Category Archives: Weather & Sky


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

Have You Ever Seen…

Fallstreak hole over Big Island Wildlife Area Ohio, March 2019 (photo by Monica Miller)

Have you ever seen a fallstreak hole?

Fallstreak holes or cavum (the formal name) are circular or elliptical holes in a blanket of cirrocumulus or altocumulus clouds. They occur when super-cooled moisture in the cloud starts to form ice crystals. In rare instances this sets off a chain reaction that causes nearby water droplets to evaporate while the ice crystals fall out of the hole. As they fall the crystals melt and evaporate.

Monica Miller and Don Weiss were at Big Island Wildlife Area, Ohio in March when this one appeared above their heads. Monica wrote, “We kept expecting the hand of God or the mothership to come down.”

Fallstreak holes are always amazing, some more stunning than others. Here are a few photos from Wikimedia Commons.

Fallstreak, Italy, Borso del Grappa, March 2019 (photo from Wikimedia Commons)

Airplanes can create elongated fallstreaks when they pass through the cloud layer. This cavum has an added bonus. There’s a sundog inside it.

Cavum are particularly dramatic near sunset.

Fallstreak hole, August 2008, Linz, Austria

Click here to see an amazing fallstreak hole over Niederlenz, Switzerland. There are birds in the photo! Probably common swifts or alpine swifts.

I don’t remember ever seeing a fallstreak hole. I can hardly wait!

(top photo by Monica Miller, remaining photos from Wikimedia Commons; click on the captions to see the originals)

Lightning Indoors

Lightning strike (photo from Wikimedia Commons)

We all know it’s unsafe outdoors in a thunderstorm but did you know that lightning can be dangerous indoors, too? It depends on what you’re doing.

When lightning strikes a building it follows the quickest path to the ground. If the building has ample lightning rods it follows their lead. Otherwise it travels through the plumbing, wiring, metal door jambs, even the metal rebar inside concrete. If a building doesn’t have plumbing or electricity it’s not safe during lightning.

Here are some indoor lightning safety tips, paraphrased from

  • Don’t touch any electrical equipment unless it’s completely cordless. Anything that’s plugged in is bad.
    • A cellphone or cordless phone is OK, but not a landline that’s got a cord.
    • A laptop is OK if it’s not plugged into any wires, not even the charger.
    • Don’t run around unplugging everything while the storm’s in progress. Lightning could strike while your hand is on the cord.
  • Avoid the plumbing. Lightning can travel in water. Don’t take a shower, wash your hands, etc.
  • Stay away from the windows, not because lightning will come through the window (it won’t), but because it will explode the glass if it hits, creating lots of flying glass.
  • This one is the hardest: Avoid metal structural components including metal window frames, door jambs, and concrete rebar. If you’re in a concrete building good luck!

Of course it goes without saying that carports, porches and balconies are not safe because they’re outdoors. Cars are safe, but only if the windows are closed.

Read some hair raising lightning stories (pun intended) in this vintage article: Looking Forward to a Little Less Lightning.

p.s. Check out the Real Time Lightning Map to see where lightning is striking right now!

(photo from Wikimedia Commons; click on the caption to see the original)

High Tide On The Allegheny

High water at Highland Park Dam, 11 July 2019, 4pm (photo by Kate St. John)

Yesterday morning heavy rain caused flash floods, road closures, landslides, basement and first floor flooding, and car accidents in Pittsburgh.

The rain gauge at the airport measured 2.32″ for the day. All but .01″ of it fell in just four hours.

Precipitation at Pittsburgh International Airport, 11 July 2019 (graph from National Weather Service)

There were three huge rain events so thick that you couldn’t see to drive.

  • Half an inch (0.53″) in 50 minutes, 7:05a-7:50a
  • More than a third of an inch (0.38″) in 25 minutes, 7:55-8:17a
  • 1.4 inches in an hour, 10:05a-11:05a

If you want to see what it was like, click here for great footage from WPXI.

Hours later, at 3:30pm, a friend and I made our way from Indiana Township (between Fox Chapel and Cheswick) to Churchill. It took over an hour to get there. The traffic was horrendous and the roads that were open were littered with debris. Nadine, Sandy Creek, and Washington Boulevard were all closed.

But I got two photos of the river in flood.

High tide on the Allegheny!

p.s. This rain didn’t even set a record at the airport though there may have been localized records. My friend Julie had 4″ in her rain gauge in her Squirrel Hill backyard. Sue Vrabel commented below that she had 4.5″ in Churchill.

(photos by Kate St. John, rain graph from the National Weather Service)

Beat The Heat With Trees

It’s been hot in Pittsburgh lately but nothing like the heat wave that’s sweeping Europe with highs above 100 degrees F. @JeremyDBarrell tweeted a long term solution with a compelling image by Meg Caffin.

Meg Caffin is an urban forest consultant from Australia who provides guidance for cities looking to beat the heat. Her image at top used an infrared camera to show the temperature difference between a paved churchyard and the trees behind it. I’ve made a Fahrenheit translation below. Yes, it’s 113oF on the pavement and only 77oF under the trees.

Tree shade is cooler than building shade because buildings merely block the sun while the trees actually lower the temperature.

Schenley Park near Bartlett entrance (photo by Kate St. John)

Trees cool the air by transpiring. They take up water from the ground and release it from the stomata in their leaves. The release doesn’t usually drip from the leaves as shown below. Instead it evaporates and that’s what cools the air.

Transpiration droplets from a leaf (photo from Wikimedia Commons)

Evaporation — changing a liquid to a gas — uses energy. According to the Transpiration blog, “Energy is absorbed into liquid water. This reduces the temperature of the surrounding plant tissue and nearby atmosphere. To evaporate 1 gram of water 590 calories of energy is required.”

So more trees mean less heat.

If that isn’t enough reason to like trees, here’s another benefit. Trees increase your property value as shown in the EPA cost-benefit analysis below.

Trees increase property value far beyond cost of maintenance (EPA)

Plan to plant a tree this fall or plan to keep one. It’s cheaper to keep an existing tree than to plant a new one and mature trees increase property value even more.

Meanwhile if you’re feeling hot right now, visit a local park.

Schenley Park Upper Trail, July 2017 (photo by Kate St. John)

Beat the heat among the trees.

(embedded Tweet from Jeremy Barrell; infrared heat image by Meg Caffin for the City of Geelong, Australia (Fahrenheit added); transpiring leaf from Wikimedia Commons; photos of Schenley Park by Kate St. John)