Frost crystals grow from water vapor in the air, just like snow crystals (see the Snowflake Primer). But while snow crystals form on suspended dust particles high in the clouds, frost crystals form near the ground -- on window panes, blades of grass, or just about any other solid surface.

Window Frost

Window frost forms when a pane of glass is exposed to below-freezing temperatures on the outside and moist air on the inside. Water vapor from the air condenses as frost on the inside surface of the window. The picture at right shows a patch of window frost about the size of an outstretched hand. Window frost often makes elaborate patterns as the crystal growth is strongly influenced by the window surface. Scratches, residual soap streaks, etc., can all change the way the crystals nucleate and grow.
Window frost was more common in the past, when houses still had single-pane windows. The newer double-pane windows are much better insulators and thus not so cold on the inside surfaces.

At left are some close-up pictures of window frost I took during a recent vacation in the California mountains. The branches you see here are tiny -- each about 3mm in length. The magnified view shows some of the same faceting and dendritic structures that you see in snowflakes.

Hoarfrost

Frost is to dew as snowflakes are to raindrops. When water vapor condenses into liquid water, you get raindrops and dew. When water vapor condenses directly into ice, then you get snowflakes and frost (see the Snowflake Primer). Snowflakes are not frozen raindrops, and likewise frost is not frozen dew.
When frost forms as minute ice crystals covering the ground, we just call it all frost. But sometimes the frost grains grow larger and are called hoarfrost crystals. Good hoarfrost is not that uncommon if you watch for it. Hoarfrost grows whenever it's cold outside and there is a ample source of water vapor nearby.

We ran across some great hoarfrost crystals while cross-country skiing early one morning in the California mountains, which you can see in these pictures. A nearby unfrozen stream was the source of water vapor, which condensed into these crystals overnight.
Some of the crystals grew pretty large (that's a ski for scale) and had the same lacy structure as dendritic snow crystals (see the Snowflake Primer). By midmorning they had all disappeared.

The pictures at left are of more hoarfrost crystals I found in northern Ontario near James Bay. The first picture shows a twig of grass that is covered with ice crystal "leaves". The second picture shows a closer view of a small stalk with some very long dendritic crystals.
One of the best places to find hoarfrost is on exposed plants near unfrozen lakes and streams. It's a nice thing to look for if you're out hiking in the woods on a winter morning.

I found lots more hoarfrost crystals to photograph in northern Japan.

Grow Your Own Frost. You can make your own hoarfrost crystals if you have a cold, wind-free location -- for example, an unheated outdoor shed. Just plug in a hotplate and heat an open pan of water over a low heat for a day or two. You'll want to heat the water without heating the whole shed, so it helps if it's good and cold outside.
You may want to provide a branch or some other artistic surface above the pan for water vapor to condense onto. How the crystals grow will depend on the air temperature (see the Snowflake Primer), so you may find different types of hoarfrost crystals on different days. If you see something interesting when you try this, take some pictures (see Photographing Snow for useful techniques) and send me an e-mail.

Surface Hoar

The most common form of hoarfrost is called surface hoar. This consists of ice crystals that form on top of snow banks, usually overnight. The sparkles you see coming from a field of snow are often reflections off the facets of surface hoar crystals.
Surface hoar typically forms when a snowbank warms up during the day and is then cooled again overnight. The night air cools the surface of the snowbank more than the inside, so that water can evaporate from inside the snowbank and recrystalize on the surface. By morning the snowbank is covered with a layer of faceted ice crystals, and they can be quite large. These usually melt again once the sun comes up, so the best time to find surface hoar is early in the morning.

Frost Flowers

   Some of the stranger ice formations you're likely to find in the woods are called "frost flowers" or "feather frost". A typical example looks like a small puff-ball of cotton candy, a few inches across, made up of clusters of thin, curved ice filaments.
   Frost flowers usually grows on a piece of water-logged wood, as shown in the pictures at right (taken by Nick Page; provided by Alan Rempel). It's something of a rare find, meaning that conditions have to be just so before it will form.
   Not much has been written on this unusual phenomenon, and to my knowledge it has never been reproduced in a controlled laboratory environment. It appears that the ice filaments are essentially pushed out from pores in the wood as they freeze.
   It's something of a misnomer to call this frost, by the way, since it freezes from liquid water, not water vapor.

Rime Formations

Snow crystals accumulate rime when they collide with water droplets in the clouds (see the Guide to Snowflakes (near the bottom of that page)). When the clouds are near the ground you have fog, and sometimes the fog is made from supercooled water droplets -- water at a temperature below the freezing point. Then the droplets freeze on contact to anything they hit, sometimes yielding some bizarre, wind-driven rime formations. The pictures of this phenomenon above were sent by Keenan and David Mackey (first picture) and by John Gibson (second).