Today the Earth reaches its furthest point from the Sun in its annual orbit.  This is Earth’s aphelion, position 1 above.

It’s a source of wonder to me that this happens at our hottest time of year.  Shouldn’t aphelion cool things off?

Apparently not by much.  The orbit determines Earth’s livability but has far less affect on temperature than the composition of our atmosphere and the tilt of the earth’s axis.  Right now the northern hemisphere is tilted toward the sun and we certainly feel it.

If we had the data and computer horsepower we could prove aphelion’s effect on climate because it hasn’t always occurred in July.

According to Wikipedia, “On a very long time scale, the dates of perihelion and aphelion progress through the seasons and make one complete cycle in 22,000 to 26,000 years. There is a corresponding movement of the position of the stars as seen from Earth that is called the apsidal precession.”

So, hey, if you’re around 12,000 years from now, aphelion will happen in December.

It’s something to look forward to.

(drawing of aphelion and perihelion by Peasron Scott Foresman via Wikimedia Commons)

Transit of Venus tonight.  Click here for a cool video describing it.

Yesterday morning the weather forecast for the eastern U.S. looked like a train wreck with severe thunderstorms, flash floods, heavy snow and freezing rain.

These events were listed as “possible” because the approaching storms were so complicated.  The depth of snow in Pittsburgh depended on the timing of two frontal systems approaching the east coast from different directions.  Would they collide and merge their forces over western Pennsylvania?  Would the coastal front stay to the east and not affect Pittsburgh?  Would the systems cause a single large storm or a prolonged one-two punch?  And where?

The National Weather Service uses many weather modeling systems to make their predictions, primarily NAM (North American Mesoscale Forecast) and GFS (Global Forecast System).  Both NAM and GFS run four times a day.  Meteorologists then analyze the results and make the forecast.  When the models agree it’s easy.  When they don’t it’s mighty hard.

Yesterday the forecast discussion for Pittsburgh said:  “Even at this close proximity to the onset of this system, subtle differences in model solutions make for a very difficult forecast.”   Then they predicted 2 to 4 inches of snow for the Pittsburgh area. Twenty-four hours later the snowfall prediction hasn’t changed but its timing has.

I feel their pain.  Because I’m in charge of computers and phones at WQED, I’m often asked to predict how a computer or phone system will behave under different conditions in the future.  Sometimes the answer is easy and sometimes… Well, suffice it say I’m glad my predictions aren’t broadcast on TV and radio news.

Meanwhile we await the results.  Rain is falling now.  It will be interesting to see how well the models predicted this one.

(weather forecast map, Sunday 22 April 2012, by the National Weather Service. Click on the image to see the current forecast map.)

Yesterday the sky attracted my attention.

In the morning I saw thin lines of rain hanging from the clouds without touching the ground.

Virga!

Virga means “rod” in Latin and is the name for precipitation that evaporates before reaching the ground.  It’s very common out West where the air is dry and virga’s rapid evaporation causes high winds.

I tried to take a picture but the best of the virga drifted behind the ballpark lights.  In the middle of the photograph you can see “rods” falling and curling from the cloud.  Moments earlier there was more separation between the rain and the ground.  I just wasn’t quick enough.  Click here for a much better picture of virga.

The sky cleared at midday, then high, thin clouds moved in ahead of a cold front.  Way up there, above 20,000 feet, the air was filled with tiny ice crystals that caused an optical effect — a halo around the sun.

Halos are circular pastel rainbows that occur when sunlight passes in one side of the hexagonal ice crystals and out another side.  The light is doing this all over the sky but we typically see halos at 22^o from the sun (or moon), though other angles are possible.

I can tell you it’s hard to take a halo’s picture because the sun confuses the camera.  I tried to block the sun with a telephone pole but that wasn’t enough.  I had to use my mitten too, so this photo is odd.

Click here for a better picture of a halo.

Keep looking up.  You may see some atmospheric effects.

(photos by Kate St. John)

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p.s. Late on Monday afternoon we had a mackerel sky (shown below).  Can you guess why it’s called that?

28 January 2012:

Last weekend’s solar flare made the news with beautiful images from NASA’s Solar Dynamics Observatory.

On January 23 at 4am UTC (January 22 11pm in Pittsburgh) a huge “burp” of charged particles and magnetic fields burst off the sun from Active Region 1402.  The wave traveled at 2,200 km/second — 150 times slower than the speed of light — so we saw it before we “felt” it on January 24 around 1400 GMT (January 24, 9am EST, plus or minus 7 hours).

Major pulses from the sun can cause outages in the electric grid and interference with radio and TV broadcasts and communication devices.  The episode I best remember was when a pulse killed Telstar 401 and stopped PBS broadcasting until they could find a new satellite and we re-pointed our station dish.

Earth’s magnetic field protects us from these “burps” but it gets distorted while doing so.  In normal times the solar wind squashes our magnetic field on the earth’s sunward (day) side and elongates on the night side.  Here’s a diagram from NASA showing how that works with the sun positioned at top left.

In a solar flare event the magnetic bulge on the night side gets longer, the loops break and they “flap in the breeze.”  When the field snaps back it releases energy that whacks the earth’s upper atmosphere, causing the beautiful northern lights and sometimes electro-magnetic interference.

Meanwhile nothing much happened here on Earth except …

On Tuesday morning January 24 around 7am, an electrical transformer at WQED blew up and burned.  It was quickly extinguished and the damage was minor, but it left us without electricity.  Thanks to our generator we remained on the air and on the web.  All day Tuesday and into the night, the electricians worked hard to hook up a temporary power feed.  Unfortunately, when they switched us back to house power on Wednesday morning at 2am an internal surge tripped a breaker on our emergency grid and we went off the air and off the web.

So it’s been an exciting week for us in technology at WQED.  The flare probably didn’t cause our electrical problem but the timing was quite a coincidence.

Watch what happened on the sun in this cool video from NASA SDO:

(All photos from NASA. Click on the images to see the originals.)

Today’s forecast in Pittsburgh calls for a rainy high of 53^o followed by a strong cold front with winds gusting to 35 mph overnight.  North and east of here the wind will be even gustier, up to 50 mph in Dubois and Johnstown.

So I wondered… What causes wind gusts?  And what will cause them tonight?

Wind gusts are quick bursts and lulls of wind (we know this) lasting 20 seconds or less.  The National Weather Service doesn’t even call it a gust until it reaches 18 mph and has a 10 mph difference between burst and lull.  If the gust lasts a minute it’s called a squall.  If it lasts longer than that it’s real wind, a gale or a hurricane.

Weather experts say gusts are caused by three things:  turbulence from friction, wind shear and solar heating.

We can rule out solar heating today but I’ve seen it in summer when rising hot air is quickly replaced by cold air dropping to fill its place.  In the desert the gusts are amazing.

Wind shear occurs at the unseen three dimensional boundaries where wind speed and direction change within a short distance.  If the wind could hold colored dots wind shear would be an amazing visual effect, an edge where a slow wind moving one way meets a faster wind moving another direction.  Aloft these gusts cause a bumpy airplane ride, but they’re dangerous near the ground where there’s no vertical distance to recover from the bump.

I don’t know if wind shear is a factor in tonight’s weather but I suspect not.  It wasn’t mentioned at all.

On the other hand I’m sure turbulence from friction is involved.  Today’s cold front is moving in very fast with 60 mph winds at 2000 feet.  At higher elevations, such as the Laurel Highlands, the 60 mph wind is a lot closer to the ground. If even a fraction of it scrapes the earth the friction will cause gusts.

Turbulence is even greater near cliffs and buildings where the wind rushes faster through narrow openings, causing whirls and eddies that raise leaves and trash high into the sky.  I experience this all the time at the Cathedral of Learning.

Tonight the peregrines won’t find it pleasant to roost up there, but they’re used to the wind.  “Ho hum,” they say.  “This wind is nothing.”

(photo by Steve F. from Wikimedia Commons. Click on the photo to see the original)

p.s. This blog post is about wind gusts but what is causing so much wind today? Read Rob Protz’ comment for an explanation.