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 vapour 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 towards an artist who decorates windows. The physics is real and rather lovely: fern frost forms when water vapour 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.