Category Archives: Weather & Sky

Nacreous Clouds

Nacreous clouds at Lake Mjosa, Norway (photo from Wikimedia Commons)

Near the poles in winter there are sometimes pearly-looking clouds glowing high above the Earth.

Polar stratospheric clouds (PSCs), also called nacreous clouds for their pearly appearance, form in the lower stratosphere at 49,000 to 82,000 feet — 1.5 to 2.6 times higher than a jet. They’re a winter phenomenon because they only form in the presence of super low temperatures, minus 108oF and colder.

Here you can see that they’re high above our usual clouds.

Nacreous clouds can cause trouble. Those made of water are benign but some are made of nitric acid + water that reacts with ozone in the stratosphere and creates a hole in the ozone layer. Stratospheric ozone protects Earth from the sun’s ultraviolet rays. It’s bad to have a hole in it!

Though we’ll never see these pearly clouds in Pittsburgh, we can appreciate their beauty from afar.

Polar Stratospheric Clouds over Scotland (photo from Wikimedia Commons)

p.s. The irony of ozone: Ground level ozone is bad; it burns plants and our lungs. Stratospheric ozone is good; it protects us from ultraviolet light. Ozone’s value depends on where it is.

Breaking Waves In The Sky

Kelvin-Helmholtz clouds in Wales (photo from Wikimedia Commons)

Have you ever seen breaking waves in the sky?

These unusual clouds are called fluctus or Kelvin-Helmholtz clouds. They occur when Kelvin-Helmholtz instability is present in the atmosphere, caused by a difference in temperature and wind. The air below the clouds is moving slowly, the air above them is moving fast. Since it’s an unstable condition, the clouds soon disappear.

Kelvin-Helmholtz instability creates fluctus clouds (photo from Wikimedia Commons)

Last summer Kelvin-Helmholtz clouds appeared over Smith Mountain Lake, Virginia on 18 June 2019. Meteorologist Chris Michaels at WSLS 10 News explains them in the video below.

Visit this vintage blog, Making Waves, for a video that shows what happens when the waves break.

(photos from Wikimedia Commons; click on the caption to see the original. Video embedded from WSLS TV)

How To Get Happy When Skies Are Gray

Fields of rapeseed on a cloudy day in France (photo from Wikimedia Commons)

Today in Pittsburgh it’s “cloudy” but a better description would be gloomy. Gray skies are depressing. How can we cope? Let’s look at yellow.

In an ongoing international survey (6,625 people in 55 countries to date) participants are asked, “What emotions do colors represent?” The data shows that most people say yellow is joyful but this isn’t true worldwide. It’s very joyful in the United States, exceptionally joyful in Finland, but in desert regions it’s not.

Researchers led by Domicele Jonauskaite at University of Lausanne, wondered if the attitude toward yellow was cultural or environmental: “What if people’s physical surroundings affect their feelings about certain colors?” They mapped the data here. (Darkest means the greatest yellow-joy; see Finland, Iceland, New Zealand.)

Likelihood of yellow-joy association (map from Science Direct, The Sun Is No Fun Without Rain)

They found that environment and cloud cover matter. The sun is not your friend in hot, dry, cloudless places. The study aptly named itself, “The sun is no fun without rain.”

Finland loves yellow. I think I know why. The first time I saw an intense field of yellow, like the one shown at top, was in Finland. I made my friends stop the car. So yellow! So happy! What is this plant? Rapeseed!

Rapeseed (Brassica napus) is the crop that makes canola oil.

Rapeseed in bloom (photo from Wikimedia Commons)

We don’t grow rapeseed in Pittsburgh, but I found a joyful yellow patch of chrysanthemums at the corner of Forbes Avenue and South Bouquet Street.

Go find some yellow and get happy.

(rapeseed photos from Wikimedia Commons; click on the captions to see the originals. Chrysanthemums by Kate St. John)

Which Personality Survives Bad Storms?

Comb-footed spider in the Everglades (photo from Wikimedia Commons)

Comb-footed spiders (Anelosimus studiosus) have a lot of personality. These social cobweb spiders live in colonies of 40-100 individuals, build their webs around branches, and hunt cooperatively to capture large prey.

The spiders exhibit either aggressive or docile personalities. If you know what to look for you can tell the difference. In the evening aggressive A.studiosus attack each other and then retire to opposite corners of the web; docile spiders rest side by side. Aggressive spiders come out to attack when their web is disturbed, the docile ones stay inside.

What happens to these spiders when they’re hit by a tropical storm or hurricane? Is there a difference in which spiders survive?

A 2018 study led by Jonathan Pruitt of U.C. Santa Barbara tracked 240 Anelosimus studiosus colonies in seven states including Florida, Alabama and the Carolinas. For baseline data they recorded the locations and personalities of the spider colonies. Later they searched for spider webs after a tropical storm or hurricane had passed.

You might think it’s futile to look for cobwebs after hurricanes, but individual spiders do survive, stay on their home territory, and rebuild. While humans are picking up the pieces, the spiders are too.

Damage from Hurricane Michael, 2018 (photo from Wikimedia Commons)

The study found that the storms always wiped out the docile spiders but the aggressive ones survived.

The relentless pressure of weather and nature is changing the spider population. Among comb-footed spiders, only the strong personalities survive.

For more information see Science Magazine: “Tropical storms are making these spiders more aggressive” and “For spiders, it’s cruel to be kind

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

Morning Glories Roll In

Morning glory clouds, Gulf of Carpentaria, Australia (photo from Wikimedia Commons)

These jelly roll clouds, called morning glories, are so rare that the only reliable place to find them is at the Gulf of Carpentaria during Australia’s spring, August to November. They form there because of Cape York Peninsula’s shape and orientation to the wind.

Map of Gulf of Carpentaria, Australia (from Wikimedia Commons)

In photographs from above these clouds look peaceful. From the ground they are awe-inspiring, rolling across the landscape on their horizontal axis as shown in this video from Leelanau State Park, Michigan in 2016.

Morning glory clouds are extremely rare in the U.S. but I have an idea why one formed at Grand Traverse Bay. Notice the shape and orientation of the Leelanau Peninsula. It’s like Cape York on a smaller scale.

Location of Leelanau State Park, Michigan (screenshot from Google maps)

Read more about morning glories and why glider pilots love them in this vintage article: Morning Glory.

(photo and Gulf of Carpentaria map from Wikimedia Commons, video by Chad Bousamra on YouTube, Michigan map screenshot from Google Maps; click on the captions to see the originals)


Dry ground in Schenley Park, 19 Sep 2019 (photo by Kate St. John)

22 September 2019

The ground in my neighborhood is parched dry and the leaves are wilting. It’s beginning to look like a drought in Pittsburgh. Is it so?

Leaves drooping on leafstem, Schenley Park, 19 Sep 2019 (photo by Kate St. John)

As of this morning Pittsburgh’s September rainfall total was 1.39 inches above normal, a statistical anomaly. We had a record rainfall event on September 1 (3.38″) and nothing to speak of since then.

If we don’t include the deluge on September 1st, precipitation is 1.9 inches below normal.

Are we in a drought? Not yet, according to the U.S. Drought Monitor. It takes more than three weeks without rain to make a drought. Indicators include stream flow, ground water and soil moisture. With a year-to-date excess of 9.44 inches, our drought severity index is zero.

U.S. Drought Monitor, 17 Sept 2019 (from

The weather forecast calls for showers tomorrow. I wonder if it will rain.

Leaves wilting in Pittsburgh, 19 Sep 2019 (photo by Kate St. John)

(photos and graph by Kate St. John, drought monitor map from

p.s. See a current map of the Long Term Palmer Drought Severity Index here.


Lenticular Cloud over Harold’s Cross Dublin Ireland, June 2015 (photo from Wikimedia Commons)

Strange as it looks, this fluffy white object is not a UFO (Unidentified Flying Object), it’s a special cloud.

Lenticular clouds form when the wind blows horizontally toward a fixed object that forces the air to rise and fall in a wave. If the air is moist it condenses at the crest of the wave and forms lenticular clouds at the top. The diagram below shows the wind flowing over a mountain, forming two mountain-shaped waves and lenticular clouds at peaks A and B.

Lee waves and windows (image from Wikimedia Commons)

Despite their calm appearance, the air around these clouds is very turbulent. Glider pilots ride the updrafts on the windward side but the downdrafts are deadly for everyone. This Weather Channel video explains more.

Because we don’t have mountains, lenticular clouds are rare in Pittsburgh. You have to travel to see a mountain wearing a hat.

Low lenticular clouds near Cook Inlet south of Homer, Alaska (photo from Wikimedia Commons)

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

Sheltering From The Storm

Immature Cooper’s hawk sheltering in a garage in Boca Raton, 2 Sep 2019, 3:44pm (photo by Natalie Mitchell)

As Hurricane Dorian approached Florida on Labor Day afternoon and a storm band dowsed the area with wind and rain, my sister-in-law went into her garage in Boca Raton and found a bird, 15-18″ tall, perched high near the ceiling. She texted me a photo. “What is it?”

This immature Cooper’s hawk is too young to have seen a hurricane before, but he knows that the weather is bad and he can feel it’s going to get worse. If he’s a skillful hunter he’s already eaten a lot in anticipation of the storm and just needs a safe place to wait out the weather, so he picked the best available option. He came indoors.

Cooper’s hawks don’t breed in South Florida but they spend the winter there (see range map). This youngster arrived recently and is improvising in bad weather. So far so good.

Cooper’s hawk range map from Wikimedia Commons (orange=breeding, purple = all year, blue=winter)

The bird may have to wait in the garage for a while. Hurricane Dorian could take 24 hours to move out of the area.

When the storm is over this bird will be glad to leave.

UPDATE, 3 Sep 2019, 8a: The storm isn’t bad in Boca Raton. The hawk left.

(photo by Natalie Mitchell, range map from Wikimedia Commons)

In The Doldrums

The doldrums appear as a band of clouds near the thermal equator (satellite image from Wikimedia Commons)

When we say we’re “in the doldrums” we feel depressed, dull and listless. Those with Seasonal Affective Disorder (SAD) — a type of depression related to the change of seasons — may be in the doldrums already though it’s only late August. They’re aware that Pittsburgh has lost an hour and 48 minutes of daylight since the June 21 solstice.

The real doldrums, whose fancy name is the Intertropical Convergence Zone (ITCZ), is dull and listless too. The ITCZ or “itch” is a band of monotonous calm where the north and south trade winds converge. As the winds meet each other they travel straight up, causing windlessness on the surface and clouds above. The calm is a real hazard for sailors who depend on wind to power their ships.

The lack of wind doesn’t mean the weather is beautiful. Rising heat and moisture lead to stacks of clouds, frequent thunderstorms and heavy rainfall. Some of the storms spin away from the doldrums as tropical depressions that become hurricanes.

You can see the ITCZ from satellite as a band of clouds near the thermal equator (photo at top). At this time of year there may be a dense circle in the line of clouds, a newly forming tropical depression.

The ITCZ moves north in the summer especially over land (which is warmer) and south in the winter, causing rainy and dry seasons in the tropics. The map below shows where the ITCZ usually goes; its path in the Pacific is affected by El Nino.

Seasonal variation of the ITCZ or doldrums (map from Wikimedia Commons)

Though the doldrums are deadly calm they can generate too much excitement (storms). Read more about them from the sailor’s point of view at: Seven things you need to know about the Doldrums.

(images from Wikimedia Commons; click on the captions to see the original)

5 Years Ago Today

Comet C/2014 Q2, Lovejoy (photo from Wikimedia Commons)

Five years ago today on 17 August 2014 Terry Lovejoy scanned the heavens from his home near Brisbane, Australia and found a new long-period comet.

Originating from the Oort Cloud, Comet C/2014 Q2 was in the southern constellation Puppis when Lovejoy first saw it. It wasn’t very bright at the time (apparent magnitude 15) but by mid-December it became so bright that you could see it naked-eye in a dark sky if you knew where to look. In January 2015 it crossed the celestial equator and became brighter still, soon passing close to the sun. Diminished by the sun’s power it won’t return for another 8,000 years.

C/2014 Q2 was the fifth comet Terry Lovejoy discovered and not even the most spectacular. His best was a Kreutz sungrazer found in December 2011. Click here to see it from the International Space Station.

As of this writing Lovejoy has discovered six comets. Find out how he does it in this article at