Archive for the 'Weather & Sky' Category

Jan 11 2014

No Snow

Snow buntings, Crawford County, Jan 2014 (photo by Shawn Collins)

Can you believe how warm it is today?

Shawn Collins found these snow buntings in Crawford County a week ago when the snow was melting.  Two days later we were in the sub-zero polar vortex.  Now it’s 60 degrees warmer and the snow is gone.

It’s a good thing snow buntings are white, brown and black. They’re camouflaged even when there’s no snow.

(photo by Shawn Collins)

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Jan 09 2014

Dowsing From Outer Space

Published by under Weather & Sky

GRACE satellite pair (drawing from NASA on Wikimedia Commons)

Here’s some old news to some of you… but new to me.

Launched in 2002, the twin Gravity Recovery and Climate Experiment (GRACE) satellites have collected data for 12 years and are already in a decaying orbit that will suck them down to Earth in 2015 or 2016.  During their run they’ve carefully measured Earth’s gravitational pull and contributed to our knowledge of aquifers, ice sheets, magma and earthquakes — all because of gravity.

Did you know that gravity is uneven around the world and can vary in the same location during the year?  Interestingly, water has a lot to do with it.  Gravity is determined by mass so an increase in groundwater causes a higher gravitational pull.  Since the GRACE satellites measure gravity, they find groundwater.  It’s as if they were dowsing (finding water using a forked stick).

George Casely dowsing for water on his Devon farm, 1942 (photo from Wikimedia Commons)

Here’s how it works.  Skimming like hockey pucks in their pole-to-pole orbit, the GRACE satellites maintain a 140-mile distance between each other which they measure constantly. Their microwave ranging system is so accurate it can record a 10 micrometre change in separation (1/10 the width of a human hair)!  When the lead satellite first encounters a stronger gravitational pull, gravity makes it speed ahead, increasing the distance between them.  When the second satellite encounters the same “bump” it speeds up too and their separation decreases.  When they’ve both passed the “bump” they both slow down.

Round and round since 2002 they measure the distance between themselves and report back to Earth.  NASA’s computers crunch the ebb and flow of gravity and create gravitational anomaly maps.  Click here to see a selection of them.

The maps help scientists understand changes in aquifers and improve groundwater management.  You can see the greatest anomalies in the tropics where there are dramatic wet and dry seasons.  On this map the Amazon basin is routinely blue (decreasing gravitational pull) in October at the end of the dry season and routinely red (higher gravity) in April at the end of the rainy season.  Bangladesh’s color cycle is the opposite because its wet-dry cycle occurs during the other half of the year.

Until gravity pulls them down and ends their mission the GRACE satellites travel above us, dowsing from outer space.

 

(image of the GRACE satellites and photo of George Casely dowsing on his farm from Wikimedia Commons. Click on the images to see the originals)

p.s.  Click here for more news from the GRACE satellites.

 

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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 climate.gov)

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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 timeanddate.com 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

Iridescent

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

Blankets?

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

Snow?

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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