Category Archives: Weather & Sky

Walking Down Vortex Street

Musings & News, Weather & Sky

I know almost nothing about fluid dynamics but my article about wingtip vortices piqued my interest in the subject.

Last weekend I learned about this amazing phenomenon, the von Kármán vortex street, animated above by Cesareo de La Rosa Siqueira.

Von Kármán vortex streets occur when a fluid flows past a stationary object and generates a long line of vortices that swirl in opposite directions.  The phenomenon was named for Theodore von Kármán, the man who described it, and is probably called a street because it looks like one.

We usually don’t see von Kármán vortex streets in the air but it’s important that engineers plan for them.  If a tall structure is uniformly straight or uniform structures are grouped too closely the vortices can make them fall down.  That’s what happened in November 1965 when three of the eight Ferrybridge cooling towers collapsed in high winds. They could individually withstand the wind speed but not the vortex.

Ferrybridge cooling towers collapse due to von Karman vortex, November 1965

On a small scale, von Kármán vortex streets make wires sing in the wind.  On a large scale they’re visible from outer space when clouds blow past a tall island.

Here’s a picture taken from the space shuttle that shows cloud cover blowing past Rishiri Island, Japan.  When the wind encounters Mt. Rishiri the clouds form a von Kármán vortex street on the downwind side.

There are more than twenty islands that reliably generate von Kármán vortex streets.  Click here to see more pictures from NASA.

(Vortex animation by Cesareo de La Rosa Siqueira via Wikimedia Commons.  Space shuttle photo from NASA via Wikimedia Commons. Click on the images to see the originals)

Open Water

Water and Shore, Weather & Sky

Female buffelhead at Bosque del Apache (photo by Steve Valasek)

The long spate of cold weather froze all our ponds and lakes.  Even the rivers were beginning to freeze until Monday’s warmth reversed the trend.

Don’t expect to see a lot of birds at Lake Arthur right now.  Waterfowl who rely on open water for food or to get airborne have left for open water.  Some are at our rivers, most have left town completely.

This female bufflehead was on the other side of the U.S. — at Bosque del Apache, New Mexico — when Steve Valasek took her picture.

No, she didn’t leave Pittsburgh for New Mexico, but Steve did.

(photo by Steve Valasek)

Wind Effect

Weather & Sky

As I write this morning before dawn, the wind is whipping around the house as a winter storm approaches from the west.

If I was at the roof peak I’d be blown away.  The wind is even faster up there (see red lines at top) where it converges to clear the house.

Outside my window on the downwind side, the air is swirling in updrafts like the turquoise lines at left.

I suppose I could find a few calm spots within the swirls if I went outdoors to experiment, but it’s not worth it.  At particularly gusty moments I hear garbage cans rattle down the alley in the dark.

 

(diagram by Barani on Wikimedia Commons.  Click the image to see the original)

The Sun As Pearls

Weather & Sky
Sun from solstice to solstice (image by Tunç Tezel, TWAN, via NASA’s Astronomy Photo Of the Day)

21 December 2012

Like a three-strand necklace of pearls, this composite photo shows the sun’s position hour by hour at the summer solstice, the vernal equinox, and the winter solstice.

It was taken at the same location in Bursa, Turkey over a period of six months by award-winning amateur astronomer and night sky photographer Tunç Tezel, a member of The World At Night.

The top strand is the sun’s transit during the summer solstice in June, the longest day of the year.  You can tell the sun was up for 15 hours because there are fifteen pearls on that strand.

The middle strand was taken during the equinox when every place on earth has 12 hours of daylight.

The lowest strand was taken on this day, the winter solstice, when there are 9 hours of sunlight in northern Turkey.

There are nine hours of daylight in Pittsburgh today, too.

Northern Turkey and western Pennsylvania are on approximately the same latitude so these sun tracks are what we see here in Pittsburgh.

The whole world shares the same sky.  We all can see the sun as pearls.

(photo copyright by Tunç Tezel, member of The World At Night (TWAN).  This photo was NASA’s Astronomy Photo Of the Day on September 23, 2012.  Click on the photo to see the original and learn more about its creation.)

Weak Winter Sun

Weather & Sky

Today’s weather is supposed to be “partly sunny” but in winter that can mean the sun looks like this for part of the day.

This photo could have been taken anywhere in western Pennsylvania in early December.  Brown fields, bare trees, power lines, crows.  Can you guess where it was taken?

Click here and then on the Google Maps link to see the photo’s location, or click on the image to read the description.  You’re in for a surprise.

 

(photo by Pauline Eccles via Wikimedia Commons. Click on the photo to see the original)

p.s. The coordinates are 51° 53? 50.63? N, 2° 29? 38.14? W

Hazy Inversion

Weather & Sky
View from the Allegheny Front Hawk Watch, 11 Nov 2012 (photo by Kate St. John)

14 November 2012

Three weeks ago I wrote about radiation fog and inversions.  We had another inversion recently, this time without fog.

Here is the view last Sunday from the Allegheny Front Hawk Watch.  It looks like a bad picture of beautiful scenery but it’s actually a good illustration of a hazy inversion.  Notice how the near trees are colorful and Wills Mountain, 10.5 miles away,  is bland and washed out.  You can’t see the fire tower on Kinton Knob.  The colors are cancelled by bad air.

This was a classic temperature inversion but the first time I was able to measure it.  As I drove to the hawk watch my car’s outdoor thermometer registered 43o in the Laurel Highland valleys and 57o on top of the mountain.  Normally the hawk watch site is far colder than anywhere else in western PA.

The weather was topsy-turvy.  Warm air aloft trapped cold air below and with it pollutants that made the air smell bad in the cold zones.

Bad air was not limited to cities and industrial zones.  On my way to the Allegheny Front I saw quite a few outdoor wood boilers creating thick white smoke that blanketed rural areas.  These relatively new devices burn wood in backyard sheds to heat water for radiators in homes.  Because outdoor wood boilers are small scale polluters they weren’t on the bad air radar at first, but their smoke is much worse than typical burning because the fire smolders when indoor heat demand is low.  I saw valleys where wood smoke enveloped nearby homes and neighbors.

At the hawk watch the air was nice and warm.

So when there’s an inversion, go to the mountain.

(photo by Kate St. John)

Mother Of Pearl

Weather & Sky
Nacreous clouds over the NASA Radome, McMurdo Station, Antarctica (photo by Alan Light, Wikimedia Commons)

7 November 2012

Here are clouds we never see at home.

Nacreous clouds are named for their iridescence.  Like nacre, the mother of pearl substance that lines oyster shells and pearls, they reflect all the colors of the rainbow.

They’re also called polar stratospheric clouds (PSCs) because they form in the lower stratosphere at 49,000 to 82,000 feet in the presence of super low temperatures, -108oF and colder.  These conditions are only found in the polar regions during winter, mostly in Antarctica.

To give you an idea of how rare these clouds are, consider that they form in an extremely dry part of the earth’s atmosphere way above most human activity. Commercial jets fly at 23,000 to 41,000 feet; these clouds are much higher.  It’s so dry up there it’s a wonder that they form at all.

The prettiest nacreous clouds contain only water droplets that glow profusely when the sun lights them from below.  These clouds are benign but others are not.  PSC clouds that form from nitric acid + water cause chemical reactions that deplete stratospheric ozone and make a hole in the ozone layer that protects Earth from the sun’s ultraviolet light.

As we head toward winter in the Northern Hemisphere, it’s summer in Antarctica so it will be many months before there are nacreous clouds again.  These were photographed in August 2009.

(Nacreous clouds in Antarctica over the NASA Radome, photo by Alan Light on Wikimedia Commons. Click on the image to see the original)

The Whole World Is Hotter

Climate Change, Musings & News, Weather & Sky

The aftermath of Hurricane Sandy has reopened the topic of climate change.  Understandably the loudest voices come from those most affected, worried that this unusual storm is just the beginning of weather as usual on a warm planet.  Mayor Bloomberg of New York City was especially forthright.

How did we get such a strong hurricane so late in the season?  Why did it hit New Jersey, a place that’s had only one hurricane make landfall in 161 years of hurricane records?  (And that was in 1903.)

I learned the answers on WESA’s Allegheny Front on Saturday. Jeff Masters of the Weather Underground explained how hot ocean temperatures, prevailing winds, and high pressure centered over Greenland spawned the storm and steered it west.  (Click here to listen to the podcast.)

And though this individual storm can’t be pinned on climate change, its causes can.  The bottom line:  The whole world is hotter.

I hadn’t realized how much hotter and how rapidly the heat has increased until I watched this NASA animation of global surface temperature anomalies from 1880 to 2011.  Using the average global temperature in the mid-20th century as baseline, the map is colored blue when colder, orange when hotter.

Play the animation and see for yourself.

The train is rolling down the track.  (Perhaps it’s naive of me to say…) we could do something if we worked together politically and individually.  Meanwhile …

Old Charlie stole the handle
And the train won’t stop going
No way to slow down.
— Jethro Tull, Locomotive Breath, 1971

(animation from Goddard Multimedia, Goddard Space Flight Center, NASA, January 2012. Click here for more information)

Shelter From The Storm

Bird Behavior, Weather & Sky
Pigeons sheltering from the storm (photo from Wikimedia Commons)

30 October 2012

Last night as Hurricane Sandy approached Pittsburgh I thought about the birds. Where will they hide from the storm?  I knew the answer but I wanted assurance.

Birds already know how to cope with bad weather.  Each species uses its own strategy to survive.

Birds that live on cliffs or buildings, like the pigeons above, shelter out of the wind and find the driest possible place to wait out the storm.  This doesn’t always keep them dry but it keeps them safe.

Birds that roost in cavities, such as woodpeckers, owls, house sparrows and starlings go indoors during bad weather.  Sometimes more than 10 bluebirds will huddle together inside a bluebird box, using their communal body heat to stay warm.

Bluebird in nestbox (photo by Marcy Cunkelman)

Robins, sparrows and cardinals roost in thickets and hunker down close to the ground when it’s windy.  If you have a brush pile, as Marcy Cunkelman does, the birds will hide there from bad weather and predators.  The Coopers hawk happens to know this, too.

Coopers hawk on a brush pile full of birds (photo by Marcy Cunkelman)

Shorebirds and ocean birds fly inland, ducks find sheltered lakes or rivers.  Yesterday afternoon as Hurricane Sandy raged across Pennsylvania (29 October 2012) Shannon Thompson found huge numbers of waterfowl sheltering at Greenlick Run Reservoir in Fayette Country. Thousands of birds had stopped there to wait out the storm including snow geese, brants, tundra swans, and nearly 5,000 ruddy ducks.

Every species has a strategy.   I’m sure most of them made it through last night’s wind in Pittsburgh.  So did we.  The electricity is still on!

For more information, including stories of birds flying in the eye of the storm, see this excellent article from the National Wildlife Federation written in response to Hurricane Irene. It explains what happens to wildlife under these circumstances.

(pigeon photo from Wikimedia Commons, click on it to see the original.  Bluebird and Coopers hawk photos by Marcy Cunkelman)

Radiation Fog

Weather & Sky
Valley fog in late autumn (photo from Shutterstock)

24 October 2012

“Radiation fog” sounds scary but it’s actually the kind of fog we get in Pennsylvania at this time of year.  We often see it in the early morning below our hawk watch sites.  (Can you see the bird in this picture?)

It forms when winds are calm overnight while the land cools.  The land’s thermal radiation lowers the air temperature and condenses moisture into fog that usually evaporates in the morning.

In hilly country it’s called “valley fog” and is more pronounced because the topography traps cold air in the valleys with warm air overlaying it in a temperature inversion.

Like fog, inversions are also common in southwestern Pennsylvania in fall and winter.  They can be deadly when air pollution is involved.  The famous Donora Smog occurred during a five day inversion in 1948, October 27 to 31.  It killed 20 people and 800 animals immediately, sickened 7,000, raised the mortality rate in Donora for at least a decade, and lowered property values (who would want to live there after that!).

Inversions still occur but our air is cleaner. 

Nowadays we take for granted that our laws will protect us from air pollution. Unfortunately the laws could be weakened because companies complain it costs money to avoid killing or sickening us.

Without protection from air pollution, the fog would be scary after all.

(photo from Shutterstock)