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 108^oF 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.

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.

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.

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)

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.)

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.

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)

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.

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.

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)

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?

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.

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

(photos and graph by Kate St. John, drought monitor map from droughtmonitor.unl.edu)

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

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.

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.

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

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 Space.com.