Archive for the 'Weather & Sky' Category

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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Feb 16 2013

Hold Onto Your Hats

Published by under Weather & Sky

Chelyabinsk meteor trace, 15 February 2013 (photo by Nikita Plekhanov via Wikipedia)

There we were, focusing our attention on an asteroid that was going to miss Earth when Bang!  a real live meteor zipped low over Russia yesterday morning.

The meteor taught me a lot more than the asteroid.  After it lit the sky, made an explosive boom, blew out windows, and injured more than 1,000 people I learned from NASA:

  • Its light was brighter than the sun.
  • Its contrail was 300 miles long. (That’s the distance from Pittsburgh to Philadelphia).
  • Eyewitnesses said the sonic boom lagged by three minutes … just long enough for everyone to go to their windows to watch.
  • The meteor was about the size of a bus (55 feet) but it weighed 10,000 tons –> 1,400 times heavier than a bus.
  • The atmosphere really did help after all.  When the meteor exploded it was still 12-15 miles up.  At least twice as high up as a jetliner.
  • If it was only the size of a bus and 2 to 4 times higher up than a jet, why did it cause such a problem?   Well, it was traveling at 40,000 mph!

So, hold onto your hats.  It’s the stuff we aren’t worried about that gets us.

Click here for scientific analysis (video) from The Telegraph UK.


(photo of the Chelyabinsk meteor’s trace by Nikita Plekhanov via Wikipedia. Click on the image to see the original)

p.s.  The meteor also taught me two things about Russian culture:  (1) Russians have dashboard cameras in their cars to protect against corrupt policemen and disputed traffic accidents, and (2) They have already made a joke about it, quoted from the Houston Chronicle: “The meteorite was supposed to fall on Dec. 21, 2012 — when many believed the Mayan calendar predicted the end of the world — but was delivered late by Russia’s notoriously inefficient postal service.”

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