I have very often in a Morning, when there has been a great hoar-frost, with an indifferently magnifying Microscope, observ'd the small Stiria, or Crystalline beard, which then usually covers the face of most bodies that lie open to the cold air, and found them to be generally Hexangular prismatical bodies…

The above passage, from Robert Hooke's Micrographia (1664), describes almost exactly what I do on sufficiently cold winter mornings. Though I don’t have a microscope, I have a Canon Power Shot camera with the macro feature that allows me to zoom in on tiny objects. (The best gift I ever got, and possibly the best thing an amateur naturalist like me could ever receive.) From afar, most hoar frost looks similar – a dusting or coating of white on any surface that can cool enough for vapor to deposit. The effect on an object can be beautiful, looking like an artist has applied a white highlighter to the small protrusions and ridges on the object, as in the rock below.

The rock is about an inch and a half across. The crystals range in size, but the larger ones are about as wide as three thick hairs (300 microns) laid side-by-side. One can see in the enlargement below that many of the crystals have a hexagon shape – like a snow crystal.

A better example showing hexagonal crystals is in the frost I saw on the roof of a black car a few days ago.

A slightly raised strip ran down the length of the roof on which the hoar appeared uniformly white to the eye. The photo shows some crystalline structure, and if you look close, you can just make out some hexagons.

Upon magnifying the image, the hexagons are clear.

Notice that a region with hexagons tends to have neighboring hexagons with the same alignment. If each crystal nucleated independently, then having the same alignment would be a coincidence. So the observation suggests that each crystal did not nucleate independently. Indeed, this and many similar observations tell me that much of what I see as hoar frost began when a liquid water film condensed and then froze. Then, with the ice coating the surface, vapor deposits, making the crystal grow up off the film. But growth is not uniform, rather it breaks up into separate columns or crystal fins that stick up off the surface. What to us appear as separate crystal hexagons side-by-side is actually one crystal that sprouted several hexagonal columns. These three stages (liquid condenses, liquid freezes, vapor deposits) is just like what happens with most snow crystals, except with snow, the water condenses as a tiny droplet, not a film. Then, once the drop freezes, vapor deposits on the crystalline sphere, causing the crystal to take on a snow-crystal shape.

- JN