21 May 2021

Discrete fluffy white clouds. When I photographed this field of rapeseed in Ohio I was struck that all the clouds are flat on the bottom. Why?

They look as if they are laid on a glass ceiling. The bottoms are at the same altitude. Here’s a similar sky in Australia.

These fair weather clouds are Cumulus humilis, “humble” clouds that are flat and wide.

If the rising air is strong enough they grow upward to become Cumulus mediocris …

… and taller into Cumulus congestus which can end a fair weather day.

All of them have flat bottoms at the same altitude because …

The flat bottom of cumulus clouds defines the exact height at which a critical combination of temperature and air pressure causes water vapor within the rising current to condense into a visible cloud.

— Chicago Tribune, Ask Tom about weather, 19 July 2015

The bottoms show where the dewpoint is.

A NOTE about the yellow field: Rapeseed (Brassica napus), in the mustard family, is grown for its oil-rich seeds. Cultivars with very low eruric acid become canola oil. “Rapeseed is the third-largest source of vegetable oil and second-largest source of protein meal in the world,” according to Wikipedia.

(photo by Kate St. John)

22 May 2021

There were clouds on the eastern horizon when the sun rose in a spectacular sun pillar on Thursday morning 20 May.

According to Wikipedia (paraphrased):

The effect is created by the reflection of light from tiny ice crystals suspended in the air or in cirrus clouds.

The crystals are usually flat hexagonal plates that are oriented horizontally as they fall through the air. Each flake acts like a tiny mirror that reflects light sources positioned below it.

The presence of flakes at a spread of altitudes causes the reflection to be elongated vertically into a column. The larger and more numerous the crystals, the more pronounced this effect becomes. 

— Wikipedia article on light pillars

Though it was 57 degrees F at sunrise that morning, the sky was cold.

Sun pillars have a lot in common with sun dogs. They are both in the family of halos.

27 March 2021

You may have heard that tomorrow’s full moon at 2:50pm EDT (28 March 2021) is called the Worm Moon after the earthworm casts that appear as the ground thaws.

Did you know it’s also called the Crow Moon?

The more northern tribes of the northeastern United States knew this as the Crow Moon, when the cawing of crows signaled the end of winter.

NASA: The Next Full Moon is a Supermoon Crow Moon

Northern tribes didn’t have worms where they lived because the Ice Age wiped them out. Instead they noticed the change in crow behavior.

Nowadays we have earthworms that were accidentally imported from Eurasia so the Worm Moon makes sense to us.

In Pittsburgh the Crow Moon happens in December. 😉

Read more at The Next Full Moon is a Supermoon Crow Moon.

(photo from Wikimedia Commons and by Kate St. John)

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.