Not all ice is created equal

Cool formations

Posted: Sunday, December 17, 2006


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  Ice extends brittle fingers of cold over water flowing in Crescent Creek near Cooper Landing earlier this winter. M. Scott Moon

Ice extends brittle fingers of cold over water flowing in Crescent Creek near Cooper Landing earlier this winter.

M. Scott Moon

When winter sets in, most of Alaska’s greenery and wildlife bows out and solidifying water takes center stage, creating an endless source of glittery spectacles to bedazzle and sometimes bewilder observers brave enough to endure winter’s sharp bite.

As fall quickly turns to winter, needle ice grows out of the ground like garlic being squeezed through a garlic press, feathery hoar frost crystals cover grasses and branches and a jumble of ice piles up in turbulent rivers.

Just feasting one’s eyes on the many ways water transitions from a liquid to a solid in the winter is enough to inspire awe, but observers reveal the true depth of wonder in the winter world when they seek explanations for what they see.

When the ground begins to freeze in early winter, for example, ice needles greet hikers as a trail-side curiosity, said Karen Henry, a research civil engineer at the U.S. Army Corps of Engineers’ Cold Regions Research and Engineering Laboratory in New Hampshire.

“That’s what makes it so interesting is that it grows,” she said. “I mean, it’s like wow, this is really cool when you’re walking around and looking at it. And then sometimes you’ll see the layers of the soil particles in the needle ice. You’ll get two or three inches of clear and then you see soil and then you see two or three inches of clear.”

Henry said the fact that needle ice grows out of the soil instead of freezing under the soil’s surface is a little mind-boggling, and that people often mistakenly think that the ice needles grow outward due to the pressure that builds up as the freezing water expands.

Instead, Henry said the answer to why the ice needles grow outward is a bit more complex, and was first discovered by a geologist trying to determine how lava solidifies in the earth.

The geologist discovered that the lava and ice needles both crystallized in the direction of heat loss. In the case of ice needles, heat is being lost from the ground’s surface. Consequently, the needles grow out of and perpendicular to the ground, she said.


Pans of ice, some larger than a warehouse, jostle for position in the shifting tide as seen from the air above Cook Inlet last winter.

M. Scott Moon

“When the ground is horizontal it grows up, when the ground is at an angle or vertical, you know that the earth itself is warmer than the air temperature so you can see that the needle ice will grow perpendicular to the ground surface,” she said.

When asked to explain why the crystallization forms in the direction of heat loss, Henry said it helps to think of the holes in the soil as straws in a glass of water.

If the water inside of one of the straws evaporates, the water in the straw will not become lower than the water surrounding the straw, instead the water will maintain equilibrium by replacing the water that evaporated, she said.

“You have a very analogous process with the needle ice,” she said. “(But) instead of evaporation occurring, it changes phase into ice. So the water doesn’t evaporate. It changes into ice, but the liquid water still wants to remain at the same height so water from below will come up and replace that ice.”

Henry said needle ice needs both freezing conditions and water that’s close to the soil’s surface — as in an area with a high water table or local subsurface source of water.

A special set of circumstance also are needed for a second winter trail-side curiosity. Hoar frost can grow under the snow, on top of snow and on the ground, but is probably most readily recognized as the delicate crystals that sometimes adorn grasses and tree branches.


Needle ice is a winter curiosity along trails.

M. Scott Moon

In order for hoar frost to grow, it needs a steep temperature gradient and source of moisture, said Mary Albert, a research engineer at CRREL.

If, for example, moist air evaporating from a river in early winter encounters a colder substrate such as an icy-cold branch, hoar frost could form on the branch. But the reverse could also happen, as is the case with hoar frost that grows on the ground, said Albert.

“Hoar frost forms on the ground when the ground’s not frozen and it has some soil moisture in it,” she said. “And then if you get a really cold night hoar frost will grow from the ground up. And in that case, it grows because the ground is warm and the air is very cold. So you need a temperature difference one way or the other and a moisture source.”

Hikers walking near turbulent rivers, such as the Kenai River, might notice a sharp contrast between the delicate hoar frost clinging to nearby vegetation and another form of solidified water — rugged chunks of river ice.

In turbulent streams, such as the Kenai River, ice formation begins its life as frazil ice, a mass of randomly oriented ice crystals that hover loosely in the water.

Individual frazil crystals are difficult to see with the naked eye, but together look like a cloud trapped beneath the water’s surface, said Dave Streubel, a hydrologist with the National Oceanic and Atmospheric Administration’s Alaska Pacific River Forecast Center.


A willow branch collects particles of hoar frost on a bitterly cold morning alongside the Kenai River.

M. Scott Moon

Frazil ice forms when turbulence prevents water from solidifying until after its temperature has dipped below freezing, creating what is called supercooled water.

“The water’s flowing down the stream and so it’s mixing around and bounces off of rocks and stuff, and the water in that case gets to the point where it’s actually below zero degrees (Celsius) and it’s just dying to freeze,” Albert said.

In a large turbulent river, such as the Kenai, the frazil ice migrates to the surface as it turns to slush and eventually forms ice pans, said Streubel. And as more and more ice pans collect along bends in the river and get caught in debris along the river’s edge they eventually accumulate and cover the river’s surface.

Sometimes, however, frazil ice does an unusual thing and forms an ice cover over the bottom of the river rather than the top, Streubel said.


Ice spreads like a spider web suspended above a puddle in a parking lot in Kenai.

M. Scott Moon

He said he has not heard of ice forming on the bed of the Kenai River, but that he has sometimes seen it in small turbulent rivers, such as the Ninilchik River.

“In some of these small streams ... the frazil ice, instead of floating to the top, adheres to the stream bed itself,” he said. “You’ll see the rocks underneath the water coated with ice. It’s really interesting to look at ... you’ll see the stream bed has ice on it and there’s running water over the top of it.”

But while frazil ice results in some curious phenomena for observers to puzzle over, it also has a more sinister side.

On days when the weather is particularly cold an abundance of frazil ice can cause flooding by jamming the river with ice.

When lots of frazil ice forms, it not only creates pans that eventually cover the river’s surface, but also builds up on the underside of the ice cover where it restricts the flow of water through the river like a clogged artery, Streubel said.

Frazil ice also is the starting point for yet another type of ice formation familiar to Alaskans. Ice pans in the sea like ice pans in rivers begin with frazil ice.

In the turbulent waters of Cook Inlet, for example, frazil ice is the first phase in the formation of sea ice pans, said Kathleen Cole, a sea ice forecaster, with NOAA’s Alaska Pacific Forecast Center.


Stationary ice frames flowing water at Juneau Falls on the Resurrection Pass Trail near Cooper Landing last winter. Gravity overcomes cold to keep water moving down the 200-foot break in Juneau Creek.

M. Scott Moon

As the water continues to cool, the frazil ice forms slush and what is known as shuga. In the ocean’s waves, the slush is barely noticeable and mixes with the water like slush in a margarita. Shuga, on the other hand, is similar to slush, but hangs out on top of the water where it forms whitish lumps and is easier to see.

Next, slush and shuga turn into a form of ice known as nilas. Nilas forms a crust over the ocean’s surface, but it’s elastic and, like a heavy blanket laid over the ocean’s surface, slows but does not stop wave action.

“It gives with the waves, but it slows the wave motion down tremendously,” she said. “And it becomes an undulating action on top of the waves ... . It’s fascinating to watch the waves move in it because they just sort of roll with it.”

As the nilas thickens, however, it loses its elasticity and starts to break up, forming young pan ice. Young pan ice is usually between 4 and 12 inches thick and has sharp edges. But as turbulent water bumps the pans against each other, their sharp edges begin to wear until the pans take on more rounded shapes and develop raised lips.

Cole said that on a cold winter in Cook Inlet, it’s not unusual to find pan ice measuring 20 feet across. Depending on how rough the water is, an ice pan can reach a width of up to 50 feet across. But most are only four to five feet across, she said.

“But there are a lot of them and they pack a lot of force,” she said.

In Cook Inlet she said there have been reports of pans measuring up to about three feet thick, but usually they don’t grow to more than two feet thick.

How strong ice is, however, depends not only on thickness, but also on purity. Clear lake ice, for example, can support more weight than white lake ice containing bubbles, because impurities weaken ice, Albert said.


Blocks of ice rise up out of the Kenai River in front of Cunningham Park earlier this winter.

M. Scott Moon

However you enjoy this winter’s icy spectacle — whether it be standing on a lake peering into the depths below or licking hoar frost from a dangling twig—there’s always room to lose oneself in wonder over the many ways that water transitions into the crystalline sculptures that make up the winter world.

Patrice Kohl can be reached at

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