6 January 2015

A fast moving cold front crossed western Pennsylvania yesterday.  The wind roared and temperatures fell from 61 degrees F Sunday morning to 19 degrees yesterday afternoon.  The weather news called it a clipper.

Technically it’s an “Alberta clipper,” described by Wikipedia as a fast moving low pressure area that typically affects the central provinces of Canada and parts of the Upper Midwest and Great Lakes.

Clippers start as warm moist wind from the Pacific that crosses the Rockies into Alberta.  When the wind hits cold air on the Canadian prairies it becomes a storm that rides the jet stream on a fast track east.  By the time clippers get to Pennsylvania, Alberta is rarely mentioned.

Though clippers sweep across the continent, they’re regional so if you live outside their zone — say in California, Colorado, or Florida — the word brings to mind the fast-moving sailing ships of the mid 19th century, famous for sailing through dangerous storms at Cape Horn (above).  The weather system is named for the ship.

Yesterday’s clipper left Pennsylvania but now we’re in for real winter — a low of 1 degree F Thursday morning.

Fill your bird feeders!  Birds need food to survive this cold.

(Clipper Ship at Cape Horn, painting by James E. Butterworth, public domain via Wikimedia Commons.  Click on the image to see the original)

p.s.  Here’s what clippers look like on eBay. 

(click on the clippers to see the original eBay photo)

Today is the earth’s perihelion, the moment each year when we’re closest to the sun.

Because the earth’s orbit is slightly elliptical, we’re always closest in early January and furthest in July (aphelion), a difference of about 3 million miles.  This sounds like a lot but it’s tiny compared to the size of our orbit.  The distance has no practical effect on our temperature.

… but …

When the earth gets close to the sun, the gravitational pull makes us speed up as you can see in the animation.  Right now we’re moving about 1 km/second faster (2,237 mph) than we do in July and this does affect our seasons.  The season surrounding early January (our winter) is 5 days shorter than the season surrounding early July.  This is nice for us but too bad for Australia where their summer is short.

This animation shows our fast and slow progress but its real purpose is to illustrate earth’s orbital precession (in an exaggerated way).

Earth’s orbit is not a closed ellipse.  Instead it tracks out a little further each time as if drawing a huge daisy in outer space without lifting its pencil.  In 21,000 years we come back to where we started and trace the same daisy again.

(animation posted by WillowW on Wikimiedia Commons. Click on the image to see the original with documentation)

Seen from the Southern Hemisphere, there’s a cat’s paw in the sky.

The Cat’s Paw Nebula, NGC 6334, was first noted by John Herschel at the Cape of Good Hope in South Africa in 1837.

At -35 degrees declination it’s hard to see this footprint even on a clear night in Pittsburgh.   Cat lovers will have to go south — far south — to get a good look.

This image was taken by the European Southern Observatory (ESO) in Chile.

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

If you live in southwestern Pennsylvania, when is the last time you saw a shadow outdoors?

A review of weather data for the past 30 days shows the sky has been overcast or foggy every day except for three days near Thanksgiving and on December 7 and December 9.

We’ve had only five days with enough sunlight to make a shadow. Otherwise, we’ve seen only brief glimpses of sun.

Oh, for a moment when the clouds are broken! It will finally happen this weekend.  Check the Pittsburgh Clear Sky Chart(*) to see how long it will last.

When the sun comes out, I’m going outdoors to make a shadow of my own.

(photo of Avenue Foch, along the Saint-Roch Square, Le Havre from Wikimedia Commons. Click on the image to see the original.)

(*) Astronomers use the Clear Sky Chart to find out when to view the stars at night.

14 December 2014

Have you ever seen these ice structures?

Needle ice forms when the soil is still warm and the air is freezing.  As ice forms on the soil’s surface, it draws up subsurface water by capillary action and builds new ice from the bottom.  The result is a structure that looks like needles or tiny barricades. Since there is very little soil above the ice it pokes into the air.

Later in the season when the soil freezes, needle ice forms underground as part of a frost heave.  Click here to see a cut-away frost heave in Vermont.

This patch of needle ice formed above a seep at Moraine State Park last weekend.  Downstream the ground was squishy but here it was forming ice that looked like tiny walls.

Watch for these ice sculptures on moist soil.

(photos by Kate St. John)

What’s the weather going to be like this winter?  NOAA’s Climate.gov has a prediction.

If you live in the U.S. West, Alaska, or northern New England, chances are you’ll be warmer than normal.  In the south-central and southeastern U.S. you’ll probably be colder.

But as the map text explains, the white zones aren’t necessarily going to be “normal.”  There’s an equal chance of being hot, cold or lukewarm in Pennsylvania.  We’ll just have to live through it to find out.

Click on the image to read this winter’s prediction at climate.gov.

(Winter temperature outlook 2014-2015 from climate.gov.  Click on the image to see the original)

18 November 2014

While we brace for 15⁰F this morning and moan that it’s 30⁰F below normal in Pittsburgh, it’s important to look at the bright side.

Last winter’s “polar vortex” put a real dent in invasive insect populations.  It reduced the hemlock woolly adelgid population in the eastern U.S. and completely wiped out adelgids in some of the infected stands in Cook Forest.  It also killed emerald ash borers and stink bugs.

This one-to-two day cold snap won’t seriously reduce invasive insects but it may zap a few bugs caught unexpectedly outside their lairs.  Every little bit helps.

And one more thought in case you aren’t convinced.  There are no poisonous snakes in Alaska and very few in Canada and northern-border U.S. states.

Winter is a great pest control system.

p.s.  It also reduces human pests.  No teenagers are “partying” tonight in the park across the street.

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