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Antarctic conservation

2 Posts tagged with the ice_crystals tag
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Sundog

Posted by Conservators Apr 2, 2012

Author: Georgina

Date: 28/03/12
Temperature: -23c
Wind Speed: 25 kts
Temp with wind chill: -38c
Sunrise: 8:39am
Sunset 7:15pm

 

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Sundog seen from Scott Base (photo by G. Evans, AHT)

 

For the past few weeks, we have been enjoying the rising and setting of the sun in something approaching normal daylight hours.  On Monday we were able to see a sundog, (scientific name: parhelion); a very special atmospheric phenomenon in which bright spots of light appear in the sky on either side of the sun.


Sundogs are created by ice crystals in the air which act as light-bending prisms. When randomly orientated, a complete halo or luminous ring around the sun is created, but at lower levels the crystals (sometimes called diamond dust) become vertically aligned, causing the light to be refracted horizontally.

 

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Detail (photo by G. Evans, AHT)

The spots we saw were like partial blurred rainbows, with the red side innermost. I didn’t get the chance to see one of these when I was last here in 2010, so I feel very lucky indeed to have seen one now.  Sundogs (also known as mock or phantom suns) can be seen anywhere in the world, but rarely as obvious or as bright as when the sun is low on the horizon and in very cold regions like Antarctica.

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Ice and Snow Formations

Posted by Conservators Jun 3, 2011

Date: 02/06/2011

 

Posted by Julie

 

Temperature: -22

Wind Speed: Gusting between 20 – 40 knots
Temp with wind chill: - 36  to  -41
Sunrise: August
Sunset August

 

 

Ever since the spectacular ice breakout in February, the ice forming over the water in front of Scott Base has steadily been growing thicker.  Last week the ice was judged thick enough to walk on. Troy, our base leader, Jane, and I set out to mark a safe route for walking through some nearby pressure ridges.

 

Troy is an experienced glacier guide, and so we got the bonus of getting Troy to talk about the snow and ice.  In a crack in the ice we discovered some spectacular, large, faceted ice crystals.  This is sometimes known as crevasse hoar. Troy explained that these ice crystals form and grow in glacial crevasses and in other cavities where a large cooled space is formed and in which water vapor can accumulate under calm, still conditions with a large temperature gradient. The vapour then attaches itself straight to the ice crystal forming a hollow hexagonal shape.

 

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Faceted ice crystal. © AHT/Julie

We also found impressively large icicles.  Icicles require liquid water and so they are notable: the temperature rarely goes above freezing, and certainly has not been above 0 degrees for many months now.

 

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Jane impersonating a radioactive rhinoceros by shining her headlamp through an icicle. © AHT/Julie


Most spectacular are the pressure ridges themselves.  Pressure ridges form because even very thick sheets of sea ice are mobile. As the sea underneath moves, or when the temperatures fluctuate, the ice shrinks and expands, cracks and shifts.  At Scott Base, the sea ice is bordered by Ross Island on one side and the permanent, immobile ice shelf (80 km thick) on the other.  The relatively thin sea ice between has nowhere to move except upwards, and so plates of ice are very slowly pushed up vertically along cracks, eventually developing into spectacular ice formations. These pressure ridges survived the recent sea ice breakout in February, meaning they are at least 14 years old, the date of the last ice breakout.

 

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Pressure ridges.  The formation in the foreground is approximately 5 meters high. © AHT/Julie