Saturday, 26 January 2013


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Pictures: The Story Behind Sun Dogs, Penitent Ice, and More
By Jane J. Lee,
National Geographic News, 25 January 2013.

Ice forms some weird and wonderful things - find out how Mother Nature does it.

1. Penitent Snow

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If you want the beauty of winter without having to brave the bone-chilling temperatures blasting much of the United States this week, snuggle into a soft blanket, grab a warm beverage, and curl up with some of these natural frozen wonders.

Nieve penitente, or penitent snow, are collections of spires that resemble robed monks - or penitents. They are flattened columns of snow wider at the base than at the tip and can range in height from 3 to 20 feet (1 to 6 meters). The picture above shows the phenomenon in central Chile. (See pictures of the patterns in snow and ice.)

Nieve penitente tend to form in shallow valleys where the snow is deep and the sun doesn't shine at too steep an angle, said Kenneth Libbrecht, a physicist at the California Institute of Technology in Pasadena who studies ice crystal formation.

As the snow melts, dirt gets mixed in with the runoff and collects in little pools here and there, he said. Since the dirt is darker in colour than the surrounding snow, the dirty areas melt faster "and you end up digging these pits," explained Libbrecht.

"They tend to form at high altitude," he said. But other than that, no one really knows the exact conditions that are needed to form penitent snow.

"They're fairly strong," Libbrecht said. "People have found [the spires] difficult to hike through."

2. Frozen Fingers

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Ice stalactites (pictured) form on the undersides of sea ice in the Arctic (interactive map) and Antarctic. (Watch a video about Antarctica's ice.)

"They're typically formed in thinner [sea] ice that's just growing," said Don Perovich, a geophysicist who studies sea ice with the Cold Regions Research and Engineering Laboratory - part of the Army Corp of Engineers - in New Hampshire. (See the animals that are threatened by vanishing sea ice.)

When ice forms from saltwater, the emerging crystal structure rejects salt particles in the seawater. That salt mixes with water that hasn't frozen yet, creating a supersalty brine, Perovich explained.

The freezing point of seawater depends on its salt content, so the saltier it is, the colder it has to get before it can freeze. This means that the brine stays liquid while ice forms around it.

Eventually the process forms a network of channels through the ice that drains the brine into the ocean, said Perovich.

Colder and saltier than normal seawater, the brine starts to sink when it hits the ocean. And when it does, it freezes the warmer surrounding water, "and you get the shell of this stalactite growing longer and longer," Perovich said.

3. Icy Bloom

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Delicate, cotton-candy-like structures like the one pictured need the perfect conditions to form. "They occur when the temperature is right around freezing," said Caltech's Libbrecht.

They usually appear on rotten, waterlogged plants, the ice crystal researcher said, and "they tend to form in Appalachia in winter because it doesn't get too cold and there's a lot of water around.

"It has to freeze very gently," he added. That's because the water contained in the vessels and tubes of woody plants needs to freeze slowly, from the top to the bottom. If temperatures get too cold, the plant will freeze too quickly.

"As water is wicked up [the tube], the ice gets pushed out the top by forces we don't really understand," he said.

The result: extruded sheets or ribbons of ice that look like frozen blooms attached to the vegetation.

Commonly known as frost flowers, they can form within hours, usually overnight.

4. Glacial Army

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Another army of "penitents" marches up a mountain slope in Bolivia.

Charles Darwin is credited with the first written account of this phenomenon. He recorded a field of penitent snow while traveling through the mountains of Chile on March 22, 1835.

After a "heavy and long climb," Darwin and his group came across a field of these "pinnacles," as he called them.

While trying to cross this field, Darwin spied a frozen horse impaled on the top of the one of the spires, "its hind legs straight up in the air." (Read Darwin's diary entry describing the pinnacles.)

5. Windchill

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Photograph by Mike Gatch, Your Shot

A lighthouse in St. Joseph, Michigan, pulls double duty as an icicle-bearing sentinel. (See pictures of winter in the U.S.)

Caltech's Libbrecht said water spray from Lake Michigan froze into these icicle shapes.

The process is similar to a phenomenon called rime ice, he said.

Rime forms when the temperature of atmospheric water droplets dips below freezing and the water comes into contact with a surface - the droplets immediately freeze, creating a coating of ice.

The Mount Washington Observatory in New Hampshire is known for the rime ice that coats many of its scientific instruments.

6. Glacial X-Ray

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Ambient light filters through the ceiling of an ice cave on Ross Island in Antarctica, creating a cracked pattern overhead.

This cave was probably a subglacial tunnel, explained Allen Pope, a doctoral student studying satellite imaging of glaciers at the University of Cambridge.

They're formed when ice melts at the surface of a glacier and then finds a moulin - a big vertical shaft - that runs down to the base of the glacier. (Watch "Chasing Ice" photographer James Balog discuss melting glaciers.)

"Once the water's at the base of the glacier it has to go somewhere, so you'll get subglacier rivers," said Pope, a National Geographic Society grantee (National Geographic News is part of the National Geographic Society). "That tunnel was probably formed in the summer at the height of the melt season."

The patterns on the tunnel's ceiling are formed when light reflects down through the glacier, highlighting the boundaries between ice crystals, he explained. "[It] looks like the crystal structure of the ice itself."

7. Halos and Sun Dogs

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Ghostly rings and arcs, such as the 22-degree halo pictured, form when sunlight or moonlight refracts off of ice crystals in the atmosphere.

The shape and alignment of these crystals will determine the appearance of various phenomena, said Caltech physicist Libbrecht. Crystals that are only a little bit aligned will produce sun dogs, or bright patches of light in the sky.

The amount of ice crystals needed to form halos, sun dogs, or other atmospheric phenomena can span hundreds of feet, he said. (See a picture of ice halos and arcs from Hurricane Sandy.)

8. Frost Flower Field

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A meadow of icy blooms coalesces out of moisture-laden air in this picture taken in the Ross Sea in Antarctica.

"[This is] a fairly rare phenomenon," said Caltech's Libbrecht. But it's essentially just frost, he explained.

Very cold water droplets in the air will attach to a spot on the surface of sea ice and freeze. More ice crystals will form on these areas and you end up with a meadow of frosty flowers.

"Conditions have to be just so," Libbrecht said. "[And] the frost has to form slowly... It can take between hours to days for these to form."

9. Underwater Ice Pillars

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This picture of ice stalactites that have grown to meet the seafloor was taken in Antarctica's McMurdo Sound.

A documentary crew for the BBC One series Frozen Planet came across an ice stalactite in the process of forming while scouting underwater locations near the Ross Archipelago in Antarctica in 2009.

The crew managed to capture a time-lapse sequence of the "ice finger of death" as it grew down from the underside of the ice and encased sea stars on the ocean floor. (See a video explaining how the filmmakers got the shot.)

[Source: National Geographic News. Edited.]

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