away. With no sun to warm it, the air outside grows colder and colder. Soon it is cold enough to freeze a puddle.

That freezing air touches the outside of your window. It takes the warmth out of the glass. Soon the whole window is icy cold, even on the inside.

Inside, the warm air of your room drifts over to the window. It carries its hidden water up to the cold glass. What do you think happens to that water when it touches the icy glass?

The water freezes onto the glass! The bits of water turn into ice crystals, which means very small pieces of ice with sharp little points. They stick tight where they land.

All night long, more water from the room reaches the window. It freezes onto the points of the ice crystals. The ice crystals grow longer and branch out like feathers.

Let's remember our special word: ice crystals. Cold night air chills the glass, and water from your room freezes onto it.

Next frosty morning, you can peek at the window and say a quiet hello to the ice crystals that grew while you slept.