The personification of frost in northern folklore — the figure credited with the patterns on cold windows, the white on the grass and the nip on your nose. English use of the name dates from the early nineteenth century.

What causes fern patterns on windows?

Water vapor freezing directly onto cold glass, branching along microscopic scratches and specks of dust. Because those imperfections differ on every pane, no two frost patterns are ever alike.

When should we send a Jack Frost letter?

The first real frost or first snow of the year, left where a child will find it at breakfast — while the windows are still decorated and before the sun clears them.

Letter from Jack Frost — First Snow Letter

The story

Jack Frost does not knock, does not give notice, and is never seen. He is the explanation for the fern patterns on the window, the white on the grass, and the sudden sting on your nose on the first properly cold morning of the year. He is mischievous rather than kind, works fast, and is gone by the time anyone is up — which is the only reason nobody has ever caught him.

Where it actually comes from

The personified frost is old across northern Europe; the Norse figure Jokul Frosti — “icicle frost” — is often cited as an ancestor, and English usage of “Jack Frost” is recorded from the early 1800s. He became a fixture of Victorian and American children’s verse and illustration, where the emphasis shifted from a cold to be feared toward an artist who decorates windows. The physics is real and rather lovely: fern frost forms when water vapor deposits directly as ice on cold glass, branching along microscopic scratches and dust, which is why no two windows are ever the same.

What it does for a child

Jack Frost is the easiest way to get a child to look closely at something ordinary. The window is right there, the pattern is genuinely extraordinary under any scrutiny, and it will be gone by mid-morning — which makes it a natural first lesson in noticing things while they are happening. Children who are taught to look at frost tend to start looking at other things.

Jack Frost Looks Forth

A story to read aloud · about 4 minutes

Long ago, in the days of candlelight and wood stoves, there lived a merry sprite called Jack Frost. He was the son of the Frost King, who rides on the winter gales, and Jack could go anywhere the cold could go. He had lived since the oldest times in the great Forest of Burzee, where the fairies and the wood nymphs live. He was a jolly rogue, full of pranks, and he dearly loved to nip noses and ears and toes.

One still, clear night, Jack Frost looked forth and whispered, “Now I shall be out of sight. Through the valley and over the height, in silence I’ll take my way. I will not go with that blustering crowd, the wind and the snow, the hail and the rain, who make so much bustle and noise for nothing. But I’ll be just as busy as they!”

Away he flew. He flew to the mountain and powdered its top with white. He landed on the trees and dressed their branches in beads that sparkled like diamonds. Over the shivering lake he spread a shining coat of ice, as smooth and strong as a knight’s armor. Along the rocks at its edge he hung rows of icicles, pointed like spears.

Then he came to the Laughing Valley, where a young man named Claus lived in a snug little house. Jack rapped at the door.

“Come in!” called Claus.

“Come out!” answered Jack. “You have a fire inside!”

So Claus came out. He had known Jack back in the Forest, and he liked the jolly rogue, even though he did not quite trust him.

“There will be rare sport for me tonight, Claus!” shouted Jack. “Isn’t this glorious weather? I shall nip dozens of noses and ears and toes before daybreak!”

“If you love me, Jack,” said Claus, “spare the children.”

“And why?” asked Jack, surprised.

“They are small and tender,” said Claus, “and they cannot fight you.”

Jack thought about it. “True,” he said at last. “Well, I will not pinch a child tonight, if I can help it! Good night, Claus!”

Then off he ran to the nearest village. He went to the windows of those who slept, and over each pane he crept, like a fairy. Wherever he breathed and wherever he stepped, beautiful pictures appeared in the light of the moon. There were flowers and trees. There were flocks of birds and swarms of bees. There were cities with temples and towers, all shining in silver.

But Jack did one thing that was hardly fair. He peeped into a cupboard and found that nobody had set out a single thing for him. “Now, just to set them thinking,” he said, “I’ll bite this basket of fruit! I’ll crack this fine pitcher in three! And the glass of water they’ve left for me shall go tchick! to tell them I’m drinking.”

In the morning the children of the village woke up and ran to the windows. They gasped, because every pane was covered with silver ferns and feathers, towers and stars. In the kitchen they found the apples nipped, the pitcher cracked and the water frozen solid in its glass. But not one child had a pinched nose. Jack Frost had kept his promise.

Jack Frost still makes his rounds every winter, as quiet as a whisper. So on a cold, clear morning, run to your window and look. If you see silver pictures on the glass, you will know that Jack Frost passed by in the night, and that he still remembers what Claus asked him long ago.

Adapted from Hannah Flagg Gould, The Frost (in Poems That Every Child Should Know, ed. Mary E. Burt, 1904) (1836). Public domain. Read the original.