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Neanderthal had low genetic diversityFollowing on from a recent post about how much of our cousin’s genetic legacy lives on in us (seehttp://tinyurl.com/mzm2h3e), another study just published in Proceedings of the National Academy of Sciences has compared over 17,000 known protein coding genes between 3 specimens excavated in Croatia, Siberia and Spain in order to assess something of the genetic variation between these widely dispersed populations.Comparing these with the average human spread, Neanderthal had very low diversity, and the results suggest low populations living in small, relatively isolated groups. They had about a quarter of the rate of African diversity, and a third of that of Asians or Europeans. Compared to ancestral humans, the genes linked to skeletal morphology (think of those huge thick bones) changed to a greater extent during their evolution than ours, while modern humans who swiftly became more geographically diverse show greater development in genes affecting skin pigmentation, demonstrating the effect of selective pressures of the different environments we evolved in.Recent research suggests that we spread out of Africa in more than one wave, starting as early as 140,000 years ago and that each wave has its own distinctive genetic legacy, partly influenced by the environment they ended up in. The two lineages split sometime between 750 and 550,000 years ago, and we were at the limit of diverging into two species when non African Homo sapiens interbred with our cousins some 40,000 years back. Humans are already less diverse than apes, suggesting genetic bottlenecks in our deep past, and Neanderthal seems to have been even less diverse, possibly as a result of repeated ice age population squeezes.LozImage credit: Aquila Gibhttp://www.livescience.com/44989-neanderthals-had-shallow-gene-pool.htmlhttp://phys.org/news/2014-04-genetic-neanderthals-diverse-modern-humans.htmlOriginal paper, paywall access: http://www.pnas.org/content/early/2014/04/16/1405138111

Neanderthal had low genetic diversity

Following on from a recent post about how much of our cousin’s genetic legacy lives on in us (seehttp://tinyurl.com/mzm2h3e), another study just published in Proceedings of the National Academy of Sciences has compared over 17,000 known protein coding genes between 3 specimens excavated in Croatia, Siberia and Spain in order to assess something of the genetic variation between these widely dispersed populations.

Comparing these with the average human spread, Neanderthal had very low diversity, and the results suggest low populations living in small, relatively isolated groups. They had about a quarter of the rate of African diversity, and a third of that of Asians or Europeans. Compared to ancestral humans, the genes linked to skeletal morphology (think of those huge thick bones) changed to a greater extent during their evolution than ours, while modern humans who swiftly became more geographically diverse show greater development in genes affecting skin pigmentation, demonstrating the effect of selective pressures of the different environments we evolved in.

Recent research suggests that we spread out of Africa in more than one wave, starting as early as 140,000 years ago and that each wave has its own distinctive genetic legacy, partly influenced by the environment they ended up in. The two lineages split sometime between 750 and 550,000 years ago, and we were at the limit of diverging into two species when non African Homo sapiens interbred with our cousins some 40,000 years back. Humans are already less diverse than apes, suggesting genetic bottlenecks in our deep past, and Neanderthal seems to have been even less diverse, possibly as a result of repeated ice age population squeezes.

Loz

Image credit: Aquila Gib

http://www.livescience.com/44989-neanderthals-had-shallow-gene-pool.html
http://phys.org/news/2014-04-genetic-neanderthals-diverse-modern-humans.html

Original paper, paywall access: http://www.pnas.org/content/early/2014/04/16/1405138111

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This is ‘The Giants Causeway’ (Clochán na bhFomhórach) in County Antrim, Northern Ireland. During the Paleogene period around 50 to 60 million years ago, Northern Ireland was subject to immense volcanic activity. Highly molten basalt intruded onto the surface to form an extensive lava plateau. There was rapid cooling; which caused contraction, cracking and the formation of pillarlike formations of different heights and widths. The estimated 40,000 interlocking basalt columns make an enchanting landscape.  What is even more enchanting is the folklore behind this beautiful area. There are many different variations to this story; but I will tell the one I was told as a child. This version has it that Finn McCoole built the causeway to get across to Scotland. There, he raided the livestock of a Scottish giant. The Scottish giant came back over the causeway to seek revenge for the raid and fight Finn. Finn’s wife; Oonagh, on seeing him cross the Causeway, and noting the size of the Scottish giant, was so afraid for her husband that she schemed to dress him up in baby clothes to protect him. When the Scottish giant came to the house, he asked the wife where the Finn was, and the wife told the Scottish giant he was out. Looking around the house, the Scottish giant came across the baby. When he saw the size of the baby, he figured the father must be truly colossal, far too large to fight, and so ran back across the Causeway to Scotland, destroying it behind him, leaving what we see today. There is something uniquely Irish about this story and I think it captures the Irish wit quite brilliantly.  It has always been known that people create stories to explain what they do not understand; I think in this instance, it is done quite nicely. I hope you enjoyed it. -Jean Image courtesy of Martin McKenna For more inforamtion see http://www.giantscausewayofficialguide.com/

This is ‘The Giants Causeway’ (Clochán na bhFomhórach) in County Antrim, Northern Ireland. During the Paleogene period around 50 to 60 million years ago, Northern Ireland was subject to immense volcanic activity. Highly molten basalt intruded onto the surface to form an extensive lava plateau. There was rapid cooling; which caused contraction, cracking and the formation of pillarlike formations of different heights and widths. The estimated 40,000 interlocking basalt columns make an enchanting landscape.

What is even more enchanting is the folklore behind this beautiful area. There are many different variations to this story; but I will tell the one I was told as a child. This version has it that Finn McCoole built the causeway to get across to Scotland. There, he raided the livestock of a Scottish giant. The Scottish giant came back over the causeway to seek revenge for the raid and fight Finn. Finn’s wife; Oonagh, on seeing him cross the Causeway, and noting the size of the Scottish giant, was so afraid for her husband that she schemed to dress him up in baby clothes to protect him. When the Scottish giant came to the house, he asked the wife where the Finn was, and the wife told the Scottish giant he was out. Looking around the house, the Scottish giant came across the baby. When he saw the size of the baby, he figured the father must be truly colossal, far too large to fight, and so ran back across the Causeway to Scotland, destroying it behind him, leaving what we see today.

There is something uniquely Irish about this story and I think it captures the Irish wit quite brilliantly.

It has always been known that people create stories to explain what they do not understand; I think in this instance, it is done quite nicely.

I hope you enjoyed it.

-Jean

Image courtesy of Martin McKenna

For more inforamtion see http://www.giantscausewayofficialguide.com/

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HABOOB A haboob (Arabic: هَبوب “strong wind”) is a wall of sand and/or dust which is carried on an atmospheric gravity current. Most form when a thunderstorm produces a downdraft of cold air. The downdraft is pushed forward by the front of the thunderstorm cell, and drags dust and debris along with it as it travels across the terrain. Strong surface winds hurl this sand and dust into the air; the leading edge of the cold and dusty air is called the thunderstorm gust front. Haboob winds can travel 35-100 km/hr. Haboobs occur worldwide, including within the Sahara desert, across the Arabian Peninsula, through Kuwait and in the most arid areas of Iraq. They have been known to occur in Central Australia, associated with cold fronts. One of the more well-known haboobs of recent years is the haboob that hit Phoenix, Arizona, USA on July 5, 2011. The storm was up to 1,500m tall when it hit Phoenix and had a 161km wall on the leading edge. The dust storm moved at speeds up to 80 km/hr, and travelled 241-322km from Tucson to Phoenix. Haboobs normally form during the summer months in Phoenix, and can last up to 3 hours. Tucson, Arizona was also the site of the Dust Bowl on July 16, 1971, a devastating sandstorm.  China has also had its share of destructive haboobs – the most destructive hit many parts of far western China’s Xinjiang region 21 March 2012, with wind speeds in the popular tourist towns of Hami and Turpan reaching 183 kilometres per hour (photo is of a haboob in China). Simple diagram of haboob formation: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter9/graphics/haboob.free.gif; http://bit.ly/Pt3XGA More complex diagram of haboob formation: http://bit.ly/TJ6JKJ -TEL Photos from large Iraqi haboob 2005: http://www.sunbelt-software.com/stu/iraq/sandstorm.htm View a timelapse of a haboob from Phoenix (USA) metro area on July 5th, 2011: http://www.scottwoodphotography.com/Weather/2011StormChasing/15913598_4nzjFs#!i=1370278891&k=RDCLM67&lb=1&s=Ahttp://www.ouramazingplanet.com/1524-haboob-dust-storm-hits-phoenix-arizona.html; http://phoenix.about.com/od/arizonamonsoon/qt/haboob.htm; http://www.newscientist.com/blogs/shortsharpscience/2011/07/howling-haboob-smothers-phoeni.html; http://www.bbc.co.uk/news/world-asia-china-17461442 Photo: China Foto Press

HABOOB

A haboob (Arabic: هَبوب “strong wind”) is a wall of sand and/or dust which is carried on an atmospheric gravity current. Most form when a thunderstorm produces a downdraft of cold air. The downdraft is pushed forward by the front of the thunderstorm cell, and drags dust and debris along with it as it travels across the terrain. Strong surface winds hurl this sand and dust into the air; the leading edge of the cold and dusty air is called the thunderstorm gust front. Haboob winds can travel 35-100 km/hr.

Haboobs occur worldwide, including within the Sahara desert, across the Arabian Peninsula, through Kuwait and in the most arid areas of Iraq. They have been known to occur in Central Australia, associated with cold fronts.

One of the more well-known haboobs of recent years is the haboob that hit Phoenix, Arizona, USA on July 5, 2011. The storm was up to 1,500m tall when it hit Phoenix and had a 161km wall on the leading edge. The dust storm moved at speeds up to 80 km/hr, and travelled 241-322km from Tucson to Phoenix. Haboobs normally form during the summer months in Phoenix, and can last up to 3 hours. Tucson, Arizona was also the site of the Dust Bowl on July 16, 1971, a devastating sandstorm.

China has also had its share of destructive haboobs – the most destructive hit many parts of far western China’s Xinjiang region 21 March 2012, with wind speeds in the popular tourist towns of Hami and Turpan reaching 183 kilometres per hour (photo is of a haboob in China).

Simple diagram of haboob formation: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter9/graphics/haboob.free.gif; http://bit.ly/Pt3XGA
More complex diagram of haboob formation: http://bit.ly/TJ6JKJ

-TEL

Photos from large Iraqi haboob 2005: http://www.sunbelt-software.com/stu/iraq/sandstorm.htm
View a timelapse of a haboob from Phoenix (USA) metro area on July 5th, 2011: http://www.scottwoodphotography.com/Weather/2011StormChasing/15913598_4nzjFs#!i=1370278891&k=RDCLM67&lb=1&s=A

http://www.ouramazingplanet.com/1524-haboob-dust-storm-hits-phoenix-arizona.html; http://phoenix.about.com/od/arizonamonsoon/qt/haboob.htm; http://www.newscientist.com/blogs/shortsharpscience/2011/07/howling-haboob-smothers-phoeni.html; http://www.bbc.co.uk/news/world-asia-china-17461442

Photo: China Foto Press

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Just some gorgeous mountain scenery

Just some gorgeous mountain scenery

(Source: rileyjanelle)

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Iridescent ammoniteWe covered the beautiful gem material known as ammolite in a recent post, (seehttp://tinyurl.com/pstqvhx). The gem consists of the fossilised nacre (the same substance that oysters deposit on pearls) of these extinct cephalopods, somewhat related to modern squid and nautili. Since they became extinct along with the dinosaurs at the end Cretaceous, the gem is a unique record of times past, complementing its surface beauty that somewhat resembles opal. This specimen of ammonite comes from Madagascar, but the nacre on these is too thin to use for jewellery.LozImage credit: Daniel CD

Iridescent ammonite

We covered the beautiful gem material known as ammolite in a recent post, (seehttp://tinyurl.com/pstqvhx). The gem consists of the fossilised nacre (the same substance that oysters deposit on pearls) of these extinct cephalopods, somewhat related to modern squid and nautili. Since they became extinct along with the dinosaurs at the end Cretaceous, the gem is a unique record of times past, complementing its surface beauty that somewhat resembles opal. This specimen of ammonite comes from Madagascar, but the nacre on these is too thin to use for jewellery.

Loz


Image credit: Daniel CD

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The Bortle Scale and Light PollutionThe Bortle Scale is used by astronomers to rate the darkness of our skies. It ranges from 1 (darkest) to 9 (brightest). For most of us, our daily lives are spent beneath a radiance level of between 5 and 8 and rarely venture into areas ranked 3 or darker- and what a shame that is.Light pollution, while a testament to our technological advances, has blanketed our view of the universe and decoupled our relationship with the cosmos. For the millions of people living in areas where less than 20 stars can be seen in the night sky, it is practically impossible to imagine a natural sky blanketed with upwards of 2,500 stars backed by great ribbons of billions of stars which can be found in our Galaxy: The Milky Way.What are the effects of light pollution?From a research perspective, studies have demonstrated that our overuse of artificial light at night can increasingly be linked to sleep disorders, diabetes and cancer. Ecologically, habitats are disturbed and species are forced to adapt to or flee from this unnatural phenomenon. Environmentally, it is a huge waste of energy. Socially, increased light pollution can lead to a decrease in safety and security.From a psychological perspective, the lack of a brilliant dark sky can affect our humility. Without the nightly reminder of a vast cosmos, it is easy to forget how small we are, how insignificant. It is simple to imagine that we are the centre of all things when confined to a solitary closed ecosystem, this tasks gets harder when faced with the immensity of the universe before us.Before we invented civilisation, our ancestors embraced the darkness of space, wondered at the stars and contemplated the universe. Today, we immerse ourselves in artificial light; a lot of which is unnecessary and the result of bad habits. If we shield, downgrade or time our existing lighting infrastructure we could open the wonders of the night sky back up again.While artificial light has undoubtedly enriched our lives, the same has always been true of darkness- we do not need to be afraid of it.In the words of Sarah Williams “I have loved the stars too fondly to be fearful of the night.”-Jean

The Bortle Scale and Light Pollution

The Bortle Scale is used by astronomers to rate the darkness of our skies. It ranges from 1 (darkest) to 9 (brightest). For most of us, our daily lives are spent beneath a radiance level of between 5 and 8 and rarely venture into areas ranked 3 or darker- and what a shame that is.

Light pollution, while a testament to our technological advances, has blanketed our view of the universe and decoupled our relationship with the cosmos. For the millions of people living in areas where less than 20 stars can be seen in the night sky, it is practically impossible to imagine a natural sky blanketed with upwards of 2,500 stars backed by great ribbons of billions of stars which can be found in our Galaxy: The Milky Way.

What are the effects of light pollution?

From a research perspective, studies have demonstrated that our overuse of artificial light at night can increasingly be linked to sleep disorders, diabetes and cancer. Ecologically, habitats are disturbed and species are forced to adapt to or flee from this unnatural phenomenon. Environmentally, it is a huge waste of energy. Socially, increased light pollution can lead to a decrease in safety and security.

From a psychological perspective, the lack of a brilliant dark sky can affect our humility. Without the nightly reminder of a vast cosmos, it is easy to forget how small we are, how insignificant. It is simple to imagine that we are the centre of all things when confined to a solitary closed ecosystem, this tasks gets harder when faced with the immensity of the universe before us.

Before we invented civilisation, our ancestors embraced the darkness of space, wondered at the stars and contemplated the universe. Today, we immerse ourselves in artificial light; a lot of which is unnecessary and the result of bad habits. If we shield, downgrade or time our existing lighting infrastructure we could open the wonders of the night sky back up again.

While artificial light has undoubtedly enriched our lives, the same has always been true of darkness- we do not need to be afraid of it.

In the words of Sarah Williams “I have loved the stars too fondly to be fearful of the night.”

-Jean

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New Species!This critter is a new species of bug, Jaera tyleri, which has been found on a whale carcass on the floor of the Southern Ocean, near Antarctica.It was actually found by accident, as UK researchers were looking for ‘black smokers’ - hydrothermal vents on the sea floor, and happened across a whale carcass, covered in these beasties - 500 to 6000 specimens per square metre - during the hunt. It is possible that this species may only be found on whale carcasses on the seafloor.Other species of Jaera have been found in shallower waters in the North Sea, but this is the only Jaera ever discovered in the Southern Hemisphere.SedgPhoto credit: Katrin Linse, British Antarctic SurveyOriginal article: http://www.antarctica.ac.uk/about_bas/news/news_story.php?id=2592

New Species!

This critter is a new species of bug, Jaera tyleri, which has been found on a whale carcass on the floor of the Southern Ocean, near Antarctica.

It was actually found by accident, as UK researchers were looking for ‘black smokers’ - hydrothermal vents on the sea floor, and happened across a whale carcass, covered in these beasties - 500 to 6000 specimens per square metre - during the hunt. It is possible that this species may only be found on whale carcasses on the seafloor.

Other species of Jaera have been found in shallower waters in the North Sea, but this is the only Jaera ever discovered in the Southern Hemisphere.

Sedg


Photo credit: Katrin Linse, British Antarctic Survey
Original article: http://www.antarctica.ac.uk/about_bas/news/news_story.php?id=2592

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Let’s get down to Bismuth.Bismuth (Bi) is a naturally occurring element with an atomic number of 83, chemically, it resembles arsenic and antimony.As you can see in the image, bismuth crystals are quite obviously angular- this is because the edges of the rhombohedral crystal structure are more energetically favourable positions than the interior of the structure. As a result, molecules continually ‘build on’ the edges but don’t fill in the centre when crystallising. The higher rate of growth on the edges forms a crystal which appears to be partially hollowed out in a rectangular-spiral stair step design.Another obvious characteristic of bismuth crystals is their beautiful colours. This is the result of surface oxidation and consequently thin film interference. The surface of the crystal oxidises to bismuth oxide in extremely thin layers. Each of these layers causes light of certain wavelengths to interfere with each other upon reflection giving rise to the colour seen on the surface. Due to variations in the thickness of the oxide layer, the crystal is not one solid colour but rather a rainbow of colours corresponding to the wavelengths of light which arise at each location. Thicker layers produce reds and greens, and thinner ones blue and violets. Interesting note: thin-film interference is also responsible for the dazzling colours in a bubble and the rainbows in an oil-spill.Jean O’DwyerThin layer interference is also explained in another post here: https://www.facebook.com/TheEarthStory/posts/665253186869062

Let’s get down to Bismuth.

Bismuth (Bi) is a naturally occurring element with an atomic number of 83, chemically, it resembles arsenic and antimony.

As you can see in the image, bismuth crystals are quite obviously angular- this is because the edges of the rhombohedral crystal structure are more energetically favourable positions than the interior of the structure. As a result, molecules continually ‘build on’ the edges but don’t fill in the centre when crystallising. The higher rate of growth on the edges forms a crystal which appears to be partially hollowed out in a rectangular-spiral stair step design.

Another obvious characteristic of bismuth crystals is their beautiful colours. This is the result of surface oxidation and consequently thin film interference. The surface of the crystal oxidises to bismuth oxide in extremely thin layers. Each of these layers causes light of certain wavelengths to interfere with each other upon reflection giving rise to the colour seen on the surface. Due to variations in the thickness of the oxide layer, the crystal is not one solid colour but rather a rainbow of colours corresponding to the wavelengths of light which arise at each location. Thicker layers produce reds and greens, and thinner ones blue and violets. 

Interesting note: thin-film interference is also responsible for the dazzling colours in a bubble and the rainbows in an oil-spill.

Jean O’Dwyer

Thin layer interference is also explained in another post here: https://www.facebook.com/TheEarthStory/posts/665253186869062

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Scattered by conesVolcanic cones (150 in total) on Marion Island almost appear are spots on a teenagers face. In addition a large portion of the island is covered in A’a and pahoehoe flows, the vegetated ones could be less than 100 years old. Marion is one of the peaks of a large submarine shield volcano that rises 500m from the bottom of the sea. Mascarin peak, the highest point on Marion is 1242m high. Eruptions have occurred in 1980 and 2004. It’s South Africa’s only historically known active volcano.Together with smaller neighbor Prince Edward Island Marion is collectively known as the Prince Edward islands and is located 1769 km from the South African mainland and a little bit further away from Antarctica.On average it rains 320 days a year. Thus Marion is rightly known as one of the cloudiest places in the world. Due to the wetness of the climate the island is covered with bog, lichen and other subarctic vegetation. There is not much difference between winter and summer on the island, it is always wet, with threat of snow and frost. The strong westerly winds that roam the island are branded the Roaring FortiesMarion was discovered by the Dutch ship Maerseveen in 1663, however it was not until 1772 that someone actually sat foot on it. Frenchman Joseph Marion du Fresne was in the preconception that he had made it to Antarctica. It took him 5 days to discover he was on two small islands. Again in 1803 the island was visited by sealers. In 1947 South Africa annexed the island and based a meteorological station there. There are no permanent inhabitants.The introduction of cats on the island (in the first place brought here to deal with the problem of mice) caused the grey petrel to become extinct. Supposedly the cats managed to eat about half a million birds in 1975 alone. The cat eradication programme finally managed to get rid of the cats in 1991.-OW-Image: NASA Erath Observatory.References:http://www.sanap.ac.za/sanap_marion/sanap_marion.htmlhttp://www.volcano.si.edu/volcano.cfm?vn=234070

Scattered by cones

Volcanic cones (150 in total) on Marion Island almost appear are spots on a teenagers face. In addition a large portion of the island is covered in A’a and pahoehoe flows, the vegetated ones could be less than 100 years old. Marion is one of the peaks of a large submarine shield volcano that rises 500m from the bottom of the sea. Mascarin peak, the highest point on Marion is 1242m high. Eruptions have occurred in 1980 and 2004. It’s South Africa’s only historically known active volcano.

Together with smaller neighbor Prince Edward Island Marion is collectively known as the Prince Edward islands and is located 1769 km from the South African mainland and a little bit further away from Antarctica.
On average it rains 320 days a year. Thus Marion is rightly known as one of the cloudiest places in the world. Due to the wetness of the climate the island is covered with bog, lichen and other subarctic vegetation. There is not much difference between winter and summer on the island, it is always wet, with threat of snow and frost. The strong westerly winds that roam the island are branded the Roaring Forties

Marion was discovered by the Dutch ship Maerseveen in 1663, however it was not until 1772 that someone actually sat foot on it. Frenchman Joseph Marion du Fresne was in the preconception that he had made it to Antarctica. It took him 5 days to discover he was on two small islands. Again in 1803 the island was visited by sealers. In 1947 South Africa annexed the island and based a meteorological station there. There are no permanent inhabitants.

The introduction of cats on the island (in the first place brought here to deal with the problem of mice) caused the grey petrel to become extinct. Supposedly the cats managed to eat about half a million birds in 1975 alone. The cat eradication programme finally managed to get rid of the cats in 1991.

-OW-

Image: NASA Erath Observatory.

References:
http://www.sanap.ac.za/sanap_marion/sanap_marion.html
http://www.volcano.si.edu/volcano.cfm?vn=234070

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VON KÁRMÁN VORTEX STREET, SELKIRK ISLAND, OFF SOUTH AMERICA The pattern shown in these swirling clouds is known as a von Kármán vortex street, named after Theodore von Kármán. They occur when a more viscous fluid flows through water and encounters a cylindrical object, creating vortices in the flow. These structures were first noticed in the laboratory by fluid dynamicists. Study of these vortices are very important in the understanding of laminar and turbulent fluid flow; these control a wide variety of real-world phenomena, like the lift under an aircraft wing and Earth’s weather. In the photo, the Alejandro Selkirk Island, off the Chilean coast, is acting like the cylinder. A vortex street is shown disrupting a layer of stratocumulus clouds which are low enough to be affected by the island, which is nearly 1.6 km above sea level and 1.5 km in diameter. When the wind-driven clouds run into the obstacle of the island, they flow around it clockwise and anticlockwise to form the beautiful spinning eddies, advecting hundreds of kilometres downwind to make a street 10,000 times longer than those made in the laboratory. A vortex street can be observed only over a given range of Reynolds numbers (Re), normally above a limiting Re value of about 90. The Reynolds number is a measure of the ratio of inertial to viscous forces in the flow of a fluid and may be defined as: Re = Vd/v where: d = the diameter of the cylinder around which the fluid is flowing. V = the steady velocity of the flow upstream of the cylinder. v = the kinematic viscosity of the fluid. You can view a simple animation of the von Kármán vortex street here: http://upload.wikimedia.org/wikipedia/commons/b/b4/Vortex-street-animation.gif A more complex animation is here (Reynolds number Re = 250): http://www.youtube.com/watch?v=IDeGDFZSYo8 -TEL The image was taken by the Landsat 7 satellite in September 1999. Credit: Bob Cahalan/NASA, USGShttp://www.wired.com/wiredscience/2010/05/gallery-clouds/; http://weathervortex.com/wakes.htm

VON KÁRMÁN VORTEX STREET, SELKIRK ISLAND, OFF SOUTH AMERICA

The pattern shown in these swirling clouds is known as a von Kármán vortex street, named after Theodore von Kármán. They occur when a more viscous fluid flows through water and encounters a cylindrical object, creating vortices in the flow. These structures were first noticed in the laboratory by fluid dynamicists. Study of these vortices are very important in the understanding of laminar and turbulent fluid flow; these control a wide variety of real-world phenomena, like the lift under an aircraft wing and Earth’s weather.

In the photo, the Alejandro Selkirk Island, off the Chilean coast, is acting like the cylinder. A vortex street is shown disrupting a layer of stratocumulus clouds which are low enough to be affected by the island, which is nearly 1.6 km above sea level and 1.5 km in diameter. When the wind-driven clouds run into the obstacle of the island, they flow around it clockwise and anticlockwise to form the beautiful spinning eddies, advecting hundreds of kilometres downwind to make a street 10,000 times longer than those made in the laboratory.

A vortex street can be observed only over a given range of Reynolds numbers (Re), normally above a limiting Re value of about 90. The Reynolds number is a measure of the ratio of inertial to viscous forces in the flow of a fluid and may be defined as:
Re = Vd/v
where:
d = the diameter of the cylinder around which the fluid is flowing.
V = the steady velocity of the flow upstream of the cylinder.
v = the kinematic viscosity of the fluid.

You can view a simple animation of the von Kármán vortex street here: http://upload.wikimedia.org/wikipedia/commons/b/b4/Vortex-street-animation.gif

A more complex animation is here (Reynolds number Re = 250): http://www.youtube.com/watch?v=IDeGDFZSYo8

-TEL

The image was taken by the Landsat 7 satellite in September 1999. Credit: Bob Cahalan/NASA, USGS
http://www.wired.com/wiredscience/2010/05/gallery-clouds/; http://weathervortex.com/wakes.htm