How does frost form?

1. On a freezing morning the window is covered in white feathers and ferns. Nobody drew them. They are frost: ice crystals that grew on the glass in the night.
2. The warm air in the room holds water vapor, an invisible gas. It comes from cooking, hot drinks, plants and your own breath. The dots here are only pretend.
3. Outside, the night is far below freezing. A thin sheet of glass cannot keep the cold out, so the glass itself drops below zero degrees Celsius.
4. Vapor molecules hit the freezing glass and lock into place as ice. They skip being liquid water. Going straight from gas to solid is called deposition.
5. Each crystal starts at a speck of dust or a tiny scratch. Its tip grows fastest because it reaches the most vapor. Side arms sprout, and the pattern spreads like a fern.
6. The same thing happens outdoors on clear, cold nights. Grass, leaves and cars cool below freezing, and vapor turns to ice on them. In the morning the sun melts it.

Tap each part to see how the frost grew on the glass.

• Freezing night. The air outside is far below freezing. It pulls heat out of the room through the glass all night.
• Cold glass. A single sheet of glass is thin, so it gets almost as cold as the air outside. Frost can grow only when the glass is below freezing.
• Warm, damp room. Warm air holds a lot of water vapor. Hot drinks, cooking and breathing keep adding more. The damper the room, the more frost can grow.
• Water vapor. Water vapor is water as an invisible gas. When it reaches the freezing glass, it turns straight into ice.
• Starting speck. Every frost fern starts somewhere: a speck of dust, a fingerprint or a tiny scratch. Clean, smooth glass frosts over more slowly.
• Main arm. The tip of each arm sticks out into the damp air, so it grows fastest and the arm gets longer and longer.
• Side branches. Side branches sprout from each arm, and smaller ones sprout from those. That is what makes frost look like ferns and feathers.

Change how cold it is outside and how damp the room is. Tap the glass to start crystals. When do you get frost, mist or nothing at all?

Experiment: A window is misty with tiny drops. What happens if the glass gets colder than freezing?

1. Start with mild weather outside and a damp room. Look for mist on the glass.
2. Make it colder outside, a little at a time.
3. Watch the thermometer pass the freezing mark. What grows on the glass?

What we found: Above freezing, damp air leaves mist: tiny drops of liquid water. Once the glass is below freezing, the vapor turns straight into ice and grows as feathery crystals. Warm the glass and the frost melts.

Turn on JavaScript to try it yourself.

Cover all five stars with frost before the sun comes up. First make the glass cold enough. You have only five crystals.

Frost looks like snow that fell in a thin layer, but it did not fall at all. It grew right where you see it. The air always holds some water vapor, which is water as an invisible gas. When that vapor touches something colder than freezing, it turns straight into ice.

Most of the time, water goes from gas to liquid to solid: vapor becomes drops, and drops freeze. Frost skips the middle step. The vapor turns directly into ice crystals. Scientists call this deposition.

A frost crystal starts at a tiny speck or scratch. Its tip pokes out into the damp air, so the tip gets the most vapor and grows fastest. Bumps along the arm grow into side branches, and those grow branches of their own. That is why frost looks like feathers and ferns.

Frost needs two things: a surface below freezing, and enough vapor in the air. A window with one thin sheet of glass gets very cold, so it frosts over on the inside. Outdoors, grass and car roofs get frosty on clear, calm nights. When the sun warms them, the frost melts or turns back into vapor.

Key words

Water vapor

Water as an invisible gas in the air.

Deposition

When a gas turns straight into a solid, as when vapor becomes frost.

Frost point

How cold a surface must be for frost to grow on it.

Crystal

A solid whose tiny particles are arranged in a regular, repeating pattern.

Insulation

Material that slows the flow of heat, such as the gas between two panes of glass.

Check your thinking

1. How does frost get onto a window?

   Show the answer

   Water vapor in the air turns straight into ice on the cold glass. The frost grows where it is. Vapor touches glass that is colder than freezing and becomes ice crystals.

2. A window is covered in mist, not frost. What does that tell you about the glass?

   Show the answer

   It is cool, but not colder than freezing. Mist is liquid drops. Vapor becomes liquid on cool glass and becomes ice only when the glass is below freezing.

3. Why does frost look like ferns and feathers?

   Show the answer

   The tips grow fastest, and side arms keep sprouting from them. A tip sticks out into the damp air, so it grows fastest. Bumps on the arm do the same and become side branches.

4. Why do windows with two sheets of glass almost never get frost ferns?

   Show the answer

   The inside sheet stays too warm for ice. The gas trapped between the sheets slows the heat escaping, so the inside sheet stays above freezing.

5. What happens to frost when the sun warms it?

   Show the answer

   It melts or turns back into vapor. Above freezing the ice melts into water, and the water soon evaporates back into the air.