30 January 2021

In winter when the moon is full and the sky is almost clear with a thin layer of cirrus clouds you may see a halo around the moon. The ring has a radius of 22^o so it’s called a 22^o halo, a moon ring, or a winter halo.

The conditions for a moon ring are relatively rare in Pittsburgh: a full moon + a clear sky + thin cirrus clouds whose ice crystals refract and reflect the moonlight. Seeing the halo is a matter of perspective based on where you stand and which ice crystals refract/reflect for you.

That’s why, like rainbows, halos around the sun – or moon – are personal. Everyone sees their own particular halo, made by their own particular ice crystals, which are different from the ice crystals making the halo of the person standing next to you.

— EarthSky.org “What makes a Halo Around the Moon”

On Wednesday night, 27 January, the nearly-full Wolf Moon rose before sunset hidden by Pittsburgh’s overcast sky. By 8pm the clouds were breaking up and made an iridescent halo around the moon. It was not a true 22^o halo but it cast enough light to make shadows. All I have to show for it is this very blurry photo.

The next morning at 4:15am the sky was absolutely clear and the moon was shining brightly without a ring. I was disappointed because the clear sky was wasted on us while Pittsburgh slept. Three hours later the sun rose in an overcast sky.

Last night was the same — a nearly full moon and an absolutely clear sky. No moon ring.

The Wolf Moon is waning and winter will too. The next full moon on 27 February is our last chance at a Winter halo. The full moon after that — 28 March — will be in the Spring.

(photo at top from Wikimedia Commons; click on the caption to see the original. blurry photo by Kate St. John)

28 January 2021

Seven years ago this week open fields in eastern Ohio and western Pennsylvania were graced with a rare winter phenomenon. Snow rollers!

Snow rollers are delicate snow balls made by the wind that look like snow bales or jelly rolls.

They can only happen when all of these weather conditions are met.

• First an icy layer(*) forms on top of old snow that new snow will not stick to.
• Then, a thin layer of wet, loose snow falls on the ice.
• After the wet snow falls, the wind blows just hard enough to start the balls rolling without destroying them.
• Snow rollers stop when they are too heavy for the wind to move. 

The center of a snow roller, the first layer, is fragile and disintegrates easily. Even before a snow roller becomes hollow it is too fragile to pick up.

This year it’s too warm and we have don’t have much snow so I doubt we’ll see snow rollers any time soon.

Read about 2014’s rare occurrence in this vintage article: Nature’s Snowballs.

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

(*) In other climates snow rollers can form on top of powder snow and can be triggered by a snowball falling on a steep hill.

25 January 2021

During Pittsburgh’s overcast winters we grumble about clouds and pray for sunshine. On Saturday after blogging about last week’s clouds, I was outdoors with Liz Spence when the clouds broke at 1pm. “Look! We have shadows!”

For real excitement, though, check out these lenticular clouds over the Rincon Mountains in Tucson, Arizona, 18 January 2021. We never see this in Pittsburgh(*).

Lenticular clouds today in Tucson over the Rincons. pic.twitter.com/Ry8uSsPOoE

— Halfastro (@halfastro) January 19, 2021

(*) because we don’t have mountains. Read more here about lenticular clouds.

(top photo by Kate St. John)

16 January 2021

Red sky at morn, sailors forewarn.

Sunrise on Friday 15 January was a deep crimson red. Though it was sunny for a couple of hours yesterday, gusty wind arrived at 9:30a and rain followed five hours later.

Mackerel sky without rain.

A mackerel sky can predict rain 6-8 hours later, but that wasn’t the case over Frick Park on Saturday 9 January 2021. The day was brilliantly sunny for two hours but became overcast by 5p. These clouds were the leading edge.

Are they a “mackerel sky” or not? What do you think?

Meanwhile, I’d say the bottom right corner is a Harbinger of Gloom.

p.s. See the comments below and this video for the definition of a mackerel sky. Indeed this is an altocumulus one.

(photos by Kate St. John)

15 January 2021

Seven years ago we were in the grips of subzero temperatures as the wobbly jet stream drove arctic air south over the U.S. It was called the arrival of the Polar Vortex.

Seven years ago on 6 January 2014 much of the U.S. was below zero as shown on the map below. Pittsburgh was above freezing that day but the warmth ended soon as the blob of cold air headed east.

This winter has been mild and the jet stream has been behaving. But a sudden stratospheric warming event occurred in the arctic in early January 2021. The warmth made the arctic vortex unstable. Meteorologists speculated: Are we in for another polar vortex in the Lower 48? Will we have a major cold snap?

Not necessarily. NEWS CENTER Maine in Portland has a good explanation of what’s going on, published on Monday 11 Jan 2021. (This news is for Mainers but it applies to us, too.)

Meanwhile it’s quite warm across the U.S. Here’s the same U.S. temperature map seven years later, published today 15 January 2021.

Read about the original polar vortex in this 2014 article: Polar Vortex.

(graphics from NOAA; click on the captions to see their origins)

6 January 2021

The desert dips below freezing on winter nights but if you want ice in the summer, how do you get it? This is a story of human innovation using natural processes.

In the 17th century BCE the Persians (Iranians) figured out how to make ice in the desert and store it through long hot summers. They used this method for 3,600+ years until electric refrigeration replaced it in the 20th century.

Though water is scarce in Iran, the method works well because the air is so dry. The process requires many steps: deliver water, make ice, store ice and keep it cool.

Deliver water to the ice-making location: The Persians built underground water channels called qanats to deliver water from uphill wells to the downhill population. Water in the qanat does not evaporate in the sun and stays cooler because it’s underground.

Make ice: To make ice the Persians built plaza-like ponds, shaded by a high wall, where the shallow water froze on winter nights. Collecting the ice before sunrise, they stored it in the ice house (dome in the background). Here are two photos of the same ice plaza no longer in use. People give it a sense of scale.

Store the ice: The ice house, called a yakhchal, stored ice underground, where conditions are naturally cooler, and protected it with an insulated dome. The hole at the top of the dome allowed hot air to escape.

Keep the ice cool: The ice was kept cool using natural ventilation to draw in cold air and remove hot air. Windcatcher towers in some towns were quite ornate.

Here’s a video that puts it all together.

Very cool!

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