With recent ice storms knocking out power across Vermont, damaging sugaring operations, sealing cars inside frozen cubes, inducing falls on slippery sidewalks, causing hours of painful shoveling and dropping huge ice chunks onto cars, it’s hard to appreciate ice right now.
Despite the trouble it causes, ice can also form beautiful and intriguing shapes.
Take, for example, hoar frost. This crust of delicate ice crystals forms on cold, clear nights with little wind, when the water vapor in air bumps into a very cold surface. The vapor cools so suddenly that it solidifies instantly into ice, without going through a liquid phase.
There are two ways in which ice forms, says atmospheric scientist Jason Shafer, a professor at Lyndon State College and the director of the Vermont Institute for Applied Meteorology.
“You can go from a liquid to a solid, like if you have a puddle that turns to ice,” he explained. “You get to a freezing temp and what happens is you reach a point where the substance starts to turn to a solid. All you need are a few molecules to start doing this and the whole system will start to cascade.”
That’s because in order to freeze, water needs something to freeze onto — some particle of dirt or an ice crystal in the air or the water.
“The other thing is you can go straight from a gas to a solid, a process called deposition, in which water vapor turns right into a solid,” Shafer said. “That occurs in hoar frost.”
Frozen vapor
Hoar frost requires particular conditions: humid air combined with extreme cold. Cold air holds less moisture than warm air — in fact, 32-degree air holds a third of the water vapor of 59-degree air, so humid air is rare in the winter. Low clouds can provide enough moisture to create hoar frost, but they are also insulating, which prevents the necessary cooling. This is why hoar frost is less common in forests, where trees insulate the forest floor.
When the conditions are right for hoar frost, it happens like this. The surface of the snow cools after sunset and water molecules in the air above begin to freeze onto the snow surface, forming crystals. The air that they vacated becomes available for more water molecules, higher up, to move down closer to the snow. The process of molecules moving into vacated space is called diffusion. It’s easiest to describe this phenomenon as if the molecules “want” to move into the empty spaces, but there is no desire involved. The molecules bounce around, bumping into each other. Eventually, after enough bumping, some are bumped into the empty space and the molecules become more evenly distributed. More molecules freeze onto the snow and the process continues, feeding the growth of surface hoar.
The same process can happen if the moisture comes from within the snowpack. Let’s say the snow melted a bit during the day. Some of that melted snow evaporated, increasing the humidity within the snowpack. As night falls, the surface of the snow cools very quickly, inducing the kind of temperature gradient that feeds hoar frost, only this time the moisture comes from below the snow surface, rather than above.
Hoar frost can also form on trees, fences, or other objects. One cause of hoar frost is animal breath. In winter, many animals bed down under the insulating snow at night, leaving an entrance hole that serves as a vent for their warm, moist breath. That moist breath hits the very cold snow around the burrow entrance, forming a crust of hoar crystals. You might find these filigreed shelters on a morning wander through the woods, and when you do, you are in the delightful position of knowing a sleeping animal is likely just a few feet away.
City-dwellers have the good fortune of being able to observe hoar frost readily in cold weather. The vents of heaters and dryers outside houses provide enough moisture to build up hoar frost on the surrounding wall, ground, and shrubbery.
Reasons for rime
Rime ice, at first glance, appears similar to hoar frost. The difference is that hoar frost is formed when water in its gaseous form (i.e., water vapor) freezes onto a cold object. Rime, on the other hand, is formed when water in its liquid form (i.e., water droplets), freezes onto a cold object, as is common when clouds move across mountaintops.
“I got this really cool picture of rime ice on my back porch a few winters ago,” Shafer said. “I hadn’t cleaned up the cobwebs and they collected all the snow and ice. Kind of creepy.”
With rime ice, says Shafer, “You usually wouldn’t see the individual crystalline structure,” whereas in hoar frost you most likely would.
Wholly rollers
Even rarer than hoar frost are snow rollers. On snowy winter days you may see herds of these little lumps peppered in sloping fields. What from a distance appear to be lumps are actually cylinders made out of snow.
Snow rollers are “basically rolled up snow the same way you and I would make a snowman,” said Shafer, “and instead of us rolling up the snow, the wind is doing it.”
These rare formations appear in very particular conditions. “You can’t make a snowman out of light, fluffy snow,” Shafer explained. “And the kind of snow we have right now is too dense and heavy.” For a snow roller to form you need “fresh snow that is just starting to get above freezing and you need a pretty strong wind as well,” about 15 to 20 miles per hour.
A snow roller starts when the wind loosens a chunk of fresh snow. The wind pushes on that chunk, rolling it over and over, leaving a trail of cleared snow behind it. As this continues, the snow chunk turns into a cylindrical roll of snow, with the inner layers thinnest. These are often blown away, making a hollow log of snow. When the snow roller is too big to be blown further, it stops growing. Snow rollers form best on slopes, where gravity helps roll the snow-logs downhill.
Other strange ice formations can be found in wintry Vermont, too. On an autumn or early winter hike, you might see needle ice extruded from the soil. These columns of ice, usually an inch or two long, appear when the air is below freezing and the ground is above freezing. These conditions usually occur overnight, particularly in the fall and spring when the air gets cold while the ground is still relatively warm. When that happens, liquid water in the soil is sucked toward the surface of the ground through capillary action.
The water freezes just below the surface and the resulting ice is pushed upward in a process called ice segregation. As ice segregation continues, thin needles, or columns, of ice form. These ice needles tend to form in clumps, where pockets of moisture exist in the soil. Together, the clumps of columnar ice push up the soil surface, so that patches of needle ice are often capped with bits of soil or pads of moss. On slopes, this tendency causes the soil to slowly creep downhill. Needle ice forms most often where the spaces between soil particles are big enough to allow water through but small enough to prevent it moving downward under gravity. Fine soils in which clay and silt dominate provide the best conditions for needle ice.
While you are chipping away at the ice rink in your driveway, or calling the insurance company about a crater in the roof of your car, you can remember that ice is not just dangerous, but strange and beautiful, too.
