Archive for the 'Weather & Sky' Category

Jan 07 2014

Polar Vortex

Published by under Weather & Sky

U.S. maximum temperature map for January 6, 2014 (graphic from NOAA)

If you’ve listened to the weather forecasters you know we’re in the grips of a polar vortex.

At first that name confused me.  I know about the single massive polar vortex that spins counterclockwise in the high northern latitudes in winter, covering 620 to 1,240 square miles and reaching into the stratosphere.  Is that vortex here now?

Not exactly.  We’re experiencing a polar vortex, not the northern polar vortex, though they’re related.

In the good old days before climate change, the winter polar vortex in the northern hemisphere was generally well behaved.  It was a persistent, strong, cold, low pressure zone surrounding the polar high at roughly the same latitude around the globe.  Its strong winds kept the jet stream in line.  Nobody got too hot or too cold.

In some years — and more frequently as the Earth gets hotter — hot air from the troposphere is forced into the stratosphere and disrupts the polar vortex.  The vortex becomes disorganized and may “collapse” into smaller pieces.  Its winds weaken and the jet stream flaps like a flag in the breeze, as shown below:

Jet stream Rossby waves (graphic from Wikimedia Commons)

(a) When the polar vortex is strong, the jet stream (pink band) circles the Earth in small amplitude Rossby waves. This keeps cold air in the north and warm air in the south.
(b) When the polar vortex weakens, the jet stream begins to wobble.
(c) A major wobble brings a tongue of polar air deep into the south, spawning a (smaller break-away) polar vortex that affects our weather.  Notice how the tongue of cold air resembles yesterday’s U.S. high temperature map above.

Disruption of the winter polar vortex used to be rare, but not any more.  Last winter the polar vortex completely collapsed because of hot air from the Himalayas. The vortex broke into pieces, one of which hovered over Siberia.  The jet stream went wobbly.  Europe had a very severe winter.

This year it’s our turn.

A lot of factors created today’s weather pattern but, yes, the weakening of the polar vortex can send us a polar vortex.

Right now only the snowy owls feel at home.  ;)


(images from NOAA and Wikimedia Commons. Click on each image to see it’s source)



UPDATE, 10 Jan 2014:  NOAA maps show the break-up of the polar vortex.  On the left, the 500mb pressure gradient on 5 Jan 2014 shows the polar vortex in bits and pieces.  On the right, the same pressure gradient on 14-16 Nov 2013 shows a nicely contained vortex.  Click on the image to see larger images and read the accompanying article at NOAA.

Polar vorrtex 500mb pressure comparison: Jan 5, 2014 to Nov 14-16, 2013 (maps by NOAA

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Dec 21 2013

Winter Solstice

Winter sunset over the Susquehanna at the Wrightsville Bridge (photo by John Beatty)

Today at 12:11pm the sun will stand still.

We call this the “winter” solstice but it’s more accurate to call it the southern solstice because the sun is going to stand still over the southern hemisphere.  The word “solstice” describes the event:  sol means sun and stice, from sistere, means to stand still.

You might be jealous of the southern hemisphere right now because they’re in the midst of summer but take heart in this: their spring and summer are shorter than ours.

That’s because the Earth doesn’t move at a constant speed in its elliptical orbit.  It takes the Earth 92.8 days to travel from the point of our vernal equinox to the location of the northern/summer solstice (March to June), 93.6 days from the summer solstice to the autumnal equinox (June to September), 89.8 days from the autumnal equinox to the winter solstice (September to December) and 89.0 days from winter solstice to vernal equinox (December to March).  Thus the seasons aren’t equal in length.

This means that in the northern hemisphere spring and summer together are 7.6 days longer than those seasons in the southern hemisphere.  We have a week’s more warmth than they do.

If this is confusing, check out the earth map and explanation at this link at whose information I paraphrased above.


(photo of the sun setting over the Susquehanna at Wrightsville, PA by John Beatty)

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Dec 17 2013

Paintballs To The Rescue

Published by under Weather & Sky

When I wrote about the lawn sprinkler asteroid on November 11, I was intrigued by this sentence in the news article:

“The asteroid could possibly have been spun up if the pressure of sunlight exerted a torque on the body.”

The pressure of sunlight?

Apparently small bodies in outer space — from dust particles to 10km wide asteroids — are affected by the relentless though tiny touch of photons.  They reflect or absorb the photons’ energy and emit what they don’t retain.  The emissions become a tiny propulsion force. However, dust and asteroids have irregular shapes and surfaces so they reflect, absorb and emit unevenly.  This affects their rotation and flight path.

There’s a lot of fancy physics that predicts what a small irregular body will do under the pressure of sunlight.  I read about the Yarkovsky effect, the YORP effect and the Poynting-Robertson effect until I got confused.  Then I googled for a simple description and found …

The United Nations’ Space Generation Advisory Council holds an annual contest to solve the problem of deflecting a killer asteroid on a collision course with Earth.  In 2012 the winning solution of the Move an Asteroid Competition was to bombard it with white paintballs.

The reason this would work is due to the Yarkovsky effect (I think).  A dirt-colored rotating asteroid absorbs photons and heats up on its daylight side, then releases energy when that side turns to night.  In a steady state the asteroid would stay on course and hit the Earth but if it’s painted white it will absorb less and emit less — and this will alter its course.

All we need for deflecting a dangerous asteroid is a 20-year lead time, a rocket, a lot of white paintballs and very good aim.

Watch the video to see how we’d paint an asteroid.  Click here to read how it works in MIT News.

Paintballs to the rescue.


(video from MIT News on YouTube)

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Dec 11 2013


Published by under Weather & Sky

Iridescent cloud (photo by "not on your nelly," Creative Commons license on Flickr)

There are a lot of iridescent things in nature:  birds, beetles, seashells, fish, minerals and clouds.

Yesterday, after a snowy start (and really bad traffic!) the wind swung around to the west and the sky cleared with a few fast-moving clouds.  At lunchtime I looked up while standing in a building’s shadow and saw a thin, beautiful, iridescent cloud blowing past the sun.

Thin is important.  Iridescence occurs when sunlight diffracts through a thin layer of water droplets (or ice crystals) of uniform size and orientation.  The glowing colors are named for the Greek goddess Iris, the personification of rainbows.

Pittsburgh’s iridescent clouds aren’t nearly as cool as the nacreous clouds in Antarctica, but we don’t have the super-cold stratospheric temperatures that cause those clouds. For which I am grateful!


(photo by “not on your nelly” on Flickr, Creative Commons license. Click on the image to see the original)

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Nov 25 2013


Published by under Weather & Sky

Cloud heat misconception (illustration from Dan Stterfield's Wild Wild Science Journal)

This morning it was extra cold (15 degrees F!).  It would have been cold anyway because an arctic air mass arrived over the weekend, but it was extra cold because the sky was mostly clear last night.  If we’d had lots of cloud cover we’d have been a little warmer.

The reason for this is not what you’d expect.  Traditionally we’ve heard that cloud cover acts like a blanket to hold the heat in.  The illustration above plays to that notion by showing heat arrows bouncing off the clouds.  But it ain’t exactly so.  Believe it or not this illustration is wrong.

The truth is that we’re warmer under cloud cover because the clouds radiate their own heat which warms the air below them.  You’ve seen this principle in action if you’ve parked your car under a leafy tree on a frosty night and found your windshield frost-free the next morning though the open ground has frost.  The tree radiated heat to keep your car just a little warmer than the open air.

I learned this from Dan Satterfield’s blog, Dan’s Wild Wild Science Journal, where I found this illustration.  In “Scientific Facts That Aren’t True” Dan writes:

The clouds do not “hold the heat in”. They absorb the heat, and radiate their own heat in all directions.  …If you’re camping, and you sleep under a tree, you will escape most of the dew compared to your buddies, who slept right out under the stars. The tree did not catch the dew, it just radiated energy to the ground around you, and kept it warmer. Warmer ground, less dew!

Click on the image to read more of Dan’s Scientific Facts That Aren’t True.

Clouds may blanket us but they aren’t blankets.


(traditional image of heat bouncing off clouds from Dan’s Wild Wild Science Journal: Scientific Facts That Aren’t True by Dan Satterfield)

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Nov 23 2013


First snow in Schenley Park, 12 Nov 2013 (photo by Kate St. John)

As of this writing we know that very cold weather is on its way (18o Sunday night!) but the question of snowfall is still up in the air.  How much will actually stick?

On November 12 the first snow of the season was quite beautiful in Schenley Park.

By now all the leaves have fallen.  Even with snow, this scene would look different if photographed today.


(photo by Kate St. John)


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Nov 11 2013

Lawn Sprinkler In The Sky

Published by under Weather & Sky

Asteroid P/2013 P5 as seen on two days in September 2013 from the Hubble Space Telescope (photos courtesy of NASA)

Back in September an amazing asteroid flew by in outer space.

It first appeared as a fuzzy dot, seen by a PanSTARRS Survey telescope in Hawaii.  Wondering what it was, astronomers directed the Hubble Space Telescope to take a look.  Boy, were they surprised.  It has six tails!

This is not a normal asteroid.  Asteroids are very tiny planets and — until now — they don’t have tails.  This one is only 700 feet across and is traveling around the sun in the asteroid belt between Mars and Jupiter.  Like it’s traveling companions in the Flora asteroid family, its probably a chunk left over from a planetary collision.

So why does it have tails?  Comets have tails because they are made of ice, dust and small rocks.  When they get near the sun the ice evaporates, causing a long streamer of debris.  But this asteroid has no ice.  It must be streaming dust.  Lots of it.

Scientists named it P/2013 P5 and ran its behavior through modelling software at the Max Planck Institute for Solar System Research.  The model showed this asteroid is spinning so fast that anything loose on the surface (dust) is traveling toward its equator.  There it accumulates and episodically escapes the asteroid’s weak gravity, arcing into outer space. Yow!  Six tails!

Why is it spinning so fast?  Scientists theorize that the pressure of sunlight could have pushed P/2013 P5 into a tail spin.

Photos, above, from the Hubble Space Telescope show it spinning like a lawn sprinkler in the sky.


Read more here at NASA’s Hubble website.


(images of Asteroid P/2013 P5 from the Hubble Space Telescope, courtesy of NASA)

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Nov 05 2013

Mackerel Sky

Published by under Weather & Sky

Clouds, 20 Oct 2013 (photo by Kate St. John)

I love it when the sky does this.

These pretty clouds won’t rain … yet.  They drift by in thin shields with gaps between and when they thicken they look like buttermilk (click here to see).

These are altocumulus clouds that form in the mid-level of the cloud deck, between 6,500 and 20,000 feet.  Their thicker version is called a mackerel sky because the effect resembles the scales on a king mackerel.

Weather sayings confuse me about the message these clouds are bringing.  One poem says, “Mackerel sky, mackerel sky – never long wet, never long dry.”   Worst case:  These are overtaken six to eight hours later by different clouds that bring rain.

On the day I took this photo I was outdoors for six hours and yes, these happy clouds were followed by thick, potential rain clouds.

But it didn’t rain until I got home.


(photo by Kate St. John)

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Apr 09 2013

Bad News For Gyrfalcons

Gyrfalcon in western Greenland (photo form Wikimedia Commons)

A new study on the future of climate change in arctic Alaska spells bad news for gyrfalcons in the U.S.

By 2050 the mean annual temperature in northernmost Alaska is expected to rise 3.10C (5.560F).  This will usher in a host of changes to ice, coastlines, tundra, plants and animals.  What will happen to the area’s breeding birds?

Scientists from the Wildlife Conservation Society, assisted by experts on each species, assessed the future of 54 of arctic Alaska’s breeding birds.  The goal was to prepare wildlife and land managers for climate change and ultimately develop plans to mitigate the effects if possible.

The study found that two species, gyrfalcons and common eiders, are highly vulnerable to the anticipated warming and likely to experience dangerous declines.  Seven others are moderately vulnerable: brant, Steller’s eider, pomerine jaeger, yellow-billed loon, buff-breasted sandpiper, red phalarope and ruddy turnstone.

Gyrfalcons are specialists and climate change is going to be rough on their niche.

  • They nest on coastal cliffs in microclimates that are a rare commodity in northern Alaska.  South-facing cliffs may become too hot, limiting the number of suitable nest sites.
  • At the start of breeding gyrfalcons eat ptarmigan almost exclusively.  When ptarmigan populations are low gyrfalcons won’t breed at all.  When climate change affects ptarmigans it will hurt gyrfalcons.
  • The gyrfalcon’s hunting style relies on open tundra but as the arctic warms shrubs will grow in formerly open land.
  • Spring storms are expected to increase. Unfortunately this will cause nest failure for gyrfalcons who require dry weather to hatch their eggs.

With all these cards stacked against them gyrfalcon numbers are expected to drop considerably from today’s 250 breeding pairs.

But the report has a silver lining.  There will be more seed eaters:  savannah sparrows, Lapland longspurs, white-crowned sparrows, American tree-sparrows and common redpolls.

Much as I like redpolls, I don’t want to trade them for gyrfalcons.


Read more about the report, Assessing Climate Change Vulnerability of Breeding Birds in Arctic Alaska, in this article in Science Daily or download it from this page on the WCS website (see the righthand column).

(photo from Wikimedia Commons.  Click on the image to see the original.)

p.s. The report was careful to point out that the study only applies to arctic Alaska, not to all breeding ranges.  The photo above was taken in western Greenland.

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Apr 03 2013

Nature’s Mushroom Cloud

Sarychev Volcano, Matua Island, 12 June 2009 (photo from the International Space Station, NASA, via Wikimedia Commons)

Imagine seeing this outside your window!

On June 12, 2009 the International Space Station was flying over the Kuril Island chain in the northwestern Pacific when they witnessed the eruption of Sarychev peak, an active volcano on Russia’s Matua Island.

Because the eruption had just begun, brown ash and steam was still rising in a mushroom cloud that had punched a hole in the cloud cover above it.  Meanwhile, dark brown ash rolled low to the ground, probably a pyroclastic flow of hot gas and rock up to 1,850oF (1000oC) and traveling at 450 mph!

The ash had just begun to spread out in the sky (light brown at top left and right).  Soon commercial air traffic was diverted to avoid engine failure from this abrasive particulate in the upper atmosphere.

The astronauts were lucky to see this eruption as it began.

Nature makes an impressive mushroom cloud.


(photo from the International Space Station, NASA)

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