Category Archives: Weather & Sky

It’s Time To Appreciate Clouds!

Wrinkled clouds in Pittsburgh, 7 Jan 2019

A week ago, 7 Jan 2019, the National Weather Service said the sky was clear at Pittsburgh International Airport but I have proof that at that moment it was overcast in the East End.

Not only was it overcast but the clouds were doing something special near the Cathedral of Learning. See that wrinkle? Is that undulation? Or is it the beginning of an asperitas formation?

Asperitas clouds are newly named, the first formation to be added to the International Cloud Atlas in 66 years. They were proposed by the Cloud Appreciation Society in 2009 and accepted in 2017. The atlas goes back to 1896 so this is a big deal.

This video of asperitas clouds in Tenerife, featured by the Cloud Appreciation Society, shows how fascinating these clouds can be.

Thick clouds say “Pittsburgh in winter” all over them. Stratus (low) and altostratus (mid-level) are our specialty but we also have altocumulus undulatus, cumulus fractus and sometimes even mammatus. Click to see what they look like.

Yes, we have overcast skies (tell me about it!) but there are cool things above us if we just take the time to look. Watch the video to get in the mood (exciting background music!).

C’mon, Pittsburgh, it’s time to appreciate clouds!

(photo by Kate St. John, video from on YouTube)

p.s. Did you know that clouds have a genus and species? At the bottom of this webpage is a cloud diagram that shows genus by height. Explore the International Cloud Atlas.

p.p.s. The sun is at the top edge of my photograph. Do you see it?

Diamond Dust

Here’s a form of ice we’re unlikely to see in Pittsburgh because our weather is rarely cold enough and it’s often overcast.

Diamond dust is clear-sky precipitation that looks like tiny diamonds in the air, falling through a ground-level cloud. The conditions for producing diamond dust are very specific:

  • The temperature has to be well below freezing — best at -13F or lower.
  • The cloud must be made of ice. It’s not a freezing fog that started wet and turned icy.
  • The cloud is in a clear sky and the sun is shining. That’s how you see the diamonds.

The best place to find diamond dust is in Antarctica where it falls 316 days of the year. Otherwise you have to be in the right place at the right time. Bundle up!

Bonus Question: Diamond dust is not associated with a pogonip. What’s a pogonip?

(video “Diamond Dust in Bellevue Washington” from Wikimedia Commons; click here to see the original)

The Snowman Of The Universe

Three images of Ultima Thule, center image is black-and-white (photos from NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute)

Last week the news broke that NASA’s New Horizons spacecraft had flown by and photographed what looks like the “snowman of the universe,” two icy chunks stuck together like a snowman and spinning out there in the Kuiper Belt beyond Neptune.

Yes, this object spins. Here’s a time lapse from NASA on New Years Day 2019.

Polar view of  Ultima Thule’s rotation over 2.5 hours (animation from NASA)

The snowman is reddish and tiny, only 20 miles long, so he can’t be seen from Earth. We wouldn’t even know about him except that a few years ago the New Horizons team looked for something interesting for the spacecraft to explore after it passed Pluto. They saw him as a dot using the Hubble Space Telescope in 2014 and chose him because he’s a classical Kuiper Belt object with low inclination and low eccentricity.

How eccentric is a snowman in outer space? It depends on his orbit.

NASA named his big chunk Ultima and his small one Thule. The combo sounds like a name from science fiction but in fact Ultima Thule was the name ancient geographers gave to the northernmost land in the inhabited world. Back then it was somewhere in Iceland, Norway, or a remote Shetland Island. Now it’s beyond Neptune. (Click here to pronounce Ultima Thule. It’s not what you think.)

Ultima Thule’s real name is (486958) 2014 MU69. It’s just the right number of digits for a phone number, but don’t call it. The long distance charges are astronomical!

For more information, including a diagram showing how the snowman formed, read NASA’s New Horizons Mission Reveals Entirely New Kind of World.

p.s. I’m sure Ultima Thule is not the only snowman out there so he’s actually “A Snowman” in the universe, not “The Snowman.”

(images from NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute)

Dogs In The Sky

Sun dog on the left side of the sun (photo from Wikimedia Commons)

Winter is an excellent time to see sun dogs, one of the many atmospheric optical phenomena caused by ice crystals in the air.

Sun dogs, also called parhelia, are bright spots to the right and left of the sun on the same horizontal level. If you know how to measure angles in the sky you’ll find them at 22 degrees from the sun, sometimes with rainbow colors.

Sun dogs are best seen when the sun is near the horizon but in Pittsburgh clouds often obscure one sun dog or even the sun itself. Pictured above is a lefthand sun dog, pictured below is one on the right. You can tell, even without seeing the sun, because the red end of the rainbow is nearest the sun.

Sun dog on righthand side of the sun (photo from Wikimedia Commons)

If you’re really lucky you’ll see the sun between two very bright sun dogs. The “three suns” effect shows the origin of their name in Greek mythology — Zeus (the sun) walking his two dogs.

Sun with both sun dogs, Germany (photo from Wikimedia Commons)

In the old days, three suns were considered an omen and not a good one. During the Wars of the Roses (England’s civil wars in the late 1400’s), the future king had to convince his panicky troops that three suns meant good news.

The prelude to the Battle of Mortimer’s Cross in HerefordshireEngland in 1461 is supposed to have involved the appearance of a halo display with three “suns”. The Yorkist commander, later Edward IV of England, convinced his initially frightened troops that it represented the three sons of the Duke of York, and Edward’s troops won a decisive victory. The event was dramatized by William Shakespeare in King Henry VI, Part 3.

Wikipedia entry for Sun dog

Look up for birds on a cloud-skimmed winter day and you may see “dogs” in the sky.

(photos from Wikimedia Commons; click on the captions to see the originals)

It Won’t Be So Wet

3-month precipitation forecast, Jan-Mar 2019, USA (map from NOAA)

Though this prediction is old news it will affect us very soon. It looks like Pittsburgh will have normal weather in the next three months, whatever that means in these crazy climate days.

According to NOAA’s precipitation outlook (above), Pittsburgh has an equal chance of being exceptionally wet or dry during January through March 2019.

The big news, but not for Pittsburgh, is that it’s going to be incredibly warm in western North America — especially in the Pacific Northwest, Alaska and Canada — and colder than normal in the Southern Appalachians. This is due to climate change and a developing El Niño in the eastern Pacific.

I won’t mind less rain in the next three months. We had record-setting rainfall in 2018: 57.83 inches, which is 19.64 inches — or 51.4% — above normal!

It won’t be so wet in early 2019. Maybe.

For more information and an interesting visual see the forecast at WeatherNation TV.

(maps of long range seasonal predictions from NOAA)

Dramatic Difference in Daylight

21 December 2018

The southern (or winter) solstice will occur in Pittsburgh this evening at 5:23pm. By then we’ll have lived through a very short day, 9 hours and 17 minutes of rainy gloomy overcast daylight.

If we were in Manchester, UK there would be even less daylight. Today they have rainy overcast skies too, but they also have fewer hours daylight, 7 hours 28 minutes. The flip side is that Manchester has more sunlight in June. 

Scott Richards decided to compare both solstices in Manchester side by side.  He filmed the entire day — sunrise to sunset — on June 21 and December 21, then sped up the film so we don’t have to watch for 20 hours. Instead it lasts six minutes.

I’ve started his video, above, near sunset on the winter solstice (right) side.  If you watch for a minute you’ll see the moon rise in winter while the summer sun is still so high that it leaves the video frame.

There’s a dramatic difference in the amount of daylight from solstice to solstice. No wonder I feel sleepy in December.

(video by Scott Richards on YouTube)

Needle Ice

Needle ice on a cold morning at Moraine State Park, Dec 2014 (photo by Kate St.John)

Tuesday 18 December 2018:

After 1.5 inches of rain fell over the weekend the ground here in Pittsburgh is super soggy.  Meanwhile it’s been warm since last Thursday but the temperature dropped below freezing last night.  These are perfect conditions for needle ice.

Needle ice forms when the soil is warm and the air is freezing.  While ice is forming on top of the soil it draws up subsurface water by capillary action. The ice grows from the bottom creating a structure that looks like needles or even tiny barricades. Since there’s little or no soil on top of the ice, it stands up in the air.

Closeup of ice needle tops (photo by Kate St. John)

From the top it doesn’t look interesting — basically a frozen lump — but seen from the side it’s very cool.  In the patch shown below some ice needles keeled over so I captured both the top and sides in one shot.

A patch of needle ice, Dec 2014 (photo by Kate St. John)

When the soil itself freezes, needle ice can form underground and create a frost heave on the surface.  I’ve never opened a frost heave but here’s what one looks like in a photo from Vermont.

As you watch your footing today, look for the delicate structures of needle ice.

(photos by Kate St. John)

It Rimes

Rime scene in Helsinki, Finland (photo from Wikimedia Commons)

We don’t have flowers in December but we do have ice and it comes in many forms.  Here’s the first in a series on ice, starting with my favorite:  Rime.

The word “rime” comes from Old English hrim which meant hoarfrost or a chill mist or fog.  Nowadays hoarfrost and rime are not the same thing. Hoarfrost doesn’t form in fog. To get rime there has to be cold fog or the location has to be in the clouds. For that reason, “Rime shouts, Mountains!”  It’s easy to find rime at high altitudes.

Rime at Jay Peak, Vermont (photo from Wikimedia Commons)

Rime forms when super-cooled water drops (in fog or a cloud) crystallize on cold objectsSoft rime is feathery and so lightweight that it can’t break the trees.  Hard rime is denser and comb-like.  Both are white because they contain trapped air.

Soft rime on a small branch (photo from Wikimedia Commons)

The best part about rime is that it can form in a light wind. When it does it points toward the wind because each new crystal is deposited on the windward side. This is counter-intuitive; it’s the opposite of rain.

What direction did the wind blow through this fence?

Rime on a chainlink fence (photo from Wikimedia Commons)

Rime can even coat snowflakes. Graupel, which looks like hail, is actually a rime-coated snowflake.  I wonder if these pop when you step on them.

Garupel in Elko, NV (photo from Wikimedia Commons)
Graupel in Elko, NV (photo from Wikimedia Commons)

This month is a good time to find rime in the Laurel Highlands. My best experience with it was during an east wind at the Allegheny Front Hawk Watch.  No one else was up there to enjoy it.  It was way too foggy!

Click here to see more rime photos including a “rime doughnut” at Summitpost.

(photos from Wikimedia Commons; click on the captions to see the originals)

Snow On Leaves

Snow on leaves, Schenley Park, 16 Nov 2018 (photo by Kate St. John)

On Thursday it rained. Then it sleeted. Then it snowed in the wee hours of Friday morning, especially north of Pittsburgh.

In the old days most of the trees would be bare by now, but this year many still have leaves.

Ice and snow made the leaves heavy and some of the trees came down, hitting power lines as they fell.  By Friday morning KDKA reported that 65,000 households north and east of the city were without electricity.  No power, no heat, and for those with well water, no water.  It may take until Sunday evening to get all of the power restored..

The City is warmer than surrounding counties so Schenley Park had snow on the leaves, but no ice.

Snow on leaves, Schenley Park, 16 Nov 2018 (photo by Kate St. John)

Here’s what my favorite hillside looked like yesterday. 50% of the trees still have leaves.

Only half of the trees are bare, Schenley Park, 16 Nov 2018 (photo by Kate St. John)

The power failures wouldn’t have been so bad if most of the trees had been bare. 

(photos by Kate St. John)

The Sun Will Rise An Hour Early

Sunrise in Pittsburgh, 13 Nov 2009 (photo by David Fulmer, CC license on Flickr)

The sun will rise an hour early tomorrow in Pittsburgh.

At 2:00am on 4 Nov 2018 the cell networks will push out the new time (1:00am Standard Time) to our mobile phones.  We’ll gain an hour of sleep and will be surprised when the sun sets just after 5pm. Rush hour will start at dusk on Monday.

Now that we’ve passed the autumnal equinox we’re losing daylight every day.  Tomorrow we’ll lose two minutes! 

Sunrise/sunset in Pittsburgh, PA, Nov 3 and 4, 2018 (screenshots from SunDroid)

Meanwhile, birds are unaffected by our trick with clocks and will still cue on daylight.  When the crows show up in Oakland at rush hour, lots of people will be surprised. 

Hundreds pf crows above Bayard (photo by Kate St. John)
Hundreds of crows above Bayard Street  at dusk, 7 Nov 2016 (photo by Kate St. John)

Monday will be the first time people and crows are on the same schedule.  😉

(photo credits: Pittsburgh sunrise by David Fulmer, CC license on Flickr, screenshots of SunDroid app & photo of crows by Kate St. John)