Back Scatter: The beauty of freezing water

All photographs by Cynthia Cummings for Physics Today.

When conditions are right in cold climates, hoarfrost can form in the early-morning hours, covering everything from blades of grass to lawn ornaments with a delicate coating of ice crystals and creating a wintry wonderland. Such a morning occurred in the Chicago suburb of Sugar Grove, Illinois, on 9 January. The temperature hovered around –5 °C, and the relative humidity level was quite high, at 90% or so. Dawn revealed a blanket of fog that gradually lifted, leaving every plant stalk and seedpod covered with fine white barbs like the flocking on a Christmas tree.

Although there are several types of frost, hoarfrost is usually characterized by its “light, airy, and delicate” appearance, according to Mary Jane Shultz , a chemistry professor at Tufts University. Hoarfrost forms in frigid, humid conditions when water vapor contacts a cold, hard surface and freezes. Fed by the plentiful moisture in the air, the nascent ice crystals expand to form whiskery spikes, which Shultz says can be “tremendously long.” The frost crystals are colorless and transparent but gain their white, hoary appearance when the sunlight that hits them is reflected.

The spectacle is ephemeral. As the sun comes up and increasingly illuminates the hoarfrost, the melting process accelerates. Even gentle breezes can further deteriorate the frost by dislodging the spikes from where they’ve formed and depositing them on the ground, as seen in the photo below.

The phenomenon of hoarfrost is just one example of the unique properties of freezing water. To learn more about crystal growth in ice and snow, see Shultz’s article in the February 2018 issue of Physics Today.